Effect of Ca on the solubility and molecular size distribution of DOC and Cu binding in soil solution samples

Flocculation of soil-derived dissolved organic carbon (DOC) was used as a tool to study the molecular size distribution of DOC and the native amount of copper bound to different size fractions. DOC was extracted from arable soils that had received varying amounts of animal manure, inorganic fertilizer, or CuSO₄. Addition of calcium to the DOC extracts resulted in flocculation of up to 50% of the DOC originally present in the samples. High performance size exclusion chromatography (HPSEC) analysis revealed that the DOC removed mainly consisted of high molecular weight (HMW) organic acids. Low molecular weight (LMW) substances remained soluble even at high calcium concentrations. Copper solution concentrations decreased almost linearly with decreasing DOC levels, suggesting that copper remained bound to the flocculated material despite increasing calcium concentrations. The amounts of copper bound to both LMW and HMW components depended on the copper content of the soil and were described by two Langmuir sorption equations. Maximum binding capacities varied between 250 μmol of copper g^-1 of C for HMW components and 450 μmol of copper g^-1 of C for LMW components, but binding affinities were higher for HMW components. The Langmuir equation described the measured copper solution concentrations from the batch experiments very well (R² = 0.994). The observation that up to 50% of the DOC in solution was not removed by calcium indicates that a significant part of copper is potentially highly mobile in soils and can be prone to DOC-facilitated transport.