|
Physicochemical Characterization of Biochar Derived From the Pyrolysis of Cotton Gin Waste and Walnut Shells
Marlene C. Ndoun1, Allan Knopf2, Heather E. Preisendanz1,3,*, Natasha Vozenilek1, Herschel A. Elliott1, Tamie L. Veith4,Michael L. Mashtare1, Stephanie B. Velegol5, Clinton F. Williams2
Published in Journal of the ASABE 66(5): 1163-1174 (doi: 10.13031/ja.15489). 2023 American Society of Agricultural and Biological Engineers.
1Department of Agricultural and Biological Engineering, Pennsylvania State University, University Park, Pennsylvania, USA.
2Arid Land Agricultural Research Center, USDA Agricultural Research Service, Maricopa, Arizona, USA.
3Institute of Sustainable Agricultural, Food, and Environmental Science, Pennsylvania State University, University Park, Pennsylvania, USA.
4Pasture Systems and Watershed Management Research Unit, USDA Agricultural Research Service, University Park, Pennsylvania, USA.
5Department of Chemical and Biological Engineering, Pennsylvania State University, University Park, Pennsylvania, USA.
*Correspondence: heg12@psu.edu
The authors have paid for open access for this article. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License https://creative commons.org/licenses/by-nc-nd/4.0/
Submitted for review on 7 December 2022 as manuscript number NRES 15489; approved for publication as a Research Article by Associate Editor Dr. Xiaoyu (Iris) Feng and Community Editor Dr. Kati Migliaccio of the Natural Resources & Environmental Systems Community of ASABE on 27 April 2023.
Highlights
Abstract. The sustainable management of agricultural waste has gained increasing attention worldwide, especially regarding the production of value-added products that are renewable and carbon-rich. Further, there is a need to provide low-cost, lower-energy alternatives to materials such as activated carbon for removing contaminants from water. The goal of this study was to characterize various physicochemical properties of biochar produced from cotton gin waste (pyrolyzed for 2 h at 700?C, CG700) and walnut shells (pyrolyzed for 2 h at 800?C, WS800) to better understand their potential to be effective in various environmental applications. The properties that were characterized are the following: (i) biochar pH; (ii) specific surface area (SSA); (iii) surface functional groups; (iv) surface elemental composition; (v) surface charge; and (vi) surface morphology. Pyrolysis led to the destruction of acidic functional groups within the parent biomass and an increase in ash content, resulting in alkaline biochars with pH values of 9.8 and 10.9 for WS800 and CG700 biochar, respectively. Zeta potential measurements demonstrated that both biochars were negatively charged at environmentally relevant pH ranges. The FT-IR spectrum and XPS results for the CG700 biochar showed the presence of several functional groups, including the OH, C=C, and C-O groups within the biochar samples. BET results demonstrated that CG700 had a low SSA (8.57–22.31 m2 g-1), and the biochar was dominated by fibrous, irregular shaped particles, according to the results from the SEM imaging. The FT-IR spectrum for the WS800 biochar showed the presence of the carbonyl group, which was inherited from the parent biomass. BET measurements for the WS800 showed a decline in SSA with a reduction in particle size, likely due to a collapse of the honeycomb structure of the WS800 biochar with crushing to reduce the particle size, as revealed by the SEM images. The results of this research will help to inform the applications of biochar produced from cotton gin waste and walnut shells, two large sources of agricultural waste materials, and promote sustainable alternatives to extend the life cycle of these materials into value-added products.
Biochar is a stable, carbon-rich, energy dense by-product synthesized through the pyrolysis of (e.g., 350–800°C) in the absence of oxygen (Tan et al., 2017). Depending on the heating rates, pyrolysis can be categorized as slow or flash/fast (Al Chami et al., 2014). Slow pyrolysis, characterized by slowly increasing heat over several hours, minimizes the formation of liquid and gaseous by-products and allows vapors produced during the secondary reaction to be removed. This has been shown to be the most efficient technique for high yields of solid carbonaceous biochar (Lehmann and Joseph, 2009; Kloss et al., 2012; Mohan et al., 2014), which is useful for soil amendment applications (Blanco-Canqui, 2019). Flash pyrolysis involves high temperatures and very short residence times (on the order of seconds) and is known to favor the production of liquids (bio-oil; Goyal et al., 2008); however, it has also been shown to produce biochar with properties that are favorable for water quality applications (e.g., Ndoun et al., 2021).
Although the pyrolysis process is similar to the method used in the production of charcoal, biochar’s use is generally intended to improve agricultural soil productivity or mitigate environmental pollution as opposed to being used for fuel, chemical reduction, or as a dye. Biochar production does not typically involve the additional high temperature or processing used to convert charcoal to activated carbon, which is a more versatile adsorbent that is particularly effective in the adsorption of organic and inorganic pollutants from air and aqueous environments (Lehmann and Joseph, 2009). Nevertheless, like activated carbon, biochar has a microporous structure, high carbon content and specific surface area, and surface carboxyl, hydroxyl, and phenolic functional groups that result in substantial adsorption capacity (Uchimiya et al., 2011a). Correspondingly, biochar has numerous potential environmental and energy-related applications, including binding heavy metals in soils (Ahmad et al., 2014), increasing soil organic matter (Chan et al., 2008; Dotaniya et al., 2016), scrubbing air emissions (Singh et al., 2010), and partially treating wastewater (Kasozi et al., 2010; Chaukura et al., 2017; Yan et al., 2017; Uchimiya et al., 2011a).
The properties, characteristics, and quality of the biochar depend both on the pyrolysis conditions (residence time, temperature, heating rate, and reactor type; Mohan et al., 2014; Tan et al., 2017) and feedstock type (crop residues, wood biomass, food waste, municipal solid waste, animal manure, and sewage sludge; Enders et al., 2012; Yahya et al., 2015). Due to the low carbon content and high molar hydrogen/carbon and oxygen/carbon ratios, biochars produced from crop residue and wood biomass exhibit higher surface areas than those made from animal litter and solid waste feedstocks (Bourke et al., 2007). The presence of more surface-active sites is generally expected to increase contaminant removal effectiveness.
Using biochar as an alternative to powdered and granular activated carbons for mitigating environmental pollution provides several economic and environmental advantages. Biochar feedstock is generally limited to waste residues, thereby repurposing the material and extending its life cycle. In addition, the production of biochar emits fewer greenhouse gases (0.9 kg CO2-eq. per kg of biochar) than the production of activated carbons (6.6 kg CO2-eq. per kg of activated carbon) and requires less energy (6.1 MJ/kg versus 97 MJ/kg; Finger Lakes Biochar, 2016; Alhashimi and Aktas, 2017). Unlike activated carbon, biochar can be used without further treatment or activation, thereby reducing production costs.
The increases in agricultural activities and industrialization that accompany the world’s population growth has generated large quantities of agricultural waste, thereby causing a variety of socioeconomic and environmental issues. An estimated 1300 Tg of waste are generated along the supply chain from agricultural to final consumption stages, resulting in approximately 3300 Tg of CO2/yr (United Nations, 2020), or 6% of annual global anthropogenic greenhouse gas emissions (Food and Agriculture Organization of the United Nations, 2015). The production of value-added products such as biochar from agricultural waste can help transform open-ended solid waste disposal systems into closed loop systems that recycle material and reduce greenhouse gas emissions.
