Pb(II), Cr(VI) and atrazine sorption behavior on sludge-derived biochar: role of humic acids

Abstract

Pyrolyzing municipal wastewater treatment sludge into biochar can be a promising sludge disposal approach, especially as the produced sludge-derived biochar (SDBC) is found to be an excellent sorbent for heavy metals and atrazine. The aim of this study was to investigate how and why the coexisting humic acids influence the sorption capacity, kinetic, and binding of these contaminants on SDBC surface. Results showed humic acids enhanced Pb(II)/Cr(VI) sorption binding, and increased the corresponding Pb(II) Langmuir sorption capacity at pH 5.0 from 197 to 233 μmol g(-1), and from 688 to 738 μmol g(-1) for Cr(VI) at pH 2.0. It can be mainly attributed to the sorbed humic acids, whose active functional groups can offer the additional sites to form stronger inner-sphere complexes with Pb(2+), and supply more reducing agent to facilitate the transformation of Cr(VI) to Cr(III). However, humic acids reduced the atrazine adsorption Freundlich constant from 1.085 to 0.616 μmol g(-1). The pore blockage, confirmed by the decreased BET-specific surface area, as well as the more hydrophilic surface with more sorbed water molecules may be the main reasons for that suppression. Therefore, the coexisting humic acids may affect heavy metal stabilization or pesticide immobilization during SDBC application to contaminated water or soils, and its role thus should be considered especially when organic residues are also added significantly to increase the humic acid content there.

Chloramphenicol interaction with functionalized biochar in water: sorptive mechanism, molecular imprinting effect and repeatable application

“…The strongest suppression occurs for large molecules with reduced access to interior pores, especially when the biochar has been coflocculated with the humic substance. Moreover, Zhou et al (2015) confirmed that pore blockage (confirmed by BET surface area measurement) as well as the more hydrophilic surface with more sorbed water molecules mainly suppressed the sorption of atrazine by biochar. In addition, hydrophobic effects and electron donor acceptor (EDA) might also be responsible for lower chloramphenicol sorption in the presence of humic acid.…”

Adsorption of Cadmium on Degraded Soils Amended with Maize-Stalk-Derived Biochar

“…Liu et al [ 25 ] found that, when adding 5% (in mass) stalk carbon with particle size of 0.25 mm in paddy soil, available contents of Pb, Zn, and Cu in the soil were reduced by 52.5%, 52.1%, and 50.1%, respectively. Adsorption of heavy metals (such as Cd, Pd, and Cr) usually increased in soil–biochar system due to the increasing sorbed sites for heavy metals which was supplied by the coated DOM (dissolved organic matter) of soil onto biochar [ 26 , 27 ]. In theory, biochar addition can improve the adsorption capacity of degraded soil and reduce the mobility and bioavailability of pollutants.…”

Highly efficient adsorption of Cr(VI) from aqueous solution by Fe3+ impregnated biochar

“…On the one hand, since negatively charged HA and Cr(VI) were simultaneously present in the adsorbent suspension, they might compete to occupy the finite adsorption sites on the Fe 3+ -BC surface, with a lower than expected adsorption of Cr(VI) onto Fe 3+ -BC; [67] Meanwhile, it was considered that HA had a better ability to chelate Fe 3+ -BC, which reduced active sites of Fe 3+ -BC available to Cr(VI); [68] Moreover, HA might form aggregates on the surface of Fe 3+ -BC, which would prevent Fe 3+ -BC from adsorbing Cr(VI); [69] Besides, it has been reported that HA has a high adsorption affinity for Cr(VI), [70] which might prevent the removal of Cr(VI) by reducing the concentration of the available Cr(VI). On the other hand, at low pH values, the negatively charged HA could be easily adsorbed, so the strong complexation of surface adsorbed HA with Cr(VI) should result in the increased adsorption of Cr(VI) on Fe 3+ -BC surface; [71] Furthermore, HA could enhance the removal of Cr(VI) via the adsorption-coupled reduction reaction, which has been widely accepted as the real mechanism of Cr(VI) adsorption through both direct and indirect reduction mechanisms under the acidic conditions. [71,72] In previous studies, thiol, phenolic, carboxylic, methoxy, and carbonyl functional groups in HA structure have been reported as effective electron-donor groups for Cr(VI) reduction.…”

“…On the other hand, at low pH values, the negatively charged HA could be easily adsorbed, so the strong complexation of surface adsorbed HA with Cr(VI) should result in the increased adsorption of Cr(VI) on Fe 3+ -BC surface; [71] Furthermore, HA could enhance the removal of Cr(VI) via the adsorption-coupled reduction reaction, which has been widely accepted as the real mechanism of Cr(VI) adsorption through both direct and indirect reduction mechanisms under the acidic conditions. [71,72] In previous studies, thiol, phenolic, carboxylic, methoxy, and carbonyl functional groups in HA structure have been reported as effective electron-donor groups for Cr(VI) reduction. [71] Hence, the presence of HA facilitated the Cr(VI) adsorption at the initial pH of 3.0 primarily by offering more positively charged groups of the adsorbed HA on Fe 3+ -BC surface for Cr(VI) binding and electron-donor groups for Cr(VI) reduction.…”

“…[71,72] In previous studies, thiol, phenolic, carboxylic, methoxy, and carbonyl functional groups in HA structure have been reported as effective electron-donor groups for Cr(VI) reduction. [71] Hence, the presence of HA facilitated the Cr(VI) adsorption at the initial pH of 3.0 primarily by offering more positively charged groups of the adsorbed HA on Fe 3+ -BC surface for Cr(VI) binding and electron-donor groups for Cr(VI) reduction. As illustrated in Figure 7, when HA was added, it was found that the adsorption of Cr(VI) on Fe 3+ -BC increased with increasing initial HA concentrations from 0 to 50 mg/L, suggesting that HA facilitated the adsorption of Cr(VI) on Fe 3+ -BC.…”