6.1. Biochar applications in organics remediation
6.1.1. Color/dye removal
The disposal of dye industry wastewater poses major problems, because effluents contain acids or bases, dissolved solids, toxic compounds, and they have color. Color is instantly recognized because it is visible. Many textile dyes are difficult to destroy by conventional waste treatment methods since they are stable to light, oxidizing agents, and resist aerobic digestion. Biochars from straw (Hameed and El-Khaiary, 2008, Qiu et al., 2009 and Xu et al., 2011), and bamboo (Mui et al., 2010 and Yang et al., 2013) were developed to remove dyes from water and wastewater.
Slow pyrolysis rice straw-biochar was applied to malachite green (MG) adsorption (Hameed and El-Khaiary, 2008). Rice straw biochar was made in a vertical tubular reactor for 2 h from 20–700 °C under nitrogen. Maximum MG was removed at pH 5 at 30 °C. About 95% of the MG was removed within 40 min from a solution with 25 mg/L of initial MG. Kinetics fitted best to a pseudo first order model, suggesting the adsorption rate is initially controlled by external mass transfer. Intraparticle diffusion was controlling at a later stage. The monolayer Langmuir adsorption capacity was 149 mg/g at 30 °C (Hameed and El-Khaiary, 2008) (Table SM2). Straw biochar was acid-treated (0.1 M HCl) to remove metals, followed by demineralization using HCl-HF (Qiu et al., 2009) according to Chun’s method (Chun et al., 2004). This biochar (BC) and a commercial activated carbon (Darco G-60, Sigma Aldrich) (AC) were used to remove reactive brilliant blue (KNR) and rhodamine B (RB) dyes from water. Surprisingly, the BC surface area (1057 m2/g) was higher than the AC’s (970 m2g−1) (Table SM2). BC was less carbonized than AC as evidenced by its high surface acidity. Biochar (at pH 6.5) adsorbed more RB than the commercial AC (at pH 3) because the dye molecules more easily accessed the larger micropores of BC than the fine pores of AC. KNR adsorption increased as its concentration increased (at pH 6.5) due to negative surface charge neutralization by Na+ cations of the dye. Two main IR bands observed after dye adsorption suggest intermolecular hydrogen bonding between O atoms of char functional groups and single bondOH groups of the dye. High RB adsorption (pH 3) with increasing ionic strength was possibly due to the formation of RB dimers. Also, Cdouble bond; length as m-dashO and single bondOH groups participated in dipole–dipole attractions between RB and BC.
Canola straw (CS), peanut straw (PS), soybean straw (SS), and rice hulls (RH) were each slowly pyrolyzed for 4 h at 350 °C in a muffle furnace, generating their respective biochars (Xu et al., 2011). The ramp heating rate was 20 °C/min. These biochars removed methyl violet from water. Sorption capacities followed the order CS > PS > SS > RH which was parallel to their respective cation exchange capacities. Methyl violet at higher concentrations adsorbed on biochar due to its low water solubility. Zeta potentials of the CS and PS chars showed their surfaces were acidic. Changes in the FT-IR phenolic single bondOH stretching and carboxylate asymmetric stretching peaks of methyl violet occur after adsorption. A drop in band intensities at 1065 and 1045 cm−1 indicated the char’s surface carbonates interacted with methyl violet. Monolayer adsorption capacities of 256, 179, and 124 mg/g were obtained for PS, SS, and RH chars, respectively.
Slow pyrolysis of waste bamboo scaffolding under N2 for 1–4 h at 400–900 °C gave biochars (Mui et al., 2010). Char yields decreased with increasing temperature. Specifically, a sharp yield decrease from 400 to 500 °C was due to lignin and hemicelluloses partial gasification. The char H/C ratios decreased versus O/C in van Krevlen plots, showing high pyrolysis temperatures caused progressive aromatization. Surface area increased with higher pyrolysis temperatures, reaching 327 m2/g at 900 °C. Char yields and %H and %O dropped on longer pyrolysis times, whereas surface area increased. High heating rates resulted in lower surface areas, pore volumes, and yields due to rapid depolymerization at char surfaces. Acid blue 25 (AB25), acid yellow 117 (AY117), and methylene blue (MB) adsorption occurred. This bamboo char had a higher adsorption capacity for MB than AY117 and AB25. Bamboo biochar also adsorbs metal complex dye acid black 172 (Yang et al., 2013).
Kenaf (Hibiscus cannabinus) fiber char (KFC) supplied by Kenaf Fiber Industries Sdn. Bhd., Malaysia was slowly pyrolyzed to acid-treated biochar (HKFC) at 1000 °C ( Mahmoud et al., 2012). This HKFC adsorbed methylene blue (MB) in a honeycomb pore network observed by SEM. Char surface area increased on acid treatment. HKFC, a mesoporous solid (average pore dia. 3 nm), had almost double the fixed carbon and a higher oxygen content than KFC. HKFC had a higher MB removal efficiency than KFC. The Langmuir adsorption capacity at pH 6–7 was 22.7 mg/g at 50 °C (MB conc. of 100 mg/L). Sorption followed pseudo second order kinetics. Both intraparticle diffusion and boundary layer diffusion controlled adsorption. Hornbeam sawdust biochars were made in a fixed bed reactor at 500, 600, 700, and 800 °C under an inert atmosphere to adsorb orange 30 ( Ates and Un, 2013). Optimum adsorption occurred at pH 2.0 on all chars. Adsorption capacity was highest on the char made at 800 °C.
