Comparing the sorptive affinity of an aluminum-mining by-product for cationic and anionic pollutants

Abstract

Oxide-rich materials, such as “red mud,” an aluminum-mining by-product, are good (ad)sorbents for soil pollutants. This study evaluated sorption reactions of selected cations/anions on aluminum-mining by-product, proposing the calculation of a new distribution constant (i.e., distribution constant at half of the maximum adsorption capacity, Kd(MAC/2)) to assess aluminum-mining by-product sorption efficiency. Adsorption was performed with increasing doses of zinc, copper, cadmium, lead, arsenic, and phosphorus, followed by subsequent desorption (pH 5.5). Data fitted well to both Langmuir and Freundlich equations. The maximum adsorption capacity estimated by the Langmuir equation and the relative adsorption capacity parameter of the Freundlich equation followed the ascending order cadmium < zinc < copper < lead < arsenic < phosphorus, whereas the distribution constant at half of the maximum adsorption capacity increased as follows: cadmium < zinc < copper < arsenic < lead < phosphorus. Anionic (ad)sorbates (i.e., arsenic and phosphorus) showed higher affinity for aluminum-mining by-product than cationic ones. Among the studied cations, lead and copper were much more adsorbed by aluminum-mining by-product than zinc and cadmium. High adsorption and low desorption occurred mostly for elements known to form inner-sphere complexes, such as phosphorus and lead. Desorption from aluminum-mining by-product was very low for all sorbates, except cadmium. The aluminum-mining by-product, a low-cost by-product, proved to be an adequate adsorbent for pollutants removal. Moreover, the distribution constant at half of the maximum adsorption capacity could be used as an alternative parameter for sorption studies with aluminum-mining by-product and conceivably other sorbents.