An effective treatment method for phosphogypsum

Phosphate mining activities affect crop rhizosphere fungal communities

Phosphate mining releases heavy metals into the surrounding environment. In this study, the effects of phosphate mining on rhizosphere soil fungi in surrounding crops, including Lactuca sativa var. angustata, Glycine max (L.) Merr., and Triticum aestivum L., were assessed. Phosphate mining significantly reduced the crop rhizosphere fungal diversity (P < 0.05). The relative abundances of Fusarium and Epicoccum increased in mining rhizosphere soil compared with the baseline. Beta diversity analysis indicated that phosphate mining led to the differentiation of fungal community structure in plant rhizospheres. Guild analysis indicated that different plant rhizosphere fungi developed various guilds in response to phosphate mining stress. Nine fungi were isolated from soil samples, with solubilization index values ranging from 1.1 to 2.5. Two efficient phosphate solubilizers, Epicoccum nigrum and Fusarium verticillioides, were enriched in phosphate mining rhizosphere soil samples. The dissolution kinetics of inorganic phosphorus and alkaline phosphatase activity assay showed strong phosphorus dissolution ability of the isolated fungi. Penicillium aculeatum, Trichoderma harzianum, Chaetomium globosum, and F. verticillioides showed strong tolerance to multiple heavy metals. This study furthers our understanding of how rhizosphere fungal ecology is affected by phosphate mining and provides important resources for the remediation of phosphate mining soil pollution.

Characteristic pollutant purification analysis of modified phosphogypsum comprehensive utilization

The waste product phosphogypsum (PG) is produced in phosphoric acid production processes. Its storage requires large amounts of land resources and poses serious environmental risks. In this work, detailed experimental research was carried out to investigate the potential reuse of PG after calcination modification as a novel building material for cast-in-place concrete products. The calcination modification mechanism was studied, and the environmental risk assessment of modified PG was presented. The results showed that the calcination modification includes crystal phase transformation, removal of impurities, and modifying the pH value. The calcination was carried out at 280 °C for 5 h, where the resulting product was a pH value of 7.1, and the soluble fluorine and phosphorus removal rates reached up to 69.2% and 71.2%, respectively. These removal rates met the requirements of the China national standard Phosphogypsum (GB/T 23456-2018). To ensure the environmental safety, ecological risk assessment methods for determining the leaching toxicity of the modified PG were employed. The toxicity of Ba and P elements in the modified PG products was assessed, as well as the leaching toxicity concentrations of all particular heavy metals, which were found well below the limits set by the national standards. All the results presented strongly suggest that the 280 °C modified PG presented here has excellent application potential as a raw component in building materials.

Surface modification of phosphogypsum and application in polyolefin composites

Phosphogypsum (PG) is one of solid wastes with large amount of yield and serious pollution, which has attracted wide attention. The aim of this study is to investigate filling performance of PG on polypropylene (PP) or high-density polyethylene (HDPE) matrix. In this work, PG was calcined initially to improve whiteness and fix impurities. X-ray diffraction (XRD) results showed that after calcined at 500 °C, the PG phase changed from CaSO₄·2H₂O to CaSO₄. The modification effects of the three modifiers were evaluated by Fourier transform infrared spectra (FTIR), oil absorption value, water floatability, and contact angle analysis. The effects of weight fraction of PG in PP and HDPE matrix on mechanics and morphology were observed by tensile test, impact test, and scanning electron microscope. Scanning electron microscope (SEM) showed that modified PG can be dispersed uniformly in the matrix at low filling content. With the increase of PG filling content, the analysis of mechanical properties showed that the tensile strength of HDPE matrix increased, while the tensile strength of PP matrix decreased gradually. The impact strength of HDPE matrix would decrease, but the impact strength of PP matrix increased first and then decreased. Compared with calcium carbonate (CC), the mechanical properties of HDPE filled with PG performed better. The apparent density showed that polymer composites filled with PG have the characteristics of light weight.

Fabrication and environmental applications of metal-containing solid waste/biochar composites: A review

The resource utilization of industrial solid waste has become a hot issue worldwide. Composites of biochar with metal-containing solid wastes (MCSWs) can not only improve the adsorption performance, but also reduce the cost of modification and promote the recycling of waste resources. Thus, the synthesis and applications of biochar composites modified by MCSWs have been attracting increasing attention. However, different MCSWs may result in metal-containing solid waste/biochar composites (MCSW-BCs) with various physicochemical properties and adsorption performance, causing distinct adsorption mechanisms and applications. Although a lot of researches have been carried out, it is still in infancy. In particular, the explanation on the adsorption mechanisms and influencing factors of pollutant onto MCSW-BCs are not comprehensive and clear enough. Therefore, a systematic review on fabrication and potential environmental applications of different MCSW-BCs is highly needed. Here we summarize the recent advances on the utilization of typical metal-containing solid wastes, preparation of MCSW-BCs, adsorption mechanisms and influencing factors of pollutants by MCSW-BCs as well as their environmental applications. Finally, comments and perspectives for future studies are proposed.

