Abstract
The influence of natural organic matters (NOMs) on strontium (Sr2+) sorption to montmorillonite was investigated in this study by using some defined structure low-weight organic species as NOMs sarrogates. With assistance of MINEQL+ code, results from Sr2+ sorption were quantified to reaction equilibrium constats (logK).It was found that stronger background electrolytes competed with Sr2+ for the structure sorption sites, and caused a decrease in logKx from 4.0 (I = 0.017 M) to 2.2 (I = 0.1 M). However, the logKs rose from -0.3 (I = 0.017 M) to 8.5 (I = 0.1 M), indicating a higher sorption affinity of the edge sorption sites toward Sr2+. Under identical background strength (I = 0.017 M) environments, when Sr2+ concentration droped from 100 ppm to 10 ppm, the corresponding logKs rose from -0.3 to 1.3. This demonstrated the important role of the edge sorptioin sites under low Sr2+ environments.By comparing Sr2+ sorption ratios obtained under varios concentrations of organic species, it was found that Sr sorption was unaffected by the presence of organic species. However, results from MINEQL+ code showed that aqueous organic species competed for the edge sorption sites against Sr2+, and reduced the values logKs from -0.3 to -1.3 (COO group) or to -2.3 (aromatic group). This indicated that the presence of organic species would reduce Sr2+ uptake under neutral and alkaline environments. Moreover, organic speices also increased the concentration of suspension solids.It was found that organic species uptake was greatly increased with the presence of Sr2+, indicating sorbed Sr2+ acts as sorption bridges for organic species. That is to say, organic species were introduced into interlayer of montmorillonite along with Sr2+. Intercalated organic species rearranged the distribution of interlayer water molecules, resulting in the various compositions of interlayer environments as indicated by XRD observations. Furthermore, FTIR results confirmed that organic species were sorbed to montmorillonite through outer-sphere complexation.