Effect of the addition of red mud on the electrochemical and adsorption properties of an oxisol

Abstract

This study aimed to characterize the waste generated in the production of alumina from bauxite refining process (red mud) and to analyze its effect on the electrochemical and adsorption properties of a tropical soil (Oxisol). The red mud (LM) is a highly alkaline residue (pH around 12), which is rich in iron oxide/hydroxides. To study the effect of red mud and calcium carbonate (CaCO3) - commonly used in agriculture to correct soil pH - on the soil, these substances were added in different proportions to Oxisol samples for 60 days. The addition of CaCO3 and LM to the soil increased the pH and the point of zero charge (PZC). However, the pH H₂0 value of the soil with LM was greater than its PCZ, enabling dispersion of soil particles. Moreover, such addition increases cation exchange capacity in soils, as observed in removal tests of Pb²⁺ in solution. It was observed that the minimal addition of 0.5% of CaCO3 and 10% of LM to the soil increased the Pb²⁺ retention capacity from 37% to 100%. The same results were observed in tests performed only with red mud. Even when subjected to an acid leaching (solution with pH 4.9), soils incubated with 1% CaCO3 and 1% LM released 7.5% and 11% of retained Pb²⁺, respectively. This low Pb²⁺ leaching rate is due to the increase in soil pH, favoring the adsorption of Pb²⁺ by Fe and A1 oxy-hydroxides present in the soil and red mud composition. It can be concluded, therefore, that the addition of red mud can change the electrochemical conditions of the soil, dispersing more clay and colloidal particles. However, by increasing the pH of the system and introducing iron and aluminum oxides and hydroxides, such material can increase the retention of heavy metals, such as Pb²⁺, in the soil.