Surface coating effects on the sorption and dissolution of ZnO nanoparticles in soil

Abstract

Soil pH and dissolved organic matter (DOM) content are among the most important factors affecting the bioavailability of Zn and the binding and dissolution of ZnO nanoparticles (NPs). To investigate the effect of NP surface chemistry and DOM on the behavior of ZnO NPs and ZnSO₄ in soil solution at pH 6 and 8, we synthesized electrostatically stabilized (bare positively charged ZnO, and negatively charged (ZnO–Zn₃(PO₄)₂ core–shell NPs)), and sterically and electrosterically stabilized (neutral dextran (DEX), and negatively charged dextran sulfate (DEX(SO₄))–ZnO NPs), respectively. We hypothesized that negatively charged ZnO NPs will have higher total Zn concentrations as opposed to neutral and positively charged ones in soil pore water at higher pH, with higher dissolution of the NPs at lower pH. At pH 8, core–shell and DEX–ZnO NP amendments had significantly higher total Zn concentration than ZnSO₄. To investigate the unexpected behavior of the neutral DEX–ZnO NPs, we performed sorption isotherm experiments which showed that DEX–ZnO NPs had the highest affinity for DOM of all ZnO NPs, which likely enhanced their colloidal stability and partitioning in soil pore water, especially at pH 8. In simple aqueous solution, with increasing ionic strength, negatively charged core–shell and DEX(SO₄) ZnO NPs were the most stable against aggregation. When DOM was introduced in to the system, the as-synthesized surface chemistry of the particles was altered, and all NPs became negatively charged. Dissolved Zn concentrations in soil extracts of NP amendments were similar while ZnSO₄ amended soils had the highest dissolved Zn among all treatments.