Cotton gin is one such agricultural waste product, as increasing population growth creates continuous worldwide demand for cotton (Gossypium hirsutum L.) to manufacture a wide range of textiles and other commodities. Cotton gin waste is a heterogenous product consisting of lint, dirt, sticks, and leaves that remain after cotton has been harvested. For every one bale of raw cotton lint (227 kg) produced, about 40-147 kg of cotton gin waste is generated (Thomasson, 1990), leading to the generation of approximately 2.7 Tg of waste annually (Maglinao et al., 2015). This huge quantity of solid waste has become a major problem for cotton mills. Cotton gin waste is rich in nutrients such as nitrogen, phosphorus, and potassium that can be added to soils to improve plant growth. Numerous studies have shown the use of cotton gin waste as a soil amendment that can be beneficially applied to land (Diaz et al., 2002; Jackson et al., 2005; Papafotiou et al., 2007; Ghosh et al., 2011). However, cotton gin waste cannot be re-used directly as a soil amendment because it contains 22% permanganate lignin, which delays decomposition (Hamawand et al., 2016). Cattle feed is another way to repurpose the cotton gin waste; however, this method has several drawbacks due to the poor digestibility, low protein content, and high lignin and ash contents of the cotton gin waste (Myer, 2007; Hamawand et al., 2016). Cotton gin waste possesses characteristics like those of other agricultural wastes (rice straw, lemongrass, cotton stalks) that have been used to produce biochar for the removal of dyes, organic contaminants, and metals from wastewater. However, the physicochemical characterization of biochar from cotton gin waste that promotes its use in environmental applications such as remediation of contaminated water and use as a soil amendment has not been extensively studied (Haque et al., 2021). Elucidating the characteristics of biochar from this large waste stream can provide a value-added waste mitigation strategy to reduce waste generated from the cotton ginning industry.
Other significant sources of agricultural waste are nutshells. Today, the Central Valley of California is the world’s principal walnut growing region, with California walnuts (Juglans regia L.) accounting for 99% of the commercial supply grown in the US and 75% of the world trade (California Walnuts, 2022). United States walnut production has steadily increased from under 0.3 Tg in 2007 to more than 0.6 Tg in 2021 (USDA-NASS, 2021). Because shells constitute 55% of the total dried fruit weight (Bujdoso and Cseke, 2021), an estimated 0.335 Tg of walnut shells are discarded each year from US production alone. Walnut shell-derived biochar has a high carbon content, large pore volume, and high surface area (Qiu et al., 2018), and it has commonly been used in wastewater treatment (Kamar et al., 2015). Walnut shell biochar has been shown to remove a wide variety of contaminants from water, such as heavy metals, pesticides, dyes, and suspended solids. Walnut shells have a very high hardness and have been used as an abrasive that is applied to the surface preparation of cementitious surfaces (Cheng et al., 2017). Recently, walnut shell biochar has been explored as the next generation of adsorbent for the removal of contaminants of emerging concern, including pharmaceuticals, veterinary drugs, herbicides, engineered nanomaterials, food additives, and industrial compounds from the environment. Recent studies such as Nazari et al. (2016), Teixeira et al. (2019), Popoola (2020), and Román et al. (2020) show that walnut shell-derived biochar has a high affinity for these compounds. However, the physicochemical characterization of walnut shell-derived biochar remains limited. Understanding these physicochemical characteristics is critical to understanding its potential for capturing contaminants of emerging concern.
The goal of this study was to characterize six physicochemical properties of biochar produced from cotton gin waste and walnut shells to better understand their potential to effectively remove contaminants from aqueous solution. The properties are as follows: (i) biochar pH, which can change the speciation of contaminants in the water passing through biochar, thereby influencing the ability of the biochar to interact with these contaminants; (ii) specific surface area (SSA), which has a positive relationship with contaminant removal; (iii) surface functional groups, which can be helpful in understanding the mechanisms by which the biochar can remove contaminants from aqueous solution; (iv) surface elemental composition, which can provide information regarding the structure and arrangement of elements and functional groups present on the biochar surface; (v) surface charge, which governs the electrostatic sorption of contaminants from aqueous solution; and (vi) surface morphology, which provides data regarding the distribution of pore sizes and particle sizes of the biochar.
The results of this research will help to inform the applications of biochar produced from cotton gin waste and walnut shells, two large sources of agricultural waste materials, and promote sustainable alternatives to extend the life cycle of these materials by creating value-added products.
Materials and Methods
Biochar Production
Cotton gin waste was obtained from Maricopa County, Arizona. The biochar produced from cotton gin waste was prepared according to Novak et al. (2013). The cotton gin waste was dried, ground, and sieved manually to pass through a 6-mm sieve. About 2.0 kg of ground cotton gin waste was placed in a stainless-steel tray and pyrolyzed slowly at low heating rates (0.05–0.1°C/second) using a gas tight retort (Lindberg/MPH, Riverside, MI) at 700°C for 2 h in a stream of N2 gas. The resulting biochar, referred to as CG700, was ground and sieved. The sieving procedure
involved placing different sieve sizes to ensure only particles of the desired size were left. For example, to pass the 600 µm mark, a 600 µm sieve was placed at the bottom and a 650 µm sieve was placed at the top. The particles in between the two sieves were collected and stored in a desiccator to prevent the adsorption of water.
Walnut shell biochar was obtained from Carbo Culture, located in the Central Valley of California. The shells were pyrolyzed using a pyrolysis oven at an elevated pressure of 10 bar. During pyrolysis, the temperature was raised to 700?C in five minutes and this was followed by slowly increasing the temperature to 800 ?C for 2 h in a non-inert carbonizer. Each carbonizer contained 1 cm3 of crushed walnuts, and the biochar obtained (WS800) was sieved to pass the 600-µm, 710-µm, and 2-mm marks and stored in a desiccator. Different particle size fractions were segregated for characterization to determine the change in physicochemical characteristics as a function of particle size. High temperatures were selected for pyrolysis because biochars produced at higher temperatures (700 ?C and higher) have been shown to exhibit properties that are most useful for contaminant removal applications, such as higher specific surface area, higher cation exchange capacity, higher anion exchange capacity, and more alkaline pH, as reviewed by Ippolito et al. (2020).
Biochar Characterization
Functional Groups: FT-IR Spectra
The functional groups of the biochars were identified using FT-IR (Bruker IFS 66/S and Bruker Vertex V70, Germany) equipped with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. Sample powder was placed in a 3 mm diameter 316 stainless steel sample cup assembly, and a total of 500 scans were averaged per spectrum at a resolution of 4 cm-1. Experiments were carried out under a constant N2 purge with a fresh, clean KBr reference acquired before each sample acquisition.
Surface Elemental Analysis: X-Ray Photoelectron Spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) was used to analyze the functional groups and surface elemental composition of the biochars. XPS analysis was performed using a Physical Electronics VersaProbe II instrument equipped with a monochromatic Al ka x-ray source (h? = 1,486.7 eV) and a concentric hemispherical analyzer. Charge neutralization was performed using both low energy electrons (<5 eV) and Ar ions. The binding energy axis was calibrated using sputter cleaned Cu (Cu 2p3/2 = 932.62 eV, Cu 3p3/2 = 75.1 eV) and Au foils (Au 4f7/2 = 83.96 eV; Seah, 2001). Peaks were charged referenced to the CHX band in the carbon 1s spectra at 284.8 eV. Measurements were made at a takeoff angle of 45° with respect to the sample surface plane. This resulted in a typical sampling depth of 3-6 nm (95% of the signal originated from this depth or shallower). Quantification was done using instrumental relative sensitivity factors (RSFs) that accounted for the x-ray cross section and inelastic mean free path of the electrons.
Surface charge: Zeta Potential
To measure zeta potential values, solutions were made at four pH levels (4, 6, 8, and 10) by adding 0.05 M HCl or NaOH to deionized water to reach the desired pH level. Biochar samples were ground, passed through a 34 µm sieve, and mixed into each pH solution in a mass-to-volume ratio of 0.015g to 30 mL. Each mixture was measured at each pH three times (150 scans per measurement) using the Malvern Zetasizer ZS (Worcestershire, UK), and scan averages are reported here.
pH
The pH of the biochar samples was measured once by a hand-held pH meter after adding biochar to de-ionized water in a mass-to-volume ratio of 1:20 (0.5 g of biochar and 10 mL of DI water) and shaking for 1 h.
Specific Surface Area (SSA)
The Brunauer-Emmett-Teller (BET) analysis was used to determine the specific surface areas and pore volumes of the biochar using the ASAP 2020 automated surface area and porosimetry system (Micrometrics, Norcross, Georgia, United States) and the accompanying t-plot analysis software. A sample mass of 0.5 g was used for BET analysis. Briefly, BET analysis was conducted with a Quantachrome NOVA 2200E BET analyzer (Germany) according to International Standard Organization Method 9277. Samples were heated prior to analysis and degassed under vacuum, and the amount of gas physically adsorbed to the biochar was determined.
