Solidification Mechanism Of Pollutants In Modified Phosphogypsum Matrix And Its Effect On Plant Growt

This article focuses on phosphogypsum as the subject of research,aiming to address the problems of environmental pollution and resource waste caused by its treatment process.The solution proposed involves creating an improved phosphogypsum matrix through the addition of materials such as biomass charcoal,fungal residue,and cow manure.The study employed a combination of pot experiments,indoor analysis,and artificial simulated rainfall methods,along with characterization,soil agrochemical analysis,and high-throughput sequencing of soil microorganisms,to analyze the effects of different ratios of improved phosphogypsum matrix on the growth status of rye grass,the concentration of F^-and PO₄^3-in leachate,heavy metal content,and substrate microbial diversity.The results showed that the improved substrate significantly increased the germination rate and growth rate of ryegrass.Addition of the improved substrate increased the p H value of the leachate,which was beneficial for plant growth.Furthermore,the concentration of F^-in the leachate was significantly reduced,and the concentration of PO₄^3-was well solidified,thus playing a certain role in environmental purification.In addition,the microbial community structure of the improved phosphogypsum substrate also changed.This study identified high-quality ratios of improved phosphogypsum matrix,providing a feasibility study for the application of phosphogypsum in lawn grass production.The following conclusions are mainly drawn:（1）This study used prickly pear residue as the raw material for biomass charcoal to prepare hydroxylated iron oxide-modified biomass charcoal（Fe OOH-BC）.Sodium fluoride was used as the simulated pollutant to investigate the effects of solution p H,adsorbent dosage,adsorption time,and initial concentration on the adsorption of fluoride ions.The results showed that under the adsorption reaction conditions of p H=6,T=25°C,initial concentration of fluoride ions of 50 mg·L^-1,and adsorbent dosage of 0.05 g,with shaking for 24 hours,Fe OOH-BC achieved a removal rate of 95.27%for fluoride ions.The study indicated that the introduction of Fe OOH can significantly improve the adsorption performance of biomass charcoal for fluoride ions,which may mainly be due to multi-layer adsorption and chemical adsorption（2）Through the use of modified biomass charcoal and other materials for composite modification,the vegetation environment of phosphogypsum has been greatly improved.The best treatment effect was achieved by mixing 60%phosphogypsum with 5%biomass charcoal,10%red clay,and 10%bacterial residue,with 5%quicklime added as the intermediate layer,and 10%bacterial residue added as the bottom layer（C treatment group）.After modification,the p H of the substrate leachate increased from 3.6 to 8.2,while EC and TDS decreased significantly.The concentrations of F^-,PO₄^3-,Mn,Cr,Cu,Pb,As,and Sb also decreased significantly.Moreover,heavy metal content in the aboveground part of rye grass decreased significantly.Following the modification treatment,the diversity and richness of the microbial community in the substrate significantly increased,promoting the establishment of a microbial ecological environment in the phosphogypsum substrate and facilitating the growth of rye grass.These findings suggest that modified biomass charcoal can effectively improve the physicochemical properties of phosphogypsum,promote plant growth,and solidify heavy metals.（3）The addition of improved substrates significantly improved the germination rate of rye grass,while there was no rye grass germination in control group A on the seventh day.The addition of materials such as modified biomass charcoal in the improved substrate increased the air permeability and water-holding capacity of the phosphogypsum substrate,making it more conducive to rapid growth of rye grass.The addition of the improved substrate also increased the p H value of the leachate to a range suitable for plant growth,and significantly reduced the concentration of F^-in the leachate.Among them,in the leachate collected on the 45th day,the treatment group BC with the addition of the improved substrate reduced the F-concentration by84.87%compared to the control group,while the concentration of PO₄^3-was well solidified after the addition of the improved substrate,and almost reached the level of undetectable in the later period.The treatment group BC showed the best effect,with a reduction of 99.68%compared to the control group.Compared with the control group A,the heavy metal content in the aboveground parts of rye grass gradually decreased after the improvement,and the difference was significant（P<0.05）.The different ratios of improved phosphogypsum substrates also had a significant impact on the microbial community structure,and there were significant differences in the microbial community structure of improved phosphogypsum substrates compared to the control group A.The microbial community structure of phosphogypsum substrates improved by different methods and ratios also differed under the growth of rye grass.（4）The original crystal surface of phosphogypsum had a smooth and slender plate-like structure,with fine crystals and impurities attached.However,after modification,small needle-shaped gypsum crystals wrapped around the phosphogypsum,giving it a more regular internal structure.Blocky hydrated products and flocculent particles were present on the surface or in the pores between fine particles,increasing the density of the entire sample.The original phosphogypsum surface was highly hydrophilic and lacked waterproofing properties,with a contact angle of 27.9°.However,after mixing with the curing and modifying agents,the contact angle increased,rendering the surface hydrophobic.The p H value of the modified phosphogypsum was also improved,which was particularly advantageous given the highly acidic nature of the original phosphogypsum（p H 2.9）.In the leaching solution,the concentrations of F^-and PO₄^3-in the original phosphogypsum were 635.00 and 12.9 mg·L^-1,respectively.However,after modification with the curing agent,both concentrations were significantly reduced and were lower than the concentration limits for hazardous components specified in the Identification of Leaching Toxicity of Hazardous Wastes（GB5085.3—2007）.This indicates that the curing agent had a good solidification effect on toxic and harmful ions.