Abstract
Phosphogypsum is a kind of solid waste that occupies land resources and harms the environment. It can be used as a solidified material, but the utilization of phosphogypsum is limited by its impurities and weak strength performance. This study aimed to use microbial-induced carbonate precipitation (MICP) to improve the water stability, permeability, and hydraulic erosion resistance of phosphogypsum and evaluate its impact on the environment. In this paper, the phosphogypsum samples and artificial slopes were prepared and solidified by spraying various concentrations of bacteria solution and cementation solution to achieve microbial modification. The water stability and permeability test were used to calculate the mass of spalling under water shaking and the permeability coefficient. A rainfall scouring test was carried out to estimate the erosion resistance. The erosion degree was quantitatively calculated using 3D laser scanning technology. The results show that the microorganism treatment can improve water stability and reduce the permeability coefficient, while the differences between the content of CaCO3 in the outermost layer and in the inner layer gradually increase with the increase in bacterial concentration, and the permeability coefficient was reduced uniformly. The sediment loss of the slope after MICP treatment was much less than that of the untreated slope, and the connection force between the particles was strengthened. By observing the morphology of the scoured samples, we found that the treated particles were aggregated and flocculated with more macropores, which led to the formation of erosion pits under scouring. The pH of the outflow of the modified slope was neutral, and the heavy metal elements were fixed by microbial action and carbonate, which is not harmful to the environment.
Highlights
IntroductionEach ton of P2 O5 produces 4.5–5.5 t of by-product phosphogypsum [1,2]
As for the phosphogypsum slope treated by higher bacterial concentrations, the CaCO3 content increases, the particles on the surface were agglomerated during cementation, which caused uneven solidification, and some macropores did not fill effectively, resulting in the infiltration of rainwater and more severe erosion during the rainfall scouring test
We used the method of cyclically spraying a bacteria solution with various concentrations (OD600 = 0.1, OD600 = 0.3, and OD600 = 0.5) and a cementation solution (0.5 M) to modify the phosphogypsum material via microbial-induced carbonate precipitation (MICP)
Summary
Each ton of P2 O5 produces 4.5–5.5 t of by-product phosphogypsum [1,2]. Worldwide PG production can reach 280 million tons per year. The annual output of phosphogypsum in China is 70 million tons, but the utilization rate is less than 15%, which causes serious land resource occupation and environmental pollution [3]. The acidity and the impurities in phosphogypsum will threaten the soil and groundwater environment, which incurs costs for its treatment and purification [4]. Phosphogypsum is a kind of gypsum material rich in phosphorus, and it contains soluble phosphorus, organic matter, and other impurities, which will affect the crystallization time of phosphogypsum crystals and reduce its mechanical properties. A single treatment method cannot meet the conditions of use, and multiple methods are often required in combination, though this increases the treatment time and cost at the same time
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