Influence of Environmental Factors on Pesticide Adsorption by Black Carbon: pH and Model Dissolved Organic Matter

Abstract

Loading two organic acids of known molecular structures onto a black carbon was conducted to study the influence of pH and dissolved organic matter on the adsorption of pesticides. Tannic acid at the loading rates of 100 and 300 micromol/g reduced the surface area of black carbon by 18 and 63%, respectively. This was due principally to the blockage of micropores, as verified by measured pore volumes and pore-size distributions. With a comparatively much smaller molecular structure, gallic acid did not apparently influence these properties. The intrinsic acidities of the two acids increased the surface acidity from 1.88 mmol/g of black carbon to 1.93-2.02 mmol/g after DOM loading, resulting in a reduction in isoelectric point pH from 1.93 to 1.66-1.82. The adsorption of propanil, 2,4-D and prometon by black carbon free and loaded of DOM was dependent on pH because major adsorptive forces were the interactions between neutral pesticide molecules and uncharged carbon surfaces. The adsorption was diminished considerably by the deprotonation of 2,4-D and protonation of prometon, as well as the surface charge change of black carbon. Tannic acid of 100 and 300 micromol/g on black carbon reduced the pesticide adsorption at the equilibrium concentration of 10 mg/L by an average of 46 and 81%, respectively, consistent with the reductions of 42 and 81% in micropore volume. At the equilibrium concentration of 30 mg/L, the mesopore surface became the additional adsorptive domain for propanil. Loading tannic acid made the mesopore surface less accessible, due presumably to the enhanced obstruction by tannic acid.

Impacts of Biochar (Black Carbon) Additions on the Sorption and Efficacy of Herbicides

“…Furhermore, the lack of information within these studies to properly characterize the biochar material used further limits the ability to compare results. However, based on the analogous observations with non-biochar materials (Table 1), the overall conclusions are that biochar additions (Table 2) (Table 2); Li et al, 2009;Qiu et al, 2009b;Wang et al, 2009, ( Herbicides are much more than just weed killers. They may exhibit beneficial or adverse effects on other organisms.…”

Sorption of halogenated phenols and pharmaceuticals to biochar: affecting factors and mechanisms

“…5). Pore blockage with humic acid or complexation with humic acid may hinder the sorption of DCP and triclosan to GAC and biochar (Pignatello et al 2006;Qiu et al 2009). …”

Impact of humic acid coating on sorption of naphthalene by biochars

“…DOM is a complex mixture, with properties varying among different types and source materials. The suppression effect of DOM coating on sorption of HOCs by biochars was reported to be dependent on its molecular size, chemical composition, and hydrophobicity [14][15][16]. One possible mechanism proposed for suppression of DOM coating on HOC sorption to biochars was the pore blockage and direct site competition with the tested compounds, thus reducing the number of effective sites for HOC sorption [17].…”