Surface Morphology: Scanning Electron Microscope (SEM) Imaging
Scanning electron microscope (SEM) imaging analysis was conducted using a Thermo Scientific Apreo S Scanning Microscope (Thermo Fisher Scientific, Waltham, Massachusetts, United States) to obtain the surface morphology and an overall view of the pore structure of each biochar. The CG700 and WS800 biochar with particle size of 710 µm were both viewed at a low maginification of 200x to determine the particle morphology. Given that the WS800 biochar showed a decline in SSA measurements as the particle sizes decreased, the different particles sizes (425, 600, 710, and 2000 µm) were viewed at a higher magnification of 2000x to obtain a better understanding of the change in pore structure as the particle sizes change.
Results and Discussion
Functional Groups: FT-IR Results
The FT-IR spectra for CG700 and WS800 revealed the functional groups present in the biochar (fig. 1, table 1). Both spectra show that CG700 and WS800 exhibited an absorbance peak between 1400 and 1500 cm-1, representing aromatic C=C ring stretching or alkenes (Gomez-Serrano et al., 1996), which is due to the presence of lignin in WS800 and CG700 (Lu et al., 2019). The bands at 1350 cm-1 showing the C-O stretching on the CG700 and WS800 spectra may be attributed to ethers (R-O-R0), esters (R-CO-O-R0), or phenol groups (Bouras et al., 2015). The peak at 870 cm-1 reflects out-of-plane bending vibrations of C-H groups located at the edges of the aromatic planes (Song et al., 2010).
 |
| Figure 1. FT-IR Spectrum of biochar produced from cotton gin waste pyrolyzed at 700ºC (CG700, primary axis) and of biochar produced from walnut shells pyrolyzed at 800ºC (WS800, secondary axis). |
Functional Groups in CG700
The CG700 biochar spectrum (fig. 1) had a band appearing at 3440 cm-1 on the CG700 spectrum, which is attributed to hydroxyl (O-H) stretching vibration (table 1). This indicates that some water was likely still present in the biochar sample (Fernandes et al., 2019). The presence of similar functional groups was shown when cotton gin residue was hydropyrolyzed at 300, 350, 400, and 500°C. The resulting biochars showed the stretching vibrations of the O-H at 3400-3500 cm-1, C-H and C-O stretching vibrations of alkanes and esters (Balagurumurthy et al., 2014).
Functional Groups in WS800
The band shown at approximately 1700 cm-1 on the W800 spectrum (fig. 1) belongs to the C=O vibration of the carbonyl group, which is consistent with other research that has shown that the carbonyl group is an aromatic ring that generally appears at 1700–1680 cm-1, and this can indicate the presence of ketones, aldehydes, or carboxylic acids (Gupta et al., 2019). Liu et al. (2020) demonstrated that walnut shell biochar produced at 550ºC contained a characteristic stretching vibration peak of the carbonyl group (C=O). The presence of this additional oxygen-rich functional group for WS800 caused the biochar to be less alkaline, which was further corroborated by the lower pH of the WS800 compared to the CG700 biochar (9.9 vs. 10.9). The WS800 biochar lacked the O-H band (fig. 1, table 1), perhaps due to the difference in pyrolysis methods (slow pyrolysis for CG700 and flash pyrolysis for WS800) and the higher pyrolysis temperature (800ºC for WS800 vs. 700ºC for CG700), which caused the sample to be more dehydrated compared to CG700. The presence of the hydroxyl and carboxyl groups on the biochar surface can act as proton donors, while the deprotonated carboxyl and hydroxyl groups can interact with cationic contaminants in solution (Yin et al., 2019). Various studies have reported the presence of C=O, C=C, C-O functional groups on walnut shell biochar pyrolyzed at different temperatures. Shagali et al. (2021) reported that walnut shell derived biochar produced at 700°C showed stretching vibrations of the C=C, C-O and C=O functional groups. When investigating the physicochemical properties of biochar derived from hard-shelled walnut, medium-shelled walnut, thin-shelled walnut, and paper-shelled walnut that was produced at 375°C, Alfattani et al. (2021) demonstrated the stretching vibrations of C-H representing the methyl/methylene group, C=C, C-H, and C-O due to the presence of cellulose, hemicellulose, and lignin in the parent biomass.
| Table 1. Summary of physicochemical and structural parameters characterized for biochar produced from cotton gin waste pyrolyzed at 700ºC (CG700) and of biochar produced from walnut shells pyrolyzed at 800ºC (WS800). | |||
| Parameter | Analysis Method | CG700 | WS800 |
| Functional Groups | Fourier-transform infrared spectroscopy (FTIR) | C-H C=C O-H | C-O C=O O-H |
| Surface Functional Groups and Elemental Composition | X-ray photoelectron spectroscopy | C-C C-O C=O O-C=O C Ca Cl K N Na O | C-C C-O C=O C O |
| Surface Charge | Zeta potential | -33.8 to -18.2 mV | -44.8 to -3.5 mV |
| pH | pH meter | 10.9 | 9.9 |
| Specific Surface Area | Brunauer-Emmett-Teller (BET) analysis | 8.57 – 22.31 m2 g-1 | 14.28 – 4.95 m2 g-1 |
| Surface morphology | Scanning electron microscope (SEM) | Heterogeneous, with fibrous, round, and polygonal shapes | More homogenous, with round and polygonal shapes and a honeycomb pore structure |
Surface Elemental Analysis: XPS Results
Surface elemental composition results (fig. 2) showed that carbon was the most dominant element present on the surface of the biochar samples, as is expected, based on Cheng et al. (2006), with C-C as the most prevalent carbon bond. The next most common element on the surface of both biochars was O. Oxygen can participate in bonds with H+ ions in solution, thereby decreasing the magnitude of negative charges on its surface. The most common functional groups on the surface of the biochars, as identified with XPS, were C-O and C=O (carbonyl) in both biochars, which is consistent with the FT-IR results discussed above, and with O-C=O (carboxyl) also present on the surface of CG700. The difference in results between FT-IR and XPS is due to XPS only being capable of identifying elements present on the surface (only 3-6 nm of depth), while the FT-IR spectra can identify elements that were present on the surfaces as well as inside the biochar samples (up to thousands of nanometers of depth), implying that the surface of the biochar differs from the bulk of the biochar sample (table 1).
Surface Chemistry of CG700
The CG700 biochar contained small percentages of additional elements on its surface, including K+, Cl-, Na+, Ca2+, and N (fig. 2, table 1). The presence of these additional elements is likely a result of the heterogeneity of the cotton gin feedstock and may help to increase contaminant removal through surface complexation, electrostatic attraction, and ion-exchange mechanisms. Rehrah et al. (2014) also showed that biochar produced from cotton gin waste can contain a variety of elements, with Ca2+, S, K+, Mg2+, P, Fe2+, and Zn2+ also present in biochar pyrolyzed at 750ºC. Similarly, biochar produced from cotton gin trash pyrolyzed at 450°C for 2 h showed the presence of 12,079 mg L-1, 13,778 mg L-1, 2,667 mg L-1, 31.45 mg L-1, 743.11 mg L-1, and 26.16 mg L-1 of Ca, K, Mg, P, Fe, and Zn, respectively (Evans et al., 2013).
