Sorption of p-nitrophenol on two Chinese soils as affected by copper

Heavy metals and organic contaminants often coexist in soils. However, very little information is available regarding the effect of metals on the sorption of organic contaminants onto soils and/or of organic contaminants on metal sorption. In the present study, the effect of Cu on the sorption of p-nitrophenol on two Chinese soils was investigated using a batch-equilibration method for three conditions: Copper and p-nitrophenol were sorbed simultaneously, either Cu or p-nitrophenol was sorbed previously, or the soil colloidal phase was removed in part previously. The results suggested that Cu suppressed the sorption of p-nitrophenol on soils, whereas p-nitrophenol had little effect on Cu sorption because of the higher affinity of Cu for soils. Mechanisms of the Cu suppression effect were suggested by the results. First, large hydrated Cu occupy the siloxane surface of soils and prevent nonspecific interaction between p-nitrophenol and soils. Second, the soil colloidal phase is an effective adsorbent of p-nitrophenol; thus, more p-nitrophenol is retained in the aqueous phase. In addition, the aggregation of the colloidal particles may occur, which blocks soil pores, thereby decreasing the sorption of p-nitrophenol on the solid soil phase. Third, x-ray absorption spectroscopy revealed that Cu forms inner-sphere complexes with the carboxyl and hydroxyl functional moieties of the soil particles (clay minerals and organic matter). Fourier-transform infrared spectroscopy study indicated that these groups also react with p-nitrophenol through H-bond formation. These results suggested that Cu and p-nitrophenol have common sorption sites, at least in the soil organic matter domain, which is partially responsible for the observed overall Cu suppression effect.