The effect of pH on dissolved organic matter fractions in solid waste

Organic matter (OM) is of high importance for the availability and mobility of nutrients and contaminants; they provide a binding surface and retain compounds when present as solid organic matter (SOM), or enhance mobility when present as dissolved organic matter (DOM). The partitioning between SOM and DOM is dependent on a variety of environmental parameters, a key parameter being the soil pH. pH dependent tests were conducted on solid waste from three different landfills. These samples contain OM of different origins, to further enhance insights on the role of pH for different types of OM. The samples were extracted at 8 different pH levels between pH 2 and 12, taking a pH range beyond environmental relevance to better elucidate mechanisms of importance, e.g. electrostatic repulsion or co-precipitation with oxides. Samples were analysed for organic matter concentration and concentrations of humic substances, the latter traditionally providing a proxy for “reactive” organic matter to which compounds can bind. All samples showed increasing DOC concentrations with increasing pH, bar between pH 2 – 4, where decreasing pH resulted in increased DOC concentration. This effect at low pH was particularly pronounced for the soil remediation waste samples relatively poor in organic matter. Humic substances showed that with increasing pH the relative contribution of humic acid increased, the relative contribution of hydrophilic acid decreased, and the relative contribution peaked around pH 7. These changes in humic substance solubility correlated with the solubility of various compounds with high OM affinity, e.g. Chromium and Copper.