The influence of pH, electrolyte type, and surface coating on arsenic(V) adsorption onto kaolinites

The adsorption of arsenate (As[V]) has been investigated through batch experiments using two solid phases: a purified kaolinite and a kaolinite coated with humic acid (HA). The influence of various chemical parameters on As(V) adsorption onto kaolinite are examined, namely initial As concentration, pH, electrolyte type (Ca and Na) and concentration, and the presence or absence of a HA coating on the kaolinite surface. In Na electrolyte, As(V) adsorption onto kaolinite and HA-coated kaolinite is pH-dependent. In Ca media, however, As(V) adsorption onto kaolinite is not pH-dependent, and pH has only a minor influence on As(V) adsorption onto kaolinite coated with HA. Depending on electrolyte type, the adsorption of As(V) respectively onto kaolinite and HA-coated kaolinite differs considerably. In NaNO3, As(V) adsorption is lower in the presence of HA regardless of the pH value, whereas in Ca(NO3)(2), adsorption of As(V) onto HA-coated kaolinite is greater than onto kaolinite alone above pH 5. The effect of Ca on As(V) adsorption thus appears complex and concerns both As(V) speciation in solution and the kaolinite adsorption site level. In addition, a decrease in the Ca concentration of the electrolyte decreases As(V) adsorption onto both solids.