Surface Chemistry of WS800
Based on the XPS data (fig. 2), the WS800 biochar was comprised of only the elements C (94.3%) and O (5.7%) (table 1). These results are similar to those reported by David (2020), who also produced biochar from walnut shells and found that the biochars were comprised of 88% C and 10% O, for biochar pyrolyzed at 800ºC at a low heating rate (5ºC/min). No other elements were reported. Additionally, the high C content of WS800 biochar is likely because walnut shells are wood-based, and wood-based feedstocks are known to produce biochars with higher C content (Ippolito et al., 2020). However, studies have reported that walnut shell biochar shows the presence of other minerals such as sodium, magnesium, potassium, etc. on their surfaces. Yin et al. (2019) demonstrated that walnut shell biochar produced at 500°C for 3 h and analyzed using X-ray fluorescence spectroscopy showed the presence of 73% C, 2% H, 15% O, 1% N, and 0.4% S. Walnut shell biochar produced at 300, 400, 500, and 600°C showed the presence of C, O, N, Na, Mg, P, Al, Si, Ca, and Fe (Gupta et al., 2019).
 |
| Figure 2. X-ray photoelectron spectroscopy results showing the distribution of functional groups and elements on the surface of the biochar produced from cotton gin waste pyrolyzed at 700ºC (CG700) and biochar produced from walnut shells pyrolyzed at 800ºC (WS800). |
Surface Charge: Zeta Potential Results
Both biochars were negatively charged and became increasingly more negatively charged as the pH increased, as shown by the decreasing zeta potential with increasing pH values (fig. 3). Similar results were shown by Taheran et al. (2016), with zeta potential decreasing from ~ 5 mV to -35 mV with increasing pH values (1-14) for both raw and activated biochar derived from pinewood. Analogous to the results in this study, Li et al. (2020) demonstrated that walnut shell activated carbon showed a zeta potential value of -13.47 mV at a pH of 7, showing that the biochar is negatively charged at neutral pH. The surface charge of WS800 was always more negative than the surface charge of CG700 (fig. 3), likely because WS800 contained a higher percentage of two oxygen-rich functional groups (C-O and C=O).
The pHpzc describes the pH at which the charge density on the surface of the biochar is zero (zeta potential = 0) and is influenced by the chemical properties of the functional groups present on its surface (Song et al., 2010). When the pH was less than pHpzc, the biochars were positively charged, and when the pH was greater than pHpzc, the biochar was negatively charged because of deprotonation of the functional groups (Bernal et al., 2017). While the pHpzc could not be determined with measured data, the results shown in figure 3 suggest that CG700 and WS800 would reach a pHpzc when the pH < 4. Therefore, both the CG700 and WS800 biochars will be negatively charged at environmentally relevant pH ranges, implying that the biochars will be more suitable for electrostatic removal of cations. However, dissimilar results were demonstrated for the pHpzc for walnut shell biochar by different studies. Lu et al. (2019) showed that walnut shell biochar had a pHpzc value of 6.0. Walnut shells pyrolyzed at 700°C for 2 h had a pHpzc of 7.1 (Liu et al., 2022), and walnut shell biochar produced at 520°C for 1 h had a pHpzc of 9.7 (Georgieva et al., 2020). These results demonstrate that the physicochemical characteristics of the biochar obtained from agro-waste can be influenced by the original source from which the waste is obtained as well as the pyrolysis conditions.
 |
| Figure 3. Zeta potential measurements at different pH values for biochar produced from cotton gin waste pyrolyzed at 700ºC (CG700) and biochar produced from walnut shells pyrolyzed at 800ºC (WS800). |
pH Results
CG700 and WS800 were both alkaline, with pH values of 10.9 and 9.9, respectively (table 1). When comparing the biochars produced from different feedstocks at temperatures ranging from 300 to 750°C, Rehrah et al. (2014) demonstrated that biochar from cotton gin had the highest pH values ranging from 8.2-9.8, and this was attributed to the high contents of Ca2+ and Mg 2+. A review of biochar produced from shredded cotton stalk and cotton straw showed biochars with pH values of 9.9 and 10.4, respectively (Makavana et al., 2021). Hard-shelled walnut, medium-shelled walnut, thin-shelled walnut, and paper-shelled walnut derived biochar produced through pyrolysis at temperatures varying from 375°C to 750°C showed alkaline characteristics with pH values between 8.1 and 8.3 (Alfattani et al., 2022). The higher alkalinity of the biochars in this study can be explained by the deprotonation of binding sites and increase in ash content as pyrolysis proceeds, leading to an increase in the pH of the biochar samples (Frišták et al., 2021). Similarly, Novak et al. (2009) reported that biochars produced at high temperatures (greater than 500°C) have high pH values because of the increase in the concentration of non-pyrolyzed inorganic elements in the feedstocks and the formation of basic surface oxides under high pyrolysis temperature. The lower pH of WS800 compared to CG700 was likely attributed to the higher percentage of C-O and C=O functional groups on the biochar’s surface.
Specific Surface Area (SSA) Results
The results of the SSA analysis for each biochar particle size are shown in table 2, with the highest SSA for CG700 of 22.31 m2 g-1 and 64.95 m2 g-1 for CG800. The surface areas observed in this study were very low compared to the reported range of 500-2000 m2 g-1 for surface areas of activated carbon and other biochars. However, similar studies have reported low surface areas for cotton gin biochars produced at high temperatures. For example, cotton gin-derived biochars produced at 750°C for 1, 2, and 3 h all showed total surface areas less than 50 m2 g-1. The low surface areas of cotton gin biochars could be due to the plugging of the pores by inorganic compounds from ash, condensed volatiles, and other amorphous compounds (Rehrah et al., 2014). Also, comparable surface areas were obtained from biochars derived from walnut shells in other studies. Lu et al. (2019) demonstrated that walnut shell-derived biochar possesses a SSA of 43.03 m2 g-1. By investigating the physicochemical characteristics of walnut shell biochar produced at 700°C, Wan et al. (2021) showed that the biochar had a SSA of 17.72 m2 g-1. Hard-shelled walnut, medium-shelled walnut, thin-shelled walnut, and paper-shelled walnut that were pyrolyzed at 375°C showed SSA ranging from 40-58 m2 g-1 (Alfattani et al., 2022). Walnut shells pyrolyzed at temperatures varying between 400-700°C showed SSA values between 74.06 and 737.98 m2 g-1 (Xu et al., 2022). The higher SSA observed for WS800 compared to CG700 with a particle size of 710 µm (table 1) may be because of the higher temperature used to produce WS800. A number of studies have established that raising the pyrolysis temperature leads to an increase in the SSA of biochar, likely due to the removal of more volatile matter inside or blocking the pores, resulting in a higher SSA (Gai et al., 2014; Geudidi et al., 2017; Huang et al., 2020; Sun et al., 2014; Uchimiya et al., 2011b; Zhang et al., 2015).
| Table 2. BET surface areas of biochar produced from cotton gin waste pyrolyzed at 700ºC (CG700) and biochar produced from walnut shells pyrolyzed at 800ºC (WS800) for three different particle sizes. | ||
| Biochar | Particle Size (µm) | Specific Surface Area (m2 g-1) |
| CG700 | 600 | 22.31 |
| 710 | 14.07 | |
| 2000 | 8.57 | |
| WS800 | 600 | 14.28 |
| 710 | 34.52 | |
| 2000 | 64.95 | |
Specific Surface Area for CG700
The SSAs for CG700 were shown to increase with decreasing particle size, such that the SSA increased more than two-fold from 8.57 m2 g-1 for a particle size of 2000 µm to 22.31 m2 g-1 for a particle size of 600 µm. The increase in SSA as the particle size decreased can be attributed to more exposure of the cracked surfaces. For example, Lui et al. (2018) showed that the biochar SSA increased from 15.88 m2 g-1 to 20.96 m2 g-1 when the particle size decreased from 0.3-2.0 mm to < 0.0075 mm. Similar results were demonstrated by Hong et al. (2020), with biochar produced from cotton stalk at 550°C showing an increase in SSA as the particle sizes decreased from coarse (2.7 m2 g-1) to fine (4.9 m2 g-1) to ultrafine (11.3 m2 g-1).
Specific Surface Area for WS800
The SSAs for WS800 were shown to decrease with decreasing particle sizes. The lowest SSA was 14.28 m2 g-1 for a particle size of 600 µm and increased more than four-fold to 64.95 m2 g-1 for a particle size of 2000 µm. For a particle size of 710 µm, the SSA was 34.52 m2 g-1, respectively, which is similar to the SSA of 34.32 m2 g-1 for walnut shell biochar produced at 500°C reported by Frištàk et al. (2021).