Study on the Kinetics of Hydration Transformation from Hemihydrate Phosphogypsum to Dihydrate Phosphogypsum in Simulated Wet Process Phosphoric Acid

The key technology of wet process phosphoric acid recrystallization is phosphogypsum phase transformation. In this study, the hydration of α-hemihydrate phosphogypsum (α-HH) to dihydrate phosphogypsum (DH) and the influence of process parameters on hydration kinetics are performed by modifying a dispersive kinetic model in the simulation of wet process phosphoric acid recrystallization. Results show that the modified dispersive kinetic model is very important in describing the entire kinetic process, indicating that α-HH-DH hydration includes induction of nucleation and growth restriction. The hydration rate of α-HH-DH substantially accelerates with the decrease of temperature and phosphoric acid concentration because the activation entropy of the reaction increases during the induction stage and the growth stage, which reduces the activation energy barrier. Moreover, the hydration rate of α-HH-DH considerably accelerates with the increase of SO₄ ^2- ion concentration. Activation entropy increases in the induction stage, causing the activation energy barrier to decrease. Activation enthalpy increases in the growth stage, causing the activation energy barrier to decrease. The influence of process parameters on the rate of the α-HH-DH hydration reaction follows the order SO₄ ^2- ion concentration > phosphoric acid concentration > temperature. Therefore, controlling the three parameters of temperature, phosphoric acid concentration, and SO₄ ^2- ion concentration are important for improving the conversion rate of α-HH-DH and the purity of DH products in the production of wet process phosphoric acid.

Distribution of uranium and thorium chains radionuclides in different fractions of phosphogypsum grains

This work presents results obtained using gamma spectrometry measurements of phosphogypsum samples on a non-fractionated (native) and fractionated phosphogypsum byproduct. The phosphogypsum was divided into particles size fractions within the range of < 0.063, 0.063-0.090, 0.090-0.125, 0.125-0.250, and over 0.250 mm and analyzed after reaching radioactive equilibrium using high-resolution gamma spectrometry technique. It was found that there is no significant differentiation between ²²⁶Ra distribution among particular grain size fractions of this material; however, tendency for preferential retention of radionuclides in particular grain size fractions is observed. The detailed analysis of results revealed that radium is preferentially retained in smaller grain size fractions, whereas lead and thorium in coarse fractions. The results indicate that overall ²²⁶Ra activity concentrations between particular fractions of phosphogypsum vary globally between - 34 and + 47% regarding non-fractionated material, and for ²¹⁰Pb activity concentration, fluctuations are found between - 26 up and + 38%. Presumably, the mechanism of radium incorporation into gypsum phase is based on a sequence of radium bearing sulfate phases formation followed by a surface adsorption of these phases on the calcium sulfate crystals, whereas for lead and thorium ions, rather incorporation into crystal lattice should be expected as more likelihood process.

Industrial Experiment of Goaf Filling Using the Filling Materials Based on Hemihydrate Phosphogypsum

The surface stockpiling of phosphogypsum not only occupies a large amount of land, but also seriously harms the surrounding ecological environment. The preparation of phosphogypsum into filling materials for mine filling can not only maintain the stability of surrounding rock, reduce surface subsidence, enhance the recovery of resources, but it can also completely solve the problem of phosphogypsum stockpiling. Under certain activation conditions, hemihydrate phosphogypsum has a strong cementing property. It is an important way to reduce the filling cost by using hemihydrate phosphogypsum instead of cement as a cementing material. Through laboratory experiments, the filling materials based on hemihydrate phosphogypsum were developed. In order to further verify its feasibility in practical filling engineering, the industrial experiment of goaf filling was carried out in a phosphorus mine. The results show that the filling system was simple, reliable, and easy to operate and manage. The strength of the filling body basically reached the expected strength target of 2.5 MPa in 3 days. The consolidation speed of the filling materials was faster, which is beneficial to the safe underground construction of the mine. The results of the industrial experiment of goaf filling indicate that the filling materials based on hemihydrate phosphogypsum are suitable for mine filling engineering practice, the work amount is small, and the filling cost is low.