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 solids chars at 250 °C under autogenic pressures for 20 h were made to remove phenanthrene (Phen), bisphenol A (BPA), and 17α-ethinyl esteradiol (EE2) from water (Sun et al., 2011). Lower H/C and O/C ratios were reported for the thermal biochars, indicating more carbonization than in the hydrothermal chars. Thermal biochars were more hydrophobic than hydrothermal chars (Sun et al., 2011) and exhibited mostly aromatic (sp2) carbons with small amounts of alkyl (sp3) carbons. Hydrothermal chars had carboxyl, methoxyl, O-aryl, and alkyl carbons in their 13C nuclear magnetic resonance spectra. More EE2 and BPA was adsorbed onto hydrothermal biochars prepared at 250 °C than onto thermal biochars. The oxygen-containing polar functional groups of hydrothermal biochars hydrogen-bond to EE2 and BPA. Organic carbon-normalized distribution coefficients (log Koc) of the hydrothermal biochars were higher than for thermal biochars. Log Koc values of Phen, EE2, and BPA followed the same order as their hydrophobicities: Phen > EE2 > BPA. BPA possesses two phenol rings. Hence weak π-H-bonding with the char and phenolic hydroxyl hydrogen bonding with the hydrothermal char’s oxygen functions occurs. Phen adsorption on chars occurs by extensive π–π interactions ( Sun et al., 2011).
6.2. Biochar applications in inorganic remediation
6.2.1. Metal ion removal
Heavy metals pose serious health threats even at very low concentrations. Some are cumulative poisons, capable of assimilation, storage and concentration by organisms exposed long periods to low concentrations. Eventual metal built-up in tissues can cause harmful physiological effects. The heavy metals appear among the main pollutants in this century (Davydova, 1999). Discharged heavy metals present a serious threat to human health and natural waters. Important biochar adsorption studies have been made with Cr, Cu, Pb, Cd, Hg, Fe, Zn, and As ions. Activated carbon has long been used to remove metal ions, but only a few milligrams of metal ions are typically adsorbed per gram of activated carbon. Regeneration problems also exist. This makes activated carbon expensive for treating wastewater, so its use in developing countries is more problematic. Low cost locally available materials with adsorption capacities comparable to activated carbon are needed. Solid biomass-derived waste is also a vexing problem. Recycling requires a suitable recycled product quality if possible. Lignocellulosic wastes have fuel value, so complete combustion, fast pyrolysis to biooil or gasification to syn gas are options. Biochar is a byproduct of biooil production in 15–25% yields. If biooil production becomes widespread, its resulting char would be widely available for water remediation use. Slow pyrolysis to biochars also converts lignocellulosic wastes to biochars. Industrial wastewater and ground/surface waters could then be widely treated with biochars to decrease metal ion removal costs.
Biochars from slow pyrolysis and hydrothermal treating of rice husks, olive pomace, orange wastes, and compost were used for Cu2+ remediation (Pellera et al., 2012). Slow pyrolysis under limited oxygen at 300 and 600 °C for 6 h was followed by demineralization by acid. HTC produced chars in a high pressure reactor by heating to 300 °C for 30 min, followed by acetone extraction to remove oils (Pellera et al., 2012). Slow 600 °C pyrolysis chars were less efficient for Cu2+ removal than those produced at 300 °C, but slow pyrolysis chars removed more Cu2+ than the hydrothermal chars (Pellera et al., 2012) (Table 2 and Table SM2). Peanut, canola, and soybean straw biochars, prepared in a 400 °C muffle furnace (ramp rate of 20 °C/min) for 3.75 h under limited oxygen, were also used for Cu2+ adsorption (Tong et al., 2011). All three biochars had higher adsorption capacities than commercial activated carbon at pH 3.5–5.0. Cu2+ sorption involved electrostatic and non-electrostatic adsorption. Cu2+ sorption capacity rose as pH went up as strong complexes formed between Cusingle bondOH and char surface functions (single bondOH and single bondCOOH). Higher phosphate contents of soybean and canola straw chars versus peanut straw char caused Cu-phosphate formation and precipitation (Tong et al., 2011). Desorption rates were canola straw > soybean straw > peanut straw. Leguminous (peanut and soybean straw) chars had higher capacities than that of non-leguminous canola straw char. Peanut straw char had a maximum Cu2+ capacity of 1.4 mol/kg at pH 5.0 (Tong et al., 2011). Each of these three straw feeds were also pyrolyzed at 300, 400, and 500 °C for use to remove Cu2+ from water (Tong et al., 2011). Cu2+ adsorption rates followed the order: peanut straw char > soybean straw char > canola straw char > rice straw char. Biochars formed at 400 °C gave the best sorption. The sorption occurred by both adsorption and surface precipitation (Tong et al., 2011) (Tables 2 and SM2).
Fast and slow pyrolyzed hardwood and corn straw biochars were reported (Chen et al., 2011b). Fast hardwood pyrolysis (HW450) was made at 450 °C in a <5 s residence time at Dynamotive Inc., Vancouver, Canada. Slow pyrolysis biochars (CS600) were made at 600 °C (residence time 2 h) at BEST Energies Inc., Madison, Wisconsin, USA. HW450 was acidic due to organic acids and phenolic compounds. Above 300 °C, alkali salts separate, raising the char’s pH. High H/C ratios of these biochars indicate lower carbonization and aromaticity than activated carbon. HW450 had a greater O/C ratio and higher polarity than CS600. CS600 possesses a higher surface area (13.08 m2/g) than HW450 (0.43 m2/g). Comparing surface areas of many biochars illustrated that feed composition can play as an important role in surface area as the synthetic technique employed. Surface area plays a critical role. SA can dominate, as confirmed by the higher Cu2+ and Zn2+ uptake by CS600 verses highly surface-functiminalized HW450. Maximum Langmuir adsorption capacities of 12.52 mg/g for Cu2+ and 11.0 mg/g for Zn2+ were achieved using CS600 char (Tables 2 and SM2).
Pig and cow manure biochars were made at 400 and 600 °C at ambient pressure under nitrogen followed by chemical and mechanical treatments (Kołodyńska et al., 2012). The chemically treated 600 °C pig manure char had the highest surface area (15.89 m2/g). These chars were used for Cu2+, Zn2+, Cd2+, and Pb2+ removal. Optimum pH values were 5.0 for Cu2+ and Zn2+ and 6.0 for Cd2+ and Pb2+ adsorption. Sorption kinetics for Cu2+ and Pb2+ was a complex combination of intraparticle pore diffusion, external mass transfer, and sorption processes (Kołodyńska et al., 2012) (Table 2 and Table SM2). Metal ion
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