A model of pH and redox buffer depletion in waste landfills

Sammanfattning: The geochemical processes governing the release of heavymetal contaminants from solid waste materials have been studiedexperimentally and modelled using a mass balance approach. Theaim of the modelling work was to develop calculation tools thatcan be used to predict the release rate of heavy metals fromwaste heaps or landfills. The leaching models are based uponthe observation that the pH and redox state of the water incontact with waste are the two master variables that controlheavy metal mobility in aqueous environments. The modelsdeveloped are intended to be simple and sufficiently flexiblethat they may be adapted to different waste types withreasonable ease.The depletion rates of pH and redox buffering constituentsin the waste material are calculated by coupling a descriptionof the geochemical processes occurring within the waste withthe in- and out-transport of reactive chemical species. Themodels of waste leaching require input data to describe thereactive properties of the waste. These data may be obtainedfrom simple laboratory experiments such as pH static titrationsand reducing capacity measurements.The simulation results indicate that oxidation of organicmaterial can improve the quality of solid wastes by way ofcarbonation processes that transform highly soluble oxide,hydroxide, and silicate forms of Ca into calcite. Calcitebuffers leachate at a circumneutral pH level that is optimalfor the minimisation of heavy metal leaching. Oxidation oforganic material can occur under oxic conditions, or underanoxic conditions using sulphate as an oxidant. Under theanoxic conditions that prevail in a fully water saturatedleaching environment, reduction of sulphate also promotes theformation of insoluble heavy metal sulphide minerals. It isconcluded that disposal of waste materials in a fully watersaturated environment is the best way of minimisingenvironmental impact from these potentially hazardousmaterials.Keywords: Leaching, chemical weathering, acidneutralisation, solid waste, combustion residues, pH buffering,redox buffering, geochemical modelling, reactive transport