Study On Adsorption And Solidification Performance Of Chelating Fibers And Their Cement-based Materials For Heavy Metal Ions

Recent years,with the rapid development of mining,electroplating,tanning and other metal-related industries,a large number of industrial wastes and industrial sewage containing heavy metals have entered the biosphere through various ways,causing serious environmental issues.Since heavy metals cannot be biodegradable and could be recycled into plants and animals through the biological chain,they would eventually harm human health.With the improvement of people's living standard and awareness of environmental protection,efficient,simple and low-cost heavy metal pollutant treatments have become a research hotspot.Cement-based materials are generally used in the solidification of solid wastes containing heavy metals,but the solidified body has some problems such as poor volume stability and heavy metal dissolution.Chelating fibers are widely used in the separation and treatment of wastewater containing heavy metals,which have the characteristics of strong selectivity and large adsorption capacity.As fibrous materials are also commonly used in civil engineering,in this study,chelating fibers are prepared by chemical method and used to effectively adsorb heavy metal ions in wastewater.Afterwards,the chelating fibers are used with cement-based materials to solidify heavy metal ions and solid waste containing heavy metal,and the mechanical properties and leaching toxicity of the solidified body are studied.The detailed research contents are highlighted as follows:Using organic polypropylene fiber and natural cellulose fiber as matrix materials,acrylonitrile was grafted on fiber surface by free radical polymerization.By optimizing experimental conditions,the grafting rate of acrylonitrile could reach33.33%and 92.18%respectively.Then the grafted fiber is reacted with hydroxylamine hydrochloride and diethylenetriamine to prepare amidoximated and aminated modified polypropylene-and cellulose-based chelating fibers.The modified polypropylene fiber has a rougher surface and increased specific surface area,while the surface of modified cellulose fiber is covered by reaction products and the specific surface area decreases.Amidoxime groups and amino groups are discovered on the surface of modified fibers and the hydrophilicity is obviously enhanced and the thermal stability is also improved.The modified polypropylene/cellulose fiber is used to simultaneously adsorb Cu²⁺,Pb²⁺,Zn²⁺in the solution.The effects of initial pH value,initial ion concentration and adsorption time on the adsorption performance of the modified fiber are discussed,and the adsorption stability and recycle performance of the modified fiber are also studied.Adsorption kinetics,adsorption isotherm,FT-IR and XPS methods are used to explore the adsorption mechanism.The results show that the modified fibers have high adsorption capacity and fast adsorption rate for Cu²⁺,Pb²⁺,Zn²⁺.The adsorption process is chemical adsorption.,as N and O in the functional groups on the fiber surface can chelate with metal ions and form stable coordination structure.The factors that affect the competitive adsorption of Cu²⁺,Pb²⁺,Zn²⁺are ion affinity,steric hindrance and ion covalent index.Compared with modified polypropylene fibers,modified cellulose fibers have better adsorption performance on heavy metal ions,but the adsorption capacity loss is greater after 20 adsorption cycles.The modified polypropylene/cellulose fibers are used with cement to solidify Cu²⁺,Pb²⁺,Zn²⁺.By testing and analyzing setting time,compressive strength,hydration heat and hydration products,the effects of heavy metal ions and modified fibers on cement properties are discussed.The solidification ability of fiber cement composites for heavy metal ions is also explored through three leaching methods.The results show that Cu²⁺,Pb²⁺and Zn²⁺can inhibit the hydration reaction of cement,reduce the compressive strength of cement and delay the setting time.The addition of modified fibers can effectively compensate for the negative effect of heavy metal ions on cement hydration,and thus enhance the solidification of heavy metal ions through dual stabilization effect.The modified fiber can improve the adsorption capacity of cement on methyl orange,achieving an adsorption efficiency of more than 99%.Based on the good adsorption and solidification performance of modified fibers and their cement-based composites for heavy metal ions,two solid wastes containing heavy metals,namely sewage sludge ash and red mud,are added into cement as mineral admixtures.The compressive strength,leaching toxicity and chemical speciation distribution of solidified body with modified fibers added are explored at different sewage sludge ash/red mud substitution ratio.It is found that increasing the substitution ratio of sewage sludge ash/red mud will reduce the compressive strength of the solidified body,increase the relative content of heavy metal ions in exchangeable and carbonate-bound fraction,and thus increase the leaching amount of heavy metal ions.By adding modified fibers,the compressive strength of solidified bodies increased,the leaching toxicity of heavy metals decreased significantly,and the relative content of heavy metal ions in residual,organo-bound and Fe-Mn oxides-bound fraction increased.Assessment models were established to evaluate the potential environmental risks of solidified body matrix and the filtrate,and change rules of the matrix and filtrate with different substitution ratio are obtained.The addition of modified fibers has positive effects on reducing the potential environmental risk of sewage sludge ash/red mud solidified matrix and filtrate.