3.1.1. Contaminated water
Sorption of organic contaminants from water onto biochar occurs due to its high surface area and microporosity (Yu et al., 2009, Yang et al., 2010 and Lou et al., 2011). Biochars produced at >400 °C are more effective for organic contaminant sorption because of their high surface area and micropore development (Uchimiya et al., 2010, Yang et al., 2010 and Ahmad et al., 2012a). Chen et al. (2008) predicted that the partitioning of organic contaminants into non-carbonized biochar fractions derived from pine needles was the major sorption mechanism at low pyrolysis temperatures (100–300 °C), whereas adsorption onto porous carbonized fractions was dominant at high temperatures (400–700 °C). Surface polarity and aromaticity are important characteristics of biochars, as they affect aqueous organic contaminant sorption (Chen et al., 2008). In general, at >500 °C, biochar surfaces become less polar and more aromatic due to the loss of O- and H-containing functional groups, which may further affect organic contaminant adsorption. Uchimiya et al. (2010) reported an increase in the sorption capacity of deisopropylatrazine with an increase in aromaticity of biochar derived from broiler litter at 700 °C. Similar observations were made for trichloroethylene sorption on biochars produced from soybean stover and peanut shells at 700 °C versus at 300 °C (Ahmad et al., 2012a). This was explained with the high aromaticity and low polarity of the biochars produced at 700 °C. In contrast, Sun et al. (2011) reported that the biochars produced at 400 °C with high polarities were more effective on the sorption of norflurazon and fluridone. These different findings are attributed to differences in the nature of the organic compounds. Polar compounds, such as norflurazon and fluridone, are adsorbed by H-bonding between the compounds and the O-containing moieties of the biochars (Sun et al., 2011), whereas non-polar compounds, such as trichloroethylene, access hydrophobic sites on biochar surfaces in the absence of H-bonding between water and O-containing functional groups (Ahmad et al., 2012a). Therefore, the functionality of the target organic contaminant critically affects biochar adsorption capacity. A higher adsorption capacity for 1-naphthol than naphthalene on biochars produced from orange peel at 200–350 °C was reported due to polar-specific interactions between hydroxyl groups in 1-naphthol and polar surfaces of the biochars (Chen and Chen, 2009).
Electrostatic attraction/repulsion between organic contaminants and biochar is another possible adsorption mechanism. Biochar surfaces are normally, negatively charged, which could facilitate the electrostatic attraction of positively charged cationic organic compounds. This electrostatic attraction was reported by Xu et al. (2011) and Qiu et al. (2009) related to the studies on the adsorption of cationic dyes including methyl violet and rhodanine from water. Aromatic π-systems in highly polar biochars, produced at 400 °C, are rich in electron-withdrawing functional groups (Keiluweit et al., 2010). They tend to be electron-deficient and may act as π-acceptors towards electron donors. Both electron rich and poor functional groups are present in high temperature derived biochars; hence, they are theoretically capable of interacting with both electron donors and electron accepters (Sun et al., 2012). The π–π electron donor–acceptor interaction between π-electron rich graphene surface of biochar and π-electron deficient positively charged organics is enhanced (Qiu et al., 2009, Teixidó et al., 2011 and Sun et al., 2012). However, an electrostatic repulsion between negatively charged anionic organic compounds and biochars could promote H-bonding and induce adsorption. This phenomenon was reported by Teixidó et al. (2011) who showed the sulfamethazine adsorption on hardwood derived biochar produced at 600 °C. It was postulated that anionic sulfamethazine deprotonates under alkaline conditions, which released OH− and resulted in the formation of strong H-bonding between sulfamethazine and carboxylate or phenolate groups available on biochar (Teixidó et al., 2011).
Solution chemistry, such as pH and ionic strength, may also affect the sorption of organics onto biochar. The sorption capacity of biochars derived from crop residue at 350 °C for methyl violet increased sharply from pH 7.7 to 8.7 (Xu et al., 2011). The electrostatic attraction between biochars and methyl violet increased with the rise in pH due to the dissociation of phenolic –OH group of biochars, thereby increasing the net negative charge on their surfaces (Xu et al., 2011). Similarly, the ionic strength of the solution also showed positive effects on the organic contaminant adsorption on biochars (Qiu et al., 2009 and Xu et al., 2011). In particular, an increase in anionic brilliant blue dye adsorption on biochars with an increase in ionic strength was due to neutralization of the negative charge of biochar with Na+ and compression of electrical double layer near the surface, which effectively reduced the electrostatic repulsion between the anionic dye and the biochar surface (Qiu et al., 2009). Biochar contains variable charged (or pH-dependent charge) surfaces. An increase in pH on these surfaces results in an increase in the negative charge (Xu et al., 2011). The relative effect of ionic strength on the adsorption onto these surfaces is pH dependent. In general, the effect of ionic strength on adsorption onto biochar can be positive or negative depending on pH or the point of zero charge of the biochar (Bolan et al., 1999).
3.1.2. Contaminated soils
Limited studies are available on biochar applications to remediate the soils contaminated with organic pollutants as compared to water remediation (Table 3). Jones et al. (2011) evaluated the long-term biochar effect on soil contaminated with simazine. Strong simazine sorption into the micropores of biochar suppresses biodegradation and leaching of simazine into groundwater (Jones et al., 2011). A high application rate (25 t ha−1) and small particle size (<2 mm) of biochar were most effective for simazine adsorption. Yang et al. (2010) and Yu et al. (2009) reported the similar findings in which the biochars produced from woodchips and cotton straw pyrolyzed at 850 °C resulted in a remarkable decrease in the dissipation of chlorpyrifos, carbofuran, and fipronil from soil due to their high sorption, which consequently reduced their bioavailability. Those authors also reported a pronounced decrease in the uptake of these pesticides by the plants grown in contaminated soils. Comparatively less efficiency was reported by the biochar produced at <450 °C. Low pesticide adsorption in soils may be attributed to the potential association of biochar with dissolved organic matter from soil, which could coat biochar particles, reducing the accessibility of pesticides to the sorption sites (Zhang et al., 2010). Sorption of atrazine onto the organic C content of biochar produced from dairy manure at 450 °C shows that the higher dissolved organic C content in soil may reduce atrazine sorption by blocking the biochar pores (Cao et al., 2011).
Overall, the biochars produced at higher temperatures exhibit higher sorption efficiency for organic contaminant remediation in soil and water. This is probably due to the high surface area and microporosity of biochars. Additional sorption mechanisms include electrostatic attractions between charged surfaces of biochars and ionic organic compounds. However, the partitioning and subsequent diffusion into the non-carbonized and carbonized fractions of biochar could be an effective sorption mechanism for non-ionic compounds. Therefore, the biochars should be produced under well-defined pyrolysis conditions. The biochar properties should also be carefully examined before the applications for the remediation of specific organic contaminants in soil or water.
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