Typically, SSA increases as particle size decreases from coarse to fine. This is because as the biochar is crushed into smaller sizes, mechanically cracked pores become exposed, and the ratio of the exposed surface area to total surface area increases (Rafiq et al., 2016; Sangani et al., 2020). However, in the current study, the opposite trend was likely observed because crushing the walnut shells after pyrolysis to reduce the particle sizes resulted in a decrease in SSA due to blocking of the pores by the debris generated from the crushing (as described by SEM). Mui et al. (2010) found that when biomass particle size was reduced from 1000-2000 µm to 500-710 µm prior to pyrolysis, the resulting biochars had SSAs of 156 m2 g-1 and 117 m2 g-1, respectively. The fine and coarse biochars had a micropore volume of 0.022 cm3 g-1 and 0.024 cm3 g-1, respectively, and an increase in mesopore volume from 0.099 to 0.258 cm3 g-1 occurred with a reduction in particle size. The decrease in surface area was due to finer biomass particles having a larger contact area with heat, which might lead to extensive pore widening and facilitate the transformation of micropores into mesopores. Therefore, when the grinding occurs in the production of the biochar (before or after pyrolysis), it appears to affect whether the SSA increases or decreases with particle size.
Surface Morphology: SEM Imaging Results
SEM images showing the morphology of CG700 and WS800 (fig. 4, table 1), both at a particle size of 710 µm and 200 × magnification, demonstrated that both biochars have irregular particles with different morphologies. Some particles exhibit a round shape, while others have polygonal shapes. The CG700 biochar still shows fibers in its structure even after pyrolysis, and this is attributed to the characteristic nature of the cotton gin waste (parent biomass), which is also very fibrous (fig. 4a). The WS800 showed a more compact structure with differences in each individual particle within the overall sample (fig. 4b). The particles with a round shape appear to be more porous, whereas on the polygonal particles, pores were rarely observed. Li et al. (2020) demonstrated through SEM that activated carbon produced from walnut shells had a compact surface with irregular particles and no cavities, which is typical for lignocellulosic materials, which usually show irregular structures and limited pores.
  | ||||
| (a) CG700 (710 µm) | (b) WS800 (710 µm) | |||
| Figure 4. Scanning electron microscope (SEM) images of biochar produced from cotton gin waste pyrolyzed at 700ºC (CG700) and biochar produced from walnut shells pyrolyzed at 800ºC (WS800) for particle sizes of (a) 600 µm, (b) 710 µm, and (c) 2000 µm. Magnifications are 200x. | ||||
Surface Morphology of WS800
The WS800 biochar with particle sizes of 425, 600, 710, and 2000 µm were viewed at a higher magnification of 2000 × (figs. 5a, b, c, and d, respectively) in an attempt to understand why the SSA decreased with a reduction in particle size. The four different particle sizes were chosen in order to obtain a better understanding of the change in pore structure due to grinding to reduce the particle sizes. Given that grinding the CG700 biochar did not result in unusual SSA values, the results from the SEM imaging of CG700 of particles sizes 425, 600, 710, and 2000 µm are not shown here. The 2000x magnification revealed a honeycomb morphology for the WS800, with the development of a porous structure and enlargement of the pores as the particle sizes became bigger. Similar results were demonstrated by Xu et al. (2022), with SEM images of walnut shells pyrolyzed at temperatures varying between 400-700°C demonstrating a honeycomb pore structure. Figure 5a shows that WS800 with particle sizes of 425 µm has smaller pores compared to the pore structure developed when the particle size increased to 600, 710, and 2000 µm. In addition, these pores have a lot of debris stuck inside them as a result of the fine particles generated during crushing. These particles can move inside the pores, blocking the pores and resulting in low overall SSA. As the particle size increases to 600, 710, and 2000 µm (figs. 5b, c, and d, respectively), the pores appear to be larger and more unencumbered.
  | ||||
| (a) WS800 (425 µm) | (b) WS800 (600 µm) | |||
  | ||||
| (c) WS800 (710 µm) | (d) WS800 (2000 µm) | |||
| Figure 5. Scanning electron microscope (SEM) images of biochar produced from walnut shells pyrolyzed at 800ºC (WS800) for particle sizes of (a) 425 µm, (b) 600 µm, (c) 710 µm, and (d) 2000 µm. Magnifications are 2000x. | ||||
With an increase in particle size, the number of fine particles in the samples decreases, and thus less blockage of the pores occurs. This therefore leads to WS800 biochar with larger particles exhibiting a larger surface area due to their pores being more accessible by the N2 gas used in surface area analysis. However, it is hard to draw a conclusion on the effects of particle size on the SSA of biochar based on current studies. Further research needs to be done in order to validate the relationship between biochar particle size and biochar SSA.
Conclusion
Agricultural wastes such as cotton gin waste and walnut shells could be successfully used as effective feedstock biomass materials for the production of biochar. The physicochemical properties of the biochars suggest their usefulness in environmental applications, including the removal of contaminants from aqueous solutions to provide improved water quality. The biochars produced via pyrolysis of walnut shells at 800°C (WS800) and cotton gin waste at 700°C (CG700) resulted in biochars with a net negative charge, with WS800 having a greater magnitude of negative charges compared to CG700. Further, given that the biochars are negatively charged at environmentally relevant pH values (pH range between 6.5-7.5), this suggests that the biochars are suitable for environmental applications that involve the sorption or retention of cationic contaminants. Both biochars are alkaline in nature (pH = 9.9-10.9) due to an increase in ash content and a reduction in acidic functional groups; therefore, these biochars also have the potential to benefit applications where a high pH is required. Although the SSA values for CG700 and WS800 biochar were comparatively low, these results do not preclude biochar from serving as effective adsorbents for the removal of neutral or charged contaminants. Moreover, the FT-IR spectrum and XPS results revealed the presence of several functional groups on the surface of the biochars, including O-H, C-H (alkyl and phenyl), C-O/ C=O and C=C bonds. These functional groups could enable biochar to be used for environmental applications where the functional groups could form bonds with contaminants in the environment, including pharmaceuticals and other emerging contaminants that are difficult to remove from domestic wastewater prior to land-application or other beneficial reuse purposes. Finally, SEM imaging showed that the biochars have irregularly shaped particles, even within the same sample. The CG700 biochar retained the fibrous nature of the parent feedstock even after pyrolysis. WS800 biochar showed a honeycomb structure with a collapse of this structure due to crushing and grinding to reduce the particle size. This resulted in a decrease in the SSA as the particle size decreased.
This study demonstrated that the parent biomass and the pyrolysis temperature and method play an important role in the morphological and physicochemical characteristics of biochar. Therefore, careful selection of the parent biomass for the production of biochar is warranted in order to produce biochar suitable for specific applications. In addition, the transformation of biowaste into biochar has the potential to convert the open-ended solid waste disposal system into a closed loop that involves the repurposing and recycling of materials.
Acknowledgments
This research is supported, in part, by the USDA National Institute of Food and Agriculture Federal Appropriations under Project PEN04574 and Accession number 1004448. C. Ndoun was supported, in part, by a cooperative agreement from USDA Agricultural Research Service and by the Penn State Department of Agricultural and Biological Engineering.
Mention of company or trade names is for description only and does not imply endorsement by the USDA. The USDA is an equal opportunity provider and employer.
