Arsenic Retention On Red Mud Treated with Gypsum.

Arsenic (As) is ranked first in the priority list of harmful substances to human health and its carcinogenic effect is known since many years. Because of this, several industry by-products have been tested, alone or in mixtures, as adsorbents for arsenic in order to reduce its mobility and bioavailability in the environment. This study aimed to characterize an aluminum mining by-product (AMB) and gypsum (G) and to evaluate the effect of the addition of G to AMB in order to improve its arsenic retention capacity. AMB and G samples were characterized by microwave furnace and sulfuric acid digestion, as well as by X-ray diffraction. Gypsum was subsequently added to AMB in the following proportions: 0, 1, 2, 5, 10 and 25%. Theses mixtures were incubated during 20 days and submitted to As adsorption and desorption and tested for their maximum adsorption capacity of As (As_MAC) by Langmuir isotherm. During adsorption experiments, adsorbents (0.3 g) were suspended in 30 mL of NaCl solution (15 mmol L^-1) at pH 5.5 with As added as Na₂HAsO₄.7H₂O in the following concentrations: 0, 100, 185, 380, 840 and 1300 µmol L^-1. Desorption was determined with 30 mL of NaCl (15 mmol L^-1). After 72 hours of reaction, the supernatant was centrifuged and collected for As analyses by graphite furnace atomic absorption spectroscopy. Arsenic adsorption fitted well to Langmuir isotherms and increased upon increasing proportion of G added to AMB. The As_MAC at pure AMB reached 909 mg kg^-1, whereas the 75% AMB + 25% G mixture sorbed up to 3333 mg kg^-1 of As, i.e., a 3.5 – fold increase in As_MAC. Sponsored by CNPq, CAPES and FAPEMIG.