Rapid conversion of red mud into soil matrix by co-hydrothermal carbonization with biomass wastes

Abstract

Red mud is the alkaline and high-volume residue produced from alumina extraction from bauxite ores. Its disposal in purpose-built areas occupies a large amount of land and poses serious threats to surrounding environment. This study proposes an innovative strategy for rapid conversion of red mud into soil matrix by co-hydrothermal carbonization (c-HTC) with biomass wastes. Effective neutralization of the alkalinity is obtained by c-HTC process, lowering the pH from 10.0 to approximate 7.0. Experiments of employing wet biomass wastes, reusing water for c-HTC and long-term pH stability verify the feasibility and practicality of c-HTC process. Change of alkaline components in red mud, formation of hydrochar, and creation of aggregates are revealed by multiple characterizations. As to soil properties, the cation exchange capacity, electrical conductivity, organic carbon, and particle size of c-HTC product is remarkably improved. Perennial ryegrass is well grown in the soil matrix. The c-HTC strategy promotes future application of phytoremediation of red mud at full field scale.