6.1.2. Phenols removal
Phenolic compounds are manufactured for plastics, dyes, drugs, antioxidants, and pesticides. They pose serious danger when entering the food chain as water pollutants. Phenols affect the taste and odor of fish and drinking water at very low concentration. Also, nitrophenols and chlorophenols are priority pollutants. Poly(acrylamide)-chicken, wood, and tire biochars (p(AAm)-CB), (p(AAm)-WB), and (p(AAm)-TB) were developed as hydrogel composites using acrylamide (AAm) monomer with N,N′-methylenebisacrylamide (MBA) as crosslinker and ammonium persulfate (APS) as initiator. P(AAm)-CB, p(AAm)-WB, and p(AAm)-TB were utilized for aqueous phenol removal (Karakoyun et al., 2011).
High sorption capacity rice husk and corncob biochars were prepared at fixed temperatures and different residence times. Biochar prepared within 1.6 s exhibited a higher phenol adsorption capacity (589 mg g−1). Adsorption via acid–base interaction and hydrogen binding between phenol and the functional groups was proposed to explain the process (Liu et al., 2011). Catechol adsorption on oak, pine, and grass biochars prepared at 250, 400, and 650 °C was reported. Catechol sorption capacity increased with rise in biochar pyrolysis temperature (Kasozi et al., 2010).
6.1.3. Pesticides and polynuclear aromatics removal
Pesticide and PAH remediation has attracted great attention. They are introduced into the environment from economic production and wide application in agriculture. Important pesticide remediation targets include organophoshorous, organochlorine, carbamate, triazine and chlorophenoxy acid compounds.
Dibromochloropropane, a soil fumigant used to control nematodes, was adsorbed from well waters onto almond shell activated biochars (Klasson et al., 2013). Almond shells were slowly pyrolyzed at 650 °C for 1 h under N2 in a Lyndberg furnace with a retort. Further steam activation at 800 °C for 45 min gave a specific surface area of 344 m2/g. The maximum adsorption capacity was 102 mg/g. Field studies were also carried out successfully (Klasson et al., 2013).
Orange peel biochars from slow pyrolysis ranging from 150 to 700 °C (OP150–OP700) were used for naphthalene and 1-naphthol adsorption (Chen and Chen, 2009). Maximum 1-naphthol and naphthalene uptake was achieved by OP200 and OP700, respectively. Naphthalene adsorption was controlled by surface coverage and partition whereas 1-naphthnol adsorption was controlled by partition, surface coverage, and surface interactions. Raw orange peels underwent large weight loses from 150 to 400 °C. The O/C ratio decreased with a rise in pyrolysis temperature. Chun et al. reported a similar trend (Chun et al., 2004).
6.1.4. Solvents removal
Biochars (WC-300, WC-400, WC-500, WC-600, and WC-700) were generated by pyrolyzing a wheat residue (Triticum aestivum L.) for 6 h between 300 °C and 700 °C and analyzing for their elemental compositions, surface areas, and surface functional groups ( Chun et al., 2004). These chars removed benzene and nitrobenzene from water. The samples made at 500–700 °C were well carbonized with high surface areas (>300 m2/g), little organic matter (<3%), and low oxygen content (⩽10%). Chars formed at 300–400 °C were only partially carbonized and exhibited <200 m2/g surface areas, 40–50% organic carbon, and >20% oxygen ( Chun et al., 2004). High-temperature chars acted via adsorption on their carbonized surfaces, Low-temperature char sorption occurred by surface adsorption and some concurrent partition into the residual organic-matter. Nitrobenzene had higher surface affinities than nonpolar benzene. Char WC-700 was highly carbonized (low H/C and low O/C) versus chars formed at lower temperatures. Maximum surface area was achieved at 600 °C. Total acidity decreased as pyrolysis temperature rose. Soybean stover, peanut shells and pine needles were pyrolyzed ( Ahmad et al., 2012 and Ahmad et al., 2013aa). Soybean stover and peanut shells were charred at 300° (SBC300, PBC300) and 700 °C (SBC700, PBC700) and used to remove trichloroethylene (TCE) from water. Chars produced at 700 °C had higher surface areas (420 and 448 m2/g) than those at 300 °C (6 and 3 m2/g). SBC700 and PBC700 had higher maximum adsorption capacities (32.02 mg/g for PBC700) than the other chars. TCE adsorption correlated well with high carbon contents and negatively with higher oxygen content ( Table SM2).
6.1.5. Miscellaneous
Poultry litter (T-PL) and wheat straw (T-WS) biochars, produced at 400 °C over 120–420 min and hydrothermal poultry litter and swine solid
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