Mechanism For Adsorption Of Organic Compounds On Condensed Carbon

Condensed carbon play an important role in the adsorption of organic compounds to soil and sediment. Relative to other forms of soil organic matter, condensed carbon exhibited stronger adsorption affinities for hydrophobic organic compounds. To evaluate the level of soil contamination, it is necessary to quantify the content of condensed carbon in soil. Heat treatment is used to quantify the content of condensed carbon in soil. However, it is not really clear of the effects of heat treatment on organic compounds adsorption to condensed carbon. Recently, biochar refers to condensed carbon has received an increasing recognition as a low-cost and excellent sorbent to affect the fate and behavior of organic compounds. Moreover, sulfonamide antibiotics residues in the soil have raised great concerns. However, limited studies have been reported to investigate the adsorption of sulfonamide compounds on biochars.In this study, batch adsorption experiments were performed for four compounds (1,3-dichlorobenzehe,1,3-dinitrobenzene,1,3,5-trichlorobenzene and 1,3,5-trinitrobenzene) to maize stalk biochar, unheated coals (lignite and anthracite), heated coals(lignite and anthracite, at 375 ℃ for 24 h) and soil humic acid. Another set of batch adsorption experiments were performed for sulfonamide compounds to biochars, which is of different graphitization degrees, content of O-functionalities and source materials. Effects of solution chemistry conditions (pH, dissolved soil humic acid, Cu2+ ion) on the adsorption were also investigated. The main results are exhibited as followed:(1) For all tested sorbates, unheated lignite and anthracite exhibited much stronger sorption than humic substances. The results indicated that the sorption of organic compounds to soil would be dominated by condensed carbon in soil; After heat treatment, the organic carbon contents of the coals (lignite and anthracite) were dramatically decreased. Moreover, the heat treatment markedly suppressed all sorbates sorption affinity to heated coals (up to 1 order of magnitude lower). This is attributable to the increased O-functionalities from oxidation on coals. Then, with 1,3-dichlorobenzene and 1,3,5-trichlorobenzene as sorbates, a dual-component model, which combines linear partition to humic substances and nonlinear adsorption on condensed carbon, was used to quantify the effects of combustion on sorption contribution of condensed carbon in soil. The results indicated that heat treatment may cause significant underestimation of content and sorption contribution of condensed carbon in soil.(2) Biochar prepared under different themochemical conditions exihited high adsorption affinity for sulfonamide compounds. The sorption trend is consistent with the condensation degree of biochars. The results indicated that the strong adsorption of sufonamide compounds on biochar is mainly driven by specific π-π electron-donor-acceptor (EDA) interacion with the graphitized surfaces of biochar; It is notable that the surface-area-normalized sorption isotherms of sulfonamides on C500 and C500-H were overlapped. The results indicated that within the tested concentration ranges, surface O-functionalities play a negligible role in the adsorption of sulfonamide compounds to both adsorbents; Adsorption affinities of sulfamethoxazole among biochars prepared under different source materials exihited a different trend. Upon the surface-area-normalization, the adsorption affinities to C400 are greater than those to C500. Sorption of sulfamethoxazole to biochars prepared not fully carbonized results from both adsorption to carbonized organic matter and partition with noncarbonized organic carbon, which partition is play an important role.(3) Under the same solution pH, the sorption of sulfonamides to biochars generated from different source materials show the same trend. The results attributed to the different speciation of the sulfonamide molecules under different pH conditions.(4) The presence of dissolved humic acids inhibited the adsorption of sulfonamides to C500/C50-H. This can be explained by the competition beween dissolved humic acid and sorbates for available surface area and/or the blockage of the pores of biochars.(5) The presence of Cu2+ ion inhibited the adsorption of both sulfonamides to C500/C50-H. This is attributable to the strong hydration reactions of Cu2+ ion, which is complexed with the O-functionalities of the biochars. The hydration shells of dense water hosted by the complexed Cu2+ ions would compete with sorbates for available surface area, or moreover, the complexed Cu2+ ions may cause inhibiting on the formation of charge-assisted H-bonds between sulfonamides and the surface O-functionalities of biochars. However, the effect of Cu2+ ion on sulfamethoxazole adsorption to biochars prepared under different source materials are not obvious. The above mentioned decreased and increased adsorption may counteract each other on all biochars. The adsorption enhancement effect is attributable to the complexation with both the solute and the O-functionalities on biochar surfaces.