An approach to minimizing the uncertainty caused by sediment washing pretreatment in phosphorus adsorption experiments

The presence of native adsorptive substances (NAS) cause uncertainties in the analysis of sediment phosphorus (P) adsorptive capacity. Therefore, HCl washing pretreatment is now used to clean sediment NAS. However, uncertainties still exist because the concentration (C) varies widely, from pH 3.5-12 mol/L, among studies. In this study, sediment samples from four sites were washed with HCl at different concentrations (C), and then analyzed to evaluate washing effects on the native absorptive substances (phosphorus, metals, and organic matter), the phosphorus equilibrium adsorption capacity (EAC), and sediment skeleton erosion. Study results reveal that washing efficiency, sediment skeleton erosion, and adsorption capacity varied with C. Micromorphology analysis showed that sediment erosion increased with increasing C; the optimum C value is 0.0-1.0 mol/L (STG1) level based on its small negative impact on the sediment skeleton. Additionally, NAS analysis further showed that washing efficiency increased with increasing C. Most native adsorptive substances (NAP, Fe, Al, Ca, and OM) were released when the HCl concentration was in the range of 0.5-1.0 mol/L. Therefore, 0.5-1.0 mol/L (in STG1) may be the best choice for sediment washing because of its fairly satisfactory washing efficiency. Finally, P equilibrium adsorption capacity (EAC) analysis showed that the sorption capacity varied greatly with C at 0.5-1.0 mol/L, due to the erosion of the sediment skeleton. Therefore, C at 0.5 mol/L is the optimal concentration because the skeleton erosion was minimized when C decreased to the minimum point in the 0.5-1.0 mol/L range. The HCl washing pretreatment (C = 0.5 mol/L) can also be used for analyzing other basic physicochemical substances (P, Ca, OM, Fe, Al). Our findings are an important step in providing important data for ecological restoration of wetlands, streams, rivers, lakes, and reservoirs, especially for those controlled by the internal source.