Maize straw-Penicillium oxalicum enhanced long-term aggregate stability in bauxite residue ameliorated with desulfurization gypsum

Abstract

To accurately identify the factors affecting the formation of stable aggregates in bauxite residue during the soil-formation process, the comprehensive effects of a combined chemical-biological amelioration strategy including solid wastes and a functional microorganism on aggregate size distribution and its stability in bauxite residue were investigated during a 365-d simulation experiment. The results showed that the combined amelioration effectively reduced the saline-alkalinity of bauxite residue, and markedly changed the contents of aggregate-associated chemical binding agents. Desulfurization gypsum and maize straw-Penicillium oxalicum (P. oxalicum) differentiated the formation of aggregates within different sizes. Maize straw-P. oxalicum stimulated the formation of water-stable macroaggregates with more durable erosion resistance by the wet-sieving and laser dynamic diffraction analysis. The Pearson correlation analysis showed that exchangeable polyvalent metal ions, pyrophosphate extractable Fe oxide, and organic carbon exhibited positive correlations with aggregate stability during the 365-d incubation. The findings in this study may provide data support and engineering practical reference for ecological restoration in the disposal areas.

1. Introduction

Bauxite residue is a highly alkaline solid waste discharged by alumina production [1−3]. Due to its low utilization rate, large amounts of bauxite residue are stacked in the disposal areas, which may cause a series of ecological and environmental issues and hinder the sustainable development of the alumina industry [4]. Ecological restoration is an effective strategy to control the environmental risks of the bauxite residue disposal areas [5].

Reconstructing the physical structure of bauxite residue and converting the bauxite residue into a soil-like growth substrate can effectively and continuously support vegetation establishment [6, 7]. However, bauxite residue produced by the Bayer process has extremely fine particles and poor physical structure. For instance, the diameter of about 50% of total particles is less than 5 μm, resulting in a high bulk density (generally 1.8 − 3.2 g/cm3), low porosity, and poor water transmissibility [8], which limit the growth of plant roots and ecological restoration.

As the basic component of soil physical structure, aggregates are complex structures of organic-inorganic-microbial composition [9], consisting of pores and solid matter produced by particle rearrangement, agglomeration, and cementation, which play important roles in the balance of water, fertilizer, gas, and heat, whilst providing an appropriate living environment for microorganisms and plants. Soil particle aggregation is affected by many factors including soil organic matter, iron-aluminum oxides, and exchangeable cations [10, 11]. Among them, organic matter is the major binding agent for the formation of the improved aggregate structure as organic matter can bind the multivalent metal cations (e.g. Al3+ and Fe3+) and clay minerals to form organic-mineral complex [12].