Interactive PHREEQ-N-AMDTreat water-quality modeling tools to evaluate performance and design of treatment systems for acid mine drainage

Highlights: • Three complementary, user-friendly tools (CausticTitration, ParallelTreatment, TreatTrainMix2) simulate water-quality changes during treatment of AMD. • PHREEQC equilibrium aqueous and surface speciation models are combined with kinetics models that simulate:Atmospheric exchange--O₂ ingassing, CO₂ outgassing, and pH. Iron and manganese oxidation--pH-dependent homogeneous and heterogeneous rate laws plus contributions by iron-oxidizing bacteria. Limestone dissolution--considers solution chemistry plus surface area of limestone fragments. Organic-carbon oxidation--reduction of sulfate and nitrate by carbon, plus Fe^III by adsorbed sulfide. • Graphical and tabular output indicates pH, concentrations of metals, TDS, and SC plus the cumulative quantity of precipitated solids. The PHREEQ-N-AMDTreat aqueous geochemical modeling tools described herein simulate changes in pH and solute concentrations resulting from passive and active treatment of acidic or alkaline mine drainage (AMD). The "user-friendly" interactive tools, which are publicly available software, utilize PHREEQC equilibrium aqueous and surface speciation models and kinetics models for O₂ ingassing and CO₂ outgassing, iron and manganese oxidation and precipitation, limestone dissolution, and organic carbon oxidation combined with reduction of nitrate, sulfate, and ferric iron. Reactions with synthetic caustic chemicals (CaO, Ca(OH)₂, NaOH, Na₂CO₃) or oxidizing agents (H₂O₂) also may be simulated separately or combined with sequential kinetic steps. A user interface facilitates input of water chemistry data for one or two (mixed) influent AMD solutions and adjustment of kinetic variables. Graphical and tabular output indicates the changes in pH, metals and other solute concentrations, total dissolved solids, and specific conductance of treated effluent plus the cumulative quantity of precipitated solids as a function of retention time or the amount of caustic agent added. By adjusting kinetic variables or chemical dosing, the effects of independent or sequential treatment steps that have different retention time (volume/flow rate), aeration rate, quantities of reactive solids, and temperature can be simulated for the specified influent quality. The size (land area) of a treatment system can then be estimated using reaction time estimates (volume for a corresponding treatment step is the product of reaction time and flow rate; area is volume divided by depth). Given the estimated system size, the AMDTreat cost-analysis model may be used to compute approximate costs for installation (capital) and annual operations and maintenance. Thus, various passive and/or active treatment strategies can be identified that could potentially achieve the desired effluent quality, but require different land area, equipment, and costs for construction and operation.