Neutralization of red mud using bio-acid generated by hydrothermal carbonization of waste biomass for potential soil application

Abstract

Red mud, a by-product generated from alumina industry, poses serious environmental issues and safety risks. Strong alkalinity of red mud is a key factor affecting its potential soil application and comprehensive utilization. A novel process employing natural biomass without addition of chemicals was proposed for remediation of red mud. Hydrothermal carbonization (HTC) of local fallen leaves was conducted to obtain solution containing bio-acids (SBA) and hydrochar. Factors of HTC and stirring process were investigated on neutralization of red mud using SBA. The presence of small-molecule compounds such as furfural and organic acids in SBA is identified by GC-MS. SBA is effective to lower the alkalinity of red mud, and the pH value of red mud is reduced from 9.88 to 6.65 under experimental conditions. Neutralization mechanism was examined by various characterizations including XRD, FTIR, TG and EPMA. Neutralization results in little change of red mud, and it is deduced that small-molecule organic acids only remove free alkali without destruction of minerals. Soil matrix prepared by mixing the neutralized red mud and hydrochar is favorable to plant growth. The developed process including HTC of waste biomass, neutralization of red mud and fabrication of soil matrix for plant growth is green, sustainable and environmentally friendly, thus highly promising for potential applications.