3.2.2. Metal contaminated soils
Biochar can have a different effect on the mobility of metals in soils compared to that in water. Beesley et al. (2010) applied hardwood-derived biochar to multi-element (As, Cu, Cd, and Zn) contaminated soil. Interestingly, Cu and As are mobilized, whereas Cd and Zn are immobilized in soils amended with biochar as compared to un-amended soil. Copper leaching is associated with high dissolved organic C contents at the increased pH induced by applying biochar, whereas As leaching was attributed to increasing the soil pH to 7.56. Similarly, Park et al. (2011a) reported Cu mobility in soil due to increased dissolved organic C with the addition of chicken manure-derived biochar. In contrast, the high pH induced by biochar results in reduced solubility of Cd and Zn. Increased mobility of As with biochar in soil was also reported by Hartley et al. (2009), and has been attributed to the rise in soil pH as well as As competition with soluble P in biochar. Biochar can also reduce As(V) to As(III), thereby enhancing As mobility (Park et al., 2011b and Zhang et al., 2013). Another oxyanion, Sb, also shows higher mobility in soil treated with broiler litter-derived biochar (Uchimiya et al., 2012). The electrostatic repulsion between anionic Sb and negatively charged biochar surfaces could have resulted in desorption of Sb. Conversely, the electrostatic attraction between positively charged Cu and negatively charged biochar is the prevailing mechanism of Cu immobilization in San Joaquin soil (Uchimiya et al., 2011c). Notably, Cu mobility/immobility is highly influenced by biochar organic C content. Generally, the biochars produced at <500 °C have high dissolved organic C content, which could facilitate the formation of soluble Cu complexes with dissolved organic C, as reported by Beesley et al. (2010) and Park et al. (2011a). Additionally, dissolved organic C can block the pores of biochars preventing Cu sorption (Bolan et al., 2010; Cao et al., 2011). However, the biochars produced at high temperatures (>600 °C) are generally deficient in dissolved organic C, which could affect Cu immobility in soil, as reported by Uchimiya et al. (2011c).
The effect of pyrolysis temperature on the retention of Pb by broiler litter-derived biochars produced at 350 and 650 °C was recently evaluated by Uchimiya et al. (2012). Those authors reported that biochar produced at a low pyrolysis temperature is favorable for immobilizing Pb. The increased release of available P, K, and Ca from biochars produced at a low temperature is associated with high Pb stabilization. Cao et al. (2011) demonstrated by XRD analysis that biochar derived from dairy manure containing a high amount of available P immobilized Pb in shooting range soil by forming insoluble hydroxypyromorphite (Pb5(PO4)3(OH)). The role of O-containing functional groups on biochar surfaces towards metal binding was predicted by Uchimiya et al. (2011b), who reported that cottonseed hull-derived biochar produced at 350 °C contains high O content resulting in high uptake of Cu, Ni, Cd, and Pb.
Soil pH is considered to greatly influence the mobility of metals. Generally biochar is alkaline, thereby inducing liming effect in soil and causes immobilization of metals and mobilization of oxyanions (Almaroai et al., 2013). As discussed earlier, biochar-induced increases in soil pH can also influence the sorption of metals. For instance, Ahmad et al. (2013) reported that in soil amended with biochar, rise in soil pH favored the sorption of Pb onto kaolinite making charge on kaolinite more negative. At pH > 5, Pb forms strong inner sphere bidentate surface complexes with kaolinite (Gräfe et al., 2007).
Biochar shows the potential to mitigate Cr contaminated soils as they are highly reactive with many functional groups and are able to donate electrons (Choppala et al., 2012). The increase in proton supply for Cr(VI) reduction may be attributed to the presence of several O-containing acidic (carbonyl, lactonic, carboxylic, hydroxyl, and phenol) and basic (chromene, ketone, and pyrone) functional groups (Goldberg, 1985 and Boehm, 1994). The resulting Cr(III) either adsorbs or participates in surface complexation with organic amendments (Hsu et al., 2009a). However, high pH biochars may prevent dissociation and oxidation of phenolic and hydroxyl groups, which may limit the supply of protons for reducing Cr(VI) (Choppala et al., 2012). Moreover, soil microbes can also cause the reduction of Cr(VI) to Cr(III) using C as an energy source from the biochar (Zimmerman, 2010). Because of the lower solubility of Cr(III) than Cr(VI), this reduction eventually results in immobilizing the Cr, thereby diminishing mobility and transport (Choppala et al., 2012).
The effect of biochar on remediation of soil and water co-contaminated with organic and inorganic contaminants has received little attention. Cao et al. (2011) reported the simultaneous immobilization of Pb and atrazine by dairy-manure biochar in soil. Those authors demonstrated that Pb was immobilized as a result of precipitation to insoluble hydroxypyromorphite due to the P content in the biochar, whereas atrazine was adsorbed onto biochar surfaces. The sorption phenomenon of co-existing organic and inorganic contaminants in aqueous solution is more complex. For example, biochar derived from soybean stalks works well for the phenanthrene and Hg(II) adsorption in a single component system, however direct competitive sorption is suppressing in a binary component system (Kong et al., 2011). The decrease in the effective surface area of biochar due to pore blockage by insoluble Hg(II) compounds results in a decrease in phenanthrene adsorption (Kong et al., 2011). In contrast, Chen et al. (2007) discounted the pore blockage mechanism for direct competitive adsorption between Cu and organic compounds (naphthalene, dichlorobenzene, and dichlorophenol) in an aqueous system. They speculated that the strong inner-sphere complexation between metal and biochar surfaces results in inhibiting organic compound adsorption around the metal-complexed moieties.
It is worth noting that sorption of organic contaminants by biochars is more favored than that of inorganic contaminants. Kong et al. (2011) reported 99.5% removal efficiency of phenanthrene by soybean stalk based biochar compared to 86.4% removal of Hg(II) from aqueous solution. This greater sorption capacity of biochar for organic contaminants is attributable to their high surface area and microporosity (Table 3). Contrarily, ion-exchange, electrostatic attraction and precipitation are prevailing mechanisms for the remediation of inorganic contaminants by biochar (Fig. 3). Since, the sorption of organic contaminants depends mainly on surface area and pore size, biochar in general shows greater sorption capacity for organic than inorganic contaminants.
The physicochemical properties of biochars as affected by pyrolysis temperature greatly influence their sorption efficiencies for both organic and inorganic contaminants. Biomass pyrolyzed at a high temperature is more effective for organic contaminants due to the high surface area and developed pore structures, whereas low temperature pyrolyzed biomass is efficient for inorganic contaminants due to the presence of more O-containing functional groups and the greater release of cations. The specific type of contaminant also impacts sorption properties of the biochar. Polar and non-polar, ionic and non-ionic organic contaminants have different affinities for biochars compared to those of cationic and anionic metals. Therefore, all biochars are not equally effective for sorbing contaminants, and care should be taken before applying biochar to remediate contaminated soil or water on a large scale. Research on biochar is contemporary, and still needs in-depth investigations to determine the long-term effects of biochar applied to contaminated areas.
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