摘要:

Under acidic pH conditions, the mobility of Cr is controlled by (Fe, Cr)(OH)3 coprecipitation in solution (homogeneous) and on soils (heterogeneous), and natural organic matter (NOM) adsorption onto soils could affect heterogeneous (Fe, Cr)(OH)3 precipitation on soils and, thus, Cr transport. Here, Suwannee River NOM (SRNOM) adsorption onto Al2O3 was investigated under varied carbon concentrations, and properties of SRNOM-coated surfaces were characterized using spectroscopic and interfacial techniques. Heterogeneous (Fe, Cr)(OH)3 precipitation on SRNOM-coated surfaces was studied at acidic pH via metal analysis and phase/size characterization. With lower NOM concentrations, preferential adsorption of aromatic moieties occurred, rendering more hydrophobic surfaces, which promoted nucleation and resulted in precipitates with higher Cr/Fe ratios. With higher NOM concentrations, NOM-coated surfaces became more negatively charged, attributed to enrichment of acidic (i.e., carboxylate) structures. Therefore, the amount of heterogeneous precipitates increased as enriched carboxylates and negative charge promoted heterogeneous nucleation and deposition. The controlling mechanisms were further validated with model OMs: For humic acid and fulvic acid, similar phenomena were observed with SRNOM. For polyacrylic acid with high acidity and no aromaticity, its adsorption onto Al2O3 made the surface highly negatively charged and hydrophilic, resulting in promoted heterogeneous precipitation with low Cr incorporation. Preferential adsorption of OMs with higher molecular weights (MWs) onto Al2O3 also occurred, but the MW did not affect either the amount or composition of heterogeneous (Fe, Cr)(OH)3 precipitates. The new knowledge learnt here could help in understanding Cr immobilization under acidic environments with diversified NOMs.

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