Nomenclature
BET = Brunauer-Emmett-Teller
CG700 = Cotton gin waste pyrolyzed at 700°C
FT-IR = Fourier Transform Infrared Spectroscopy
GHG = Greenhouse gas
pHpzc = pH point of zero charge
SEM = Scanning Electron Microscopy
SSA = Specific Surface Area
WS800 = Walnut shells pyrolyzed at 800° C
References
Ahmad, M., Lee, S. S., Lim, J. E., Lee, S.-E., Cho, J. S., Moon, D. H.,... Ok, Y. S. (2014). Speciation and phytoavailability of lead and antimony in a small arms range soil amended with mussel shell, cow bone and biochar: EXAFS spectroscopy and chemical extractions. Chemosphere, 95, 433-441. https://doi.org/10.1016/j.chemosphere.2013.09.077
Al Chami, Z., Amer, N., Smets, K., Yperman, J., Carleer, R., Dumontet, S., & Vangronsveld, J. (2014). Evaluation of flash and slow pyrolysis applied on heavy metal contaminated Sorghum bicolor shoots resulting from phytoremediation. Biomass Bioenergy, 63, 268-279. https://doi.org/10.1016/j.biombioe.2014.02.027
Alfattani, R., Shah, M. A., Siddiqui, M. I., Ali, M. A., & Alnaser, I. A. (2021). Bio-char characterization produced from walnut shell biomass through slow pyrolysis: Sustainable for soil amendment and an alternate bio-fuel. Energies, 15(1), 1. https://doi.org/10.3390/en15010001
Alhashimi, H. A., & Aktas, C. B. (2017). Life cycle environmental and economic performance of biochar compared with activated carbon: A meta-analysis. Resour. Conserv. Recycl., 118, 13-26. https://doi.org/10.1016/j.resconrec.2016.11.016
Balagurumurthy, B., Singh, R., Oza, T. S., Shiva Kumar, K. L., Saran, S., Bahuguna, G. M.,... Bhaskar, T. (2014). Effect of pressure and temperature on the hydropyrolysis of cotton residue. J. Mater. Cycles Waste Manag., 16(3), 442-448. https://doi.org/10.1007/s10163-014-0250-1
Bernal, V., Erto, A., Giraldo, L., & Moreno-Piraján, J. C. (2017). Effect of solution pH on the adsorption of paracetamol on chemically modified activated carbons. Molecules, 22(7), 1032. https://doi.org/10.3390/molecules22071032
Blanco-Canqui, H. (2019). Biochar and water quality. J. Environ. Qual., 48(1), 2-15. https://doi.org/10.2134/jeq2018.06.0248
Bouras, H. D., Benturki, O., Bouras, N., Attou, M., Donnot, A., Merlin, A.,... Holtz, M. D. (2015). The use of an agricultural waste material from Ziziphus jujuba as a novel adsorbent for humic acid removal from aqueous solutions. J. Mol. Liq., 211, 1039-1046. https://doi.org/10.1016/j.molliq.2015.08.028
Bourke, J., Manley-Harris, M., Fushimi, C., Dowaki, K., Nunoura, T., & Antal, M. J. (2007). Do all carbonized charcoals have the same chemical structure? A model of the chemical structure of carbonized charcoal. Ind. Eng. Chem. Res., 46(18), 5954-5967. https://doi.org/10.1021/ie070415u
Bujdoso, G., & Cseke, K. (2021). The Persian (English) walnut (Juglans regia L.) assortment of Hungary: Nut characteristics and origin. Sci. Hortic., 283, 110035. https://doi.org/10.1016/j.scienta.2021.110035
California Walnuts. (2022). About walnuts: History. Retrieved from https://walnuts.org/about-walnuts/history/
Chan, K. Y., Van Zwieten, L., Meszaros, I., Downie, A., & Joseph, S. (2008). Using poultry litter biochars as soil amendments. Aust. J. Soil Res., 46(5), 437-444. https://doi.org/10.1071/SR08036
Chaukura, N., Murimba, E. C., & Gwenzi, W. (2017). Sorptive removal of methylene blue from simulated wastewater using biochars derived from pulp and paper sludge. Environ. Technol. Innov., 8, 132-140. https://doi.org/10.1016/j.eti.2017.06.004
Cheng, C.-H., Lehmann, J., Thies, J. E., Burton, S. D., & Engelhard, M. H. (2006). Oxidation of black carbon by biotic and abiotic processes. Org. Geochem., 37(11), 1477-1488. https://doi.org/10.1016/j.orggeochem.2006.06.022
Cheng, W., Liu, G., & Chen, L. (2017). PET fiber reinforced wet-mix shotcrete with walnut shell as replaced aggregate. Appl. Sci., 7(4), 345. https://doi.org/10.3390/app7040345
David, E. (2020). Evaluation of hydrogen yield evolution in gaseous fraction and biochar structure resulting from walnut shells pyrolysis. Energies, 13(23), 6359. https://doi.org/10.3390/en13236359
Di´az, M. J., Madejón, E., López, F., López, R., & Cabrera, F. (2002). Composting of vinasse and cotton gin waste by using two different systems. Resour. Conserv. Recycl., 34(4), 235-248. https://doi.org/10.1016/S0921-3449(01)00109-4
Dotaniya, M. L., Datta, S. C., Biswas, D. R., Dotaniya, C. K., Meena, B. L., Rajendiran, S.,... Lata, M. (2016). Use of sugarcane industrial by-products for improving sugarcane productivity and soil health. Int. J. Recycl. Organic Waste Agric., 5(3), 185-194. https://doi.org/10.1007/s40093-016-0132-8
Enders, A., Hanley, K., Whitman, T., Joseph, S., & Lehmann, J. (2012). Characterization of biochars to evaluate recalcitrance and agronomic performance. Bioresour. Technol., 114, 644-653. https://doi.org/10.1016/j.biortech.2012.03.022
Evans, M. R., Jackson, B. E., Popp, M., & Sadaka, S. (2013). Chemical properties of biochar materials manufactured from agricultural products common to the southeast United States. HortTechnology, 27(1), 16-23. https://doi.org/10.21273/horttech03481-16
Fernandes, M. J., Moreira, M. M., Paíga, P., Dias, D., Bernardo, M., Carvalho, M.,... Delerue-Matos, C. (2019). Evaluation of the adsorption potential of biochars prepared from forest and agri-food wastes for the removal of fluoxetine. Bioresour. Technol., 292, 121973. https://doi.org/10.1016/j.biortech.2019.121973
Finger Lakes Biochar. (2016). Biochar versus activated carbon. Retrieved from http://fingerlakesbiochar.com/biochar-vs-activated-carbon/
Food and Agriculture Organization of the United Nations. (2015). Food wastage footprint and climate change. Retrieved from http://www.fao.org/nr/sustainability/food-loss-and-waste
Frišták, V., Pipíška, M., Turcan, V., Bell, S. M., Laughinghouse, H. D., Duriška, L., & Soja, G. (2021). Preparation and characterization of novel magnesium composite/walnut shells-derived biochar for As and P sorption from aqueous solutions. Agriculture, 11(8), 714. https://doi.org/10.3390/agriculture11080714
Gai, X., Wang, H., Liu, J., Zhai, L., Liu, S., Ren, T., & Liu, H. (2014). Effects of feedstock and pyrolysis temperature on biochar adsorption of ammonium and nitrate. PLoS One, 9(12), e113888. https://doi.org/10.1371/journal.pone.0113888
Georgieva, V. G., Gonsalvesh, L., & Tavlieva, M. P. (2020). Thermodynamics and kinetics of the removal of nickel (II) ions from aqueous solutions by biochar adsorbent made from agro-waste walnut shells. J. Mol. Liq., 312, 112788. https://doi.org/10.1016/j.molliq.2020.112788
Ghosh, S., Hulugalle, N., Lockwood, P., Daniel, H., & McCorkell, B. E. (2011). Applying composted cotton gin trash to a vertisol in Southeastern Queensland, Australia. Commun. Soil Sci. Plant Anal., 42(15), 1855-1861. https://doi.org/10.1080/00103624.2011.587576
Gomez-Serrano, V., Pastor-Villegas, J., Perez-Florindo, A., Duran-Valle, C. (1996). FT-IR study of rockrose and of char and activated carbon. J., Anal., Appl., Pyrol., 36, 71– 80.
