AWWA ACE63078

The objectives of this work were to quantify the effects of pH and competing ions in the presence and absence of other competing ions on arsenic removal during coagulation with ferric chloride. An additional objective was to compare in-situ formed ferric hydroxides vs. granular ferric oxide media as arsenic adsorbents on the basis of µg As sorbed/g Fe(III). Finally, attempts were made to use the MINEQL+ chemical equilibrium modeling program to predict arsenic adsorption as a function of pH and competing ions. The removal of As(V) was highly pH dependent while As(III) was largely independent of pH in the NSFI-53 challenge water. The adsorption of As(V) which is charged in the pH range of 6.5-8.5 was significantly higher than uncharged As(III). The in-situ formed hydroxides had significantly higher adsorption capacities compared to the granular media due to high effective surface area of FeOOH coagulant. All three competing ions--silicate, phosphate, and vanadate--interfered with arsenic adsorption, and the level of interference depended on the competing ion concentration and the presence or absence of other competing ions. The unmodified Mineql+ chemical equilibrium modeling program could not simulate As(III) or As(V) adsorption onto Fe(OH)₃ during FeCl₃ coagulation. Includes figures.