Haloalkalitolerant plants drive alkaline mineral weathering and dealkalization of seawater-treated bauxite residue

Background

Ecological engineering of tailings into soil-like growth media (i.e., technosol) has emerged as a nature-based solution for soilless rehabilitation of alkaline bauxite residue. However, field applications often result in spatial heterogeneity, where uneven amendment leaves behind highly alkaline and saline BR pockets that limit sustainable rehabilitation. It is hypothesized that haloalkalitolerant plants with roots partially grown in the improved niches render their strong tolerance of BR matrix to form extensive roots-mineral interfaces, generate physical and biochemical modification, and irreversibly neutralize the extremely alkaline pH in BR niches.

Methods

In this study, seawater-treated BR was used to simulate these residual alkaline matrices. Four plant species, including two halophytes, one haloalkalitolerant native acacia and one glycophytic sorghum grass were cultured in a sand-BR compartment system to simulate field heterogeneity.

Results

Massive roots-BR interfaces were formed. Root activities at the interfaces accelerated the weathering of alkaline minerals and neutralized alkaline pH to circumneutral. Halophytes (Atriplex nummularia and Chloris gayana) were found to be the most effective in weathering and neutralizing BR, due to their higher capacity to exude low-molecular-weight organic acids rich in carboxyl groups in roots-BR interfaces. The LMWOAs facilitated Na⁺-H⁺ exchange with sodalite and disrupted its structure.

Conclusion

These interactions stimulated the formation of nanosized amorphous Al-Si-Fe minerals on root surfaces. Plant uptake of liberated Na led to Na depletion in the rhizosphere. Therefore, halophytic species should be included in the field application to eliminate substrate heterogeneity of eco-engineering soil formation for soilless BR rehabilitation.

Graphical Abstract

The root activities of pioneer plants effectively weather alkaline minerals and neutralize alkaline pH in bauxite residue.