Goyal, H. B., Seal, D., & Saxena, R. C. (2008). Bio-fuels from thermochemical conversion of renewable resources: A review. Renew. Sustain. Energy Rev., 12(2), 504-517. https://doi.org/10.1016/j.rser.2006.07.014
Guedidi, H., Reinert, L., Soneda, Y., Bellakhal, N., & Duclaux, L. (2017). Adsorption of ibuprofen from aqueous solution on chemically surface-modified activated carbon cloths. Arab. J. Chem., 10, S3584-S3594. https://doi.org/10.1016/j.arabjc.2014.03.007
Gupta, S., Gupta, G. K., & Mondal, M. K. (2019). Slow pyrolysis of chemically treated walnut shell for valuable products: Effect of process parameters and in-depth product analysis. Energy, 181, 665-676. https://doi.org/10.1016/j.energy.2019.05.214
Hamawand, I., Sandell, G., Pittaway, P., Chakrabarty, S., Yusaf, T., Chen, G.,... Hopf, J. (2016). Bioenergy from cotton industry wastes: A review and potential. Renew. Sustain. Energy Rev., 66, 435-448. https://doi.org/10.1016/j.rser.2016.08.033
Haque, A. N., Remadevi, R., & Naebe, M. (2021). A review on cotton gin trash: Sustainable commodity for material fabrication. J. Cleaner Prod., 281, 125300. https://doi.org/10.1016/j.jclepro.2020.125300
Hong, Z., Zhong, F., Niu, W., Zhang, K., Su, J., Liu, J.,... Wu, F. (2020). Effects of temperature and particle size on the compositions, energy conversions and structural characteristics of pyrolysis products from different crop residues. Energy, 190, 116413. https://doi.org/10.1016/j.energy.2019.116413
Huang, J., Zimmerman, A. R., Chen, H., & Gao, B. (2020). Ball milled biochar effectively removes sulfamethoxazole and sulfapyridine antibiotics from water and wastewater. Environ. Pollut., 258, 113809. https://doi.org/10.1016/j.envpol.2019.113809
Ippolito, J. A., Cui, L., Kammann, C., Wrage-Mönnig, N., Estavillo, J. M., Fuertes-Mendizabal, T.,... Borchard, N. (2020). Feedstock choice, pyrolysis temperature and type influence biochar characteristics: A comprehensive meta-data analysis review. Biochar, 2(4), 421-438. https://doi.org/10.1007/s42773-020-00067-x
Jackson, B. E., Wright, A. N., Cole, D. M., & Sibley, J. L. (2005). Cotton gin compost as a substrate component in container production of nursery crops. J. Environ. Hortic., 23(3), 118-122. https://doi.org/10.24266/0738-2898-23.3.118
Kamar, F. H., Nechifori, A. C., Alwan, G. M., Craciun, M. E., & Nechifor, G. (2015). Comparative removal of lead, copper and cadmium ions from wastewater in single and ternary batch biosorption systems onto dry walnut shells. Rev. Chim., 66(8), 1083-1087. Retrieved from http://bch.ro/pdfRC/FIRAS%20K.pdf%208%2015.pdf
Kasozi, G. N., Zimmerman, A. R., Nkedi-Kizza, P., & Gao, B. (2010). Catechol and humic acid sorption onto a range of laboratory-produced black carbons (biochars). Environ. Sci. Technol., 44(16), 6189-6195. https://doi.org/10.1021/es1014423
Kloss, S., Zehetner, F., Dellantonio, A., Hamid, R., Ottner, F., Liedtke, V.,... Soja, G. (2012). Characterization of slow pyrolysis biochars: Effects of feedstocks and pyrolysis temperature on biochar properties. J. Environ. Qual., 41(4), 990-1000. https://doi.org/10.2134/jeq2011.0070
Lehmann, J., Joseph, S. (2009). Biochar for Environmental Management: Science and Technology. Earthscan Press, ISBN 978-1-84407-658-1.
Li, Z., Hanafy, H., Zhang, L., Sellaoui, L., Netto, M. S., Oliveira, M. L.,... Li, Q. (2020). Adsorption of congo red and methylene blue dyes on an ashitaba waste and a walnut shell-based activated carbon from aqueous solutions: Experiments, characterization and physical interpretations. Chem. Eng. J., 388, 124263. https://doi.org/10.1016/j.cej.2020.124263
Liu, C., Wang, W., Wu, R., Liu, Y., Lin, X., Kan, H., & Zheng, Y. (2020). Preparation of acid- and alkali-modified biochar for removal of methylene blue pigment. ACS Omega, 5(48), 30906-30922. https://doi.org/10.1021/acsomega.0c03688
Liu, L., Li, X., Wang, X., Wang, Y., Shao, Z., Liu, X.,... Dai, Y. (2022). Metolachlor adsorption using walnut shell biochar modified by soil minerals. Environ. Pollut., 308, 119610. https://doi.org/10.1016/j.envpol.2022.119610
Liu, R., Liu, G., Yousaf, B., & Abbas, Q. (2018). Operating conditions-induced changes in product yield and characteristics during thermal-conversion of peanut shell to biochar in relation to economic analysis. J. Cleaner Prod., 193, 479-490. https://doi.org/10.1016/j.jclepro.2018.05.034
Lu, X., Wu, J., & Guo, Y. (2019). Removal of Cd (II) from aqueous solution by sulfur-functionalized walnut shell: Adsorption performance and micro-structural morphology. Desalin. Water Treat., 169, 322-332. https://doi.org/10.5004/dwt.2019.24742
Maglinao, A. L., Capareda, S. C., & Nam, H. (2015). Fluidized bed gasification of high tonnage sorghum, cotton gin trash and beef cattle manure: Evaluation of synthesis gas production. Energy Convers. Manag., 105, 578-587. https://doi.org/10.1016/j.enconman.2015.08.005
Makavana, J. M., Sarsavadia, P. N., Chauhan, P. M., Dulawat, M. S., Dobariya, U. D., & Yadav, R. (2021). A review pyrolysis: Different agricultural residues and their bio-char characteristics. Int. J. Environ. Clim. Change, 11(7), 80-88. https://doi.org/10.9734/ijecc/2021/v11i730442
Meyer, R. O. (2007). Cotton gin trash: Alternative roughage feed for beef cattle. AN177. Gainesville, FL: University of Florida, IFAS Extension. Retrieved from https://original-ufdc.uflib.ufl.edu/IR00003767/00001
Mohan, D., Sarswat, A., Ok, Y. S., & Pittman, C. U. (2014). Organic and inorganic contaminants removal from water with biochar, a renewable, low cost and sustainable adsorbent – A critical review. Bioresour. Technol., 160, 191-202. https://doi.org/10.1016/j.biortech.2014.01.120
Mui, E. L., Cheung, W. H., & McKay, G. (2010). Tyre char preparation from waste tyre rubber for dye removal from effluents. J. Hazard. Mater., 175(1-3), 151-158. https://doi.org/10.1016/j.jhazmat.2009.09.142
Nazari, G., Abolghasemi, H., & Esmaieli, M. (2016). Batch adsorption of cephalexin antibiotic from aqueous solution by walnut shell-based activated carbon. J. Taiwan Inst. Chem. Eng., 58, 357-365. https://doi.org/10.1016/j.jtice.2015.06.006
Ndoun, M. C., Elliott, H. A., Preisendanz, H. E., Williams, C. F., Knopf, A., & Watson, J. E. (2021). Adsorption of pharmaceuticals from aqueous solutions using biochar derived from cotton gin waste and guayule bagasse. Biochar, 3(1), 89-104. https://doi.org/10.1007/s42773-020-00070-2
Novak, J. M., Cantrell, K. B., & Watts, D. W. (2012). Compositional and thermal evaluation of lignocellulosic and poultry litter chars via high and low temperature pyrolysis. Bioenergy Res., 6(1), 114-130. https://doi.org/10.1007/s12155-012-9228-9
Novak, J. M., Lima, I., Xing, B., Gaskin, J. W., Steiner, C., Das, K. C.,... Schomberg, H. (2009). Characterization of designer biochar produced at different temperatures and their effects on a loamy sand. Ann. Environ. Sci., 3, 195-206. Retrieved from https://www.researchgate.net/publication/38444634
