Mechanisms controlling the release of inorganic contaminants, organic matter fractions, and ammonium from solid landfill waste: pH dependent leaching experiments and geochemical modelling

Abstract

With the intention of making landfilling more sustainable, three full-scale projects have been set up to investigate the effects of leachate recirculation and aeration on the stabilization of contaminants. Prior to this (an)aerobic stabilization, solid waste was sampled from all three landfills to identify the mechanisms behind contaminant leaching and stabilization, as a reference point for evaluation of the treatment effects at a later stage of the project. Processes underlying the leaching of a wide range of inorganic contaminants were identified using a combined approach of pH dependent leaching experiments, organic matter fractionation, and geochemical modelling. We additionally focused on organic matter, being an important surface controlling the leaching of inorganic elements, and ammonium, for which compliance with intended environmental limit values for sustainable landfill management is challenging for many landfills. Three types of pH dependent leaching behaviour could be identified for the investigated elements: non-reactive behaviour, anionic behaviour, and cationic behaviour. Reactive elements, including ammonium, showed maximum dissolved concentrations around the upper and lower ends of the pH range, while leaching of non-reactive elements was pH-independent. Organic matter became increasingly soluble with increasing pH, while its relative composition became increasingly enriched in fractions reactive for metal binding, most notably humic acid, only for the most organic landfill. The organic signature of the waste samples, together with their varying organic matter content, resulted in distinct differences in leaching behaviour between landfills. These differences result from the seemingly strong influence of organic matter on the pH at which minimum solubility of elements occurs, being lower for elements with higher organic matter binding affinity, an effect being more pronounced for more organic-rich samples. The model, based on independently determined generic sorption parameters, provided adequate descriptions of most elements, yet arsenic and ammonium were poorly described due to mechanisms not captured by the model while chromium was inadequately described due to limitations of the current binding parameters. As the three landfills have been shown to differ strongly with respect to waste properties (e.g. organic versus. inorganic) an important outcome of this study is that landfill-specific combinations of proposed treatment strategies (aeration, leachate recirculation) are necessary to reduce the leaching of specific contaminant groups to below limit values and are anticipated to be most effective for less mature landfills characterized by a lower ratio of solid to dissolved OM.