Papafotiou, M., Avajianneli, B., Michos, C., & Chatzipavlidis, I. (2007). Coloration, anthocyanin concentration, and growth of croton (Codiaeum variegatum L.) as affected by cotton gin trash compost use in the potting medium. HortScience, 42(1), 83-87. https://doi.org/10.21273/hortsci.42.1.83
Popoola, L. T. (2020). Tetracycline and sulfamethoxazole adsorption onto nanomagnetic walnut shell-rice husk: Isotherm, kinetic, mechanistic and thermodynamic studies. Int. J. Environ. Anal. Chem., 100(9), 1021-1043. https://doi.org/10.1080/03067319.2019.1646739
Qiu, Z., Chen, J., Tang, J., & Zhang, Q. (2018). A study of cadmium remediation and mechanisms: Improvements in the stability of walnut shell-derived biochar. Sci. Total Environ., 636, 80-84. https://doi.org/10.1016/j.scitotenv.2018.04.215
Rafiq, M. K., Bachmann, R. T., Rafiq, M. T., Shang, Z., Joseph, S., & Long, R. (2016). Influence of pyrolysis temperature on physico-chemical properties of corn stover (Zea mays L.) biochar and feasibility for carbon capture and energy balance. PLoS One, 11(6), e0156894. https://doi.org/10.1371/journal.pone.0156894
Rehrah, D., Reddy, M. R., Novak, J. M., Bansode, R. R., Schimmel, K. A., Yu, J.,... Ahmedna, M. (2014). Production and characterization of biochars from agricultural by-products for use in soil quality enhancement. J. Anal. Appl. Pyrolysis, 108, 301-309. https://doi.org/10.1016/j.jaap.2014.03.008
Román, S., Nabais, J. M., Ledesma, B., Laginhas, C., & Titirici, M.-M. (2020). Surface interactions during the removal of emerging contaminants by hydrochar-based adsorbents. Molecules, 25(9), 2264. https://doi.org/10.3390/molecules25092264
Sangani, M. F., Abrishamkesh, S., & Owens, G. (2020). Physicochemical characteristics of biochars can be beneficially manipulated using post-pyrolyzed particle size modification. Bioresour. Technol., 306, 123157. https://doi.org/10.1016/j.biortech.2020.123157
Seah, M. P. (2001). Summary of ISO/TC 201 Standard: VII ISO 15472 : 2001 — Surface chemical analysis — x-ray photoelectron spectrometers — calibration of energy scales. Surf. Interface Anal., 31(8), 721-723. https://doi.org/10.1002/sia.1076
Shagali, A. A., Hu, S., Wang, Y., Li, H., Wang, Y., Su, S., & Xiang, J. (2021). Comparative study on one-step pyrolysis activation of walnut shells to biochar at different heating rates. Energy Rep., 7, 388-396. https://doi.org/10.1016/j.egyr.2021.10.021
Singh, B. P., Hatton, B. J., Singh, B., Cowie, A. L., & Kathuria, A. (2010). Influence of biochars on nitrous oxide emission and nitrogen leaching from two contrasting soils. J. Environ. Qual., 39(4), 1224-1235. https://doi.org/10.2134/jeq2009.0138
Song, X., Liu, H., Cheng, L., & Qu, Y. (2010). Surface modification of coconut-based activated carbon by liquid-phase oxidation and its effects on lead ion adsorption. Desalination, 255(1), 78-83. https://doi.org/10.1016/j.desal.2010.01.011
Sun, Y., Gao, B., Yao, Y., Fang, J., Zhang, M., Zhou, Y.,... Yang, L. (2014). Effects of feedstock type, production method, and pyrolysis temperature on biochar and hydrochar properties. Chem. Eng. J., 240, 574-578. https://doi.org/10.1016/j.cej.2013.10.081
Taheran, M., Naghdi, M., Brar, S. K., Knystautas, E. J., Verma, M., Ramirez, A. A.,... Valero, J. R. (2016). Adsorption study of environmentally relevant concentrations of chlortetracycline on pinewood biochar. Sci. Total Environ., 571, 772-777. https://doi.org/10.1016/j.scitotenv.2016.07.050
Tan, Z., Lin, C. S., Ji, X., & Rainey, T. J. (2017). Returning biochar to fields: A review. Appl. Soil Ecol., 116, 1-11. https://doi.org/10.1016/j.apsoil.2017.03.017
Teixeira, S., Delerue-Matos, C., & Santos, L. (2019). Application of experimental design methodology to optimize antibiotics removal by walnut shell based activated carbon. Sci. Total Environ., 646, 168-176. https://doi.org/10.1016/j.scitotenv.2018.07.204
Thomasson, J. A. (1990). A review of cotton gin trash disposal and utilization. In C. P. Duggar, & D. A. Richter (Eds.), Proceedings: Beltwide Cotton Production Research Conferences (pp. 689-705). Memphis, TN: National Cotton Council. Retrieved from https://www.cotton.org/beltwide/proceedings/8395/abstracts/2697.cfm
Uchimiya, M., Chang, S., & Klasson, K. T. (2011b). Screening biochars for heavy metal retention in soil: Role of oxygen functional groups. J. Hazard. Mater., 190(1), 432-441. https://doi.org/10.1016/j.jhazmat.2011.03.063
Uchimiya, M., Klasson, K. T., Wartelle, L. H., & Lima, I. M. (2011a). Influence of soil properties on heavy metal sequestration by biochar amendment: Copper sorption isotherms and the release of cations. Chemosphere, 82(10), 1431-1437. https://doi.org/10.1016/j.chemosphere.2010.11.050
United Nations. (2020). The sustainable development goals report. New York: United Nations Publ. ISBN: 978-92-1-101425-9, https://unstats.un.org/sdgs/report/2020/The-Sustainable-Development-Goals-Report-2020.pdf
USDA-NASS. (2021). Quick Stats. English walnut production yearly, nationwide. Washington, DC. Washington, DC: United States Department of Agriculture-National Agricultural Statistics Service. Retrieved from https://quickstats.nass.usda.gov/results/B6B990A3-0FCE-388A-8244-478BF5EC9C78#2402B188-4899-3CB8-8D20-163B0E01045C
Wan, J., Liu, F., Wang, G., Liang, W., Peng, C., Zhang, W.,... Yang, J. (2021). Exploring different mechanisms of biochars in removing hexavalent chromium: Sorption, reduction and electron shuttle. Bioresour. Technol., 337, 125382. https://doi.org/10.1016/j.biortech.2021.125382
Xu, H., Han, Y., Wang, G., Deng, P., & Feng, L. (2022). Walnut shell biochar based sorptive remediation of estrogens polluted simulated wastewater: Characterization, adsorption mechanism and degradation by persistent free radicals. Environ. Technol. Innov., 28, 102870. https://doi.org/10.1016/j.eti.2022.102870
Yahya, M. A., Al-Qodah, Z., & Ngah, C. W. (2015). Agricultural bio-waste materials as potential sustainable precursors used for activated carbon production: A review. Renew. Sustain. Energy Rev., 46, 218-235. https://doi.org/10.1016/j.rser.2015.02.051
Yan, Y., Ma, M., Liu, X., Ma, W., Li, M., & Yan, L. (2017). Effect of biochar on anaerobic degradation of pentabromodiphenyl ether (BDE-99) by archaea during natural groundwater recharge with treated municipal wastewater. Int. Biodeterior. Biodegrad., 124, 119-127. https://doi.org/10.1016/j.ibiod.2017.04.019
Yin, Q., Liu, M., & Ren, H. (2019). Biochar produced from the co-pyrolysis of sewage sludge and walnut shell for ammonium and phosphate adsorption from water. J. Environ. Manag., 249, 109410. https://doi.org/10.1016/j.jenvman.2019.109410
Zhang, J., Liu, J., & Liu, R. (2015). Effects of pyrolysis temperature and heating time on biochar obtained from the pyrolysis of straw and lignosulfonate. Bioresour. Technol., 176, 288-291. https://doi.org/10.1016/j.biortech.2014.11.011