Solidification/Stabilization Of Clay Soil Contaminated By Nickel And Zinc

Green sustainable remediation and securely recycling of industrially contaminated site soils are not only currently an emergent concern in geoenvironmental engineering practice,but also an urgent need for winning the victory in the battle against soil pollution encountered in China.However,the conventional Portland cement-based binder used in soil solidification/stabilization is characterized as an intensive consumer of unrenewable natural resources and energy,and a high carbon footprint,which conflicts with the green sustainable remediation strategy.Therefore,it is necessary to develop novel sustainable binders to improve the ecological environment quality of the contaminated soils so as they can be reused as green construction materials.With financial supports provided by the National Key Research and Development Program of China(No.2019YFC1806000),National Natural Science Foundation of China(Nos.41877248 and41472258),National High Technology Research and Development Program of China(No.2013AA06A206),and Environmental Protection Scientific Research Project of Jiangsu Province,China(No.2016031),the dissertation presented a comprehensive investigation on the sustainable binder development,performance evaluation and field application by combining laboratory tests,field tests and theoretical analysis.The main finding and conclusions are incorporated as follows:(1)A sustainable steel slag-based binder was developed,and the geoenvironmental properties of nickel(Ni)and zinc(Zn)contaminated soil solidified/stabilized with this binder were investigated.The geoenvironmental properties include unconfined compressive strength,leachability,p H,EC and basic physicochemical properties.The results indicated that,with the addition of the sustainable steel slag-based binder,the unconfined compressive strength and p H value remarkably increased,but the leached concentrations of target heavy metals of Ni and Zn and EC value of the contaminated soil significantly decreased.Furthermore,the basic physicochemical properties of the contaminated soil significantly changed as follows,liquid limit,specific surface area,organic matter content and clay content of the contaminated soil decreased,but the cation exchange capacity,specific gravity,maximum dry density and sand content increased.(2)The mechanisms leading to the changes of geoenvironmental properties of the steel slag-based binder solidified/stabilized soil were revealed.Variations in microscopic properties of the contaminated soil and phase identification of reaction products formed in the solidified/stabilized soil were analyzed by soil pore-size distribution,acid neutralization capacity,chemical speciations of target heavy metals,XRD and SEM-EDS tests.The results indicated that the cementation property of calcium silicate hydrate(C-S-H)and the soil pore filling effects of ettringite(AFt),portlandite(CH),and heavy metal-bearing precipitates lead to the improvement in soil strength.The formation of Ni(OH)₂,Ni-Fe layered double hydroxides,Ca Zn₂(OH)₆·2H₂O and Zn₅(OH)₈Cl₂·H₂O contributed to the enhanced binding intensity of heavy metals.Furthermore,the absorptivity of C-S-H,heavy metals encapsulation by C-S-H,and the ion-exchange of heavy metals with AFt facilitated the immobilization of heavy metals in the solidified/stabilized soil.The formation of alkaline reaction products improved the acid neutralization capacity.The increased chemical stability of the target heavy metals and the acid neutralization capacity resulted in the decreased leached concentrations of target heavy metals.(3)Influences of the initial water content of contaminated soil and compaction degree of solidified/stabilized soil on the solidification/stabilization effectiveness were investigated.The leached concentrations and observed diffusion coefficients of target heavy metals were studied by leaching toxicity and semi-dynamic leaching tests.The results indicated that both the initial water content(17%to 26%)of the contaminated soil and the degree of compaction of the solidified/stabilized soil had remarkable effects on the solidification/stabilization effectiveness of the steel slag-based binder.The leached concentrations of target heavy metals significantly decreased by50%until the initial water content of the contaminated soil increased to the range of 17%to 20%,which was close to that of the optimum moisture content obtained from standard Proctor compaction tests.The leached concentrations and observed diffusion coefficients of target heavy metals decreased with the degree of compaction increasing from 75%to 100%.The water content-dependent leachability of the solidified/stabilized soil resulted from the variations in chemical speciation of target heavy metals and pore-size distribution of the solidified/stabilized soil.The chemical speciation of target heavy metals and particle-size distribution of the solidified/stabilized soil contributed to the compaction degree-dependent leachability of the steel slag-based binder solidified/stabilized soil.(4)The geoenvironmental properties of the steel slag-based binder solidified/stabilized soil subjected to drying-wetting cycles were investigated.The variations in relative cumulative mass loss,unconfined compressive strength,leachability of solidified/stabilized soil at different drying-wetting cycles were studied by modified ASTM D4843,and the corresponding mechanisms were revealed.The results indicated that the relative cumulative mass loss and unconfined compressive strength of the solidified/stabilized soil exhibited a firstly decreased and then increased trend.Meanwhile,the leached concentrations of target heavy metals exhibited an opposite trend.The threshold cycles relevant to the turning point for relative cumulative mass loss,unconfined compressive strength were both 18 cycles.However,for leached concentrations of target heavy metals,the threshold cycles were 6 cycles.The variations in unconfined compressive strength,leachability of solidified/stabilized soil corresponding to drying-wetting cycles were attributed to the changes of the pore-size distribution and chemical speciation of target heavy metals.(5)The diffusion and advection migration characteristics of target heavy metals were investigated by one-dimensional diffusion equipment and flexible wall permeameter,and the effective diffusion coefficient(D^*)and partition coefficient(k_p)of heavy metals,and hydraulic conductivity coefficient(k)of the contaminated and solidified/stabilized soils were discussed.The results indicated that the concentrations of heavy metals in the distilled water reservoir of the diffusion test increased with extending testing time,but the concentrations of heavy metals in the effluent leachate of infiltration test decreased with extending testing time.The steel slag-based binder had a remarkable influence on the D^*and k_p of target heavy metals,and hydraulic conductivity(k)of the contaminated soil.With 8%binder addition,the D^*value of Ni and Zn decreased to 3.75%and 3.60%;meanwhile,the k_p value of Ni and Zn increased by 169 and 175times,respectively.On the other hand,the k value of the 8%binder solidified/stabilized soil lower than that of the untreated contaminated soil by approximately 2 order of magnitudes.(6)The engineering,environmental and economic performances of the steel slag-based binder solidified/stabilized soil reused as filling materials of roadway subgrade were systematically evaluated.A complete set of construction procedures for ex-situ solidification/stabilization were developed,and the feasibility of recycling steel slag-based binder solidified/stabilized soil as filling materials of roadway subgrade was verified by series of field tests.Portland cement and quicklime was selected as control binders in the field trial for comparison purposes.The results indicated that the steel slag-based binder solidified/stabilized soil was a promising filling material for roadway subgrade.The resilient modulus of the steel slag-based binder solidified/stabilized soil well met the construction standards specified by the China Ministry of Housing and Urban-Rural Development in Code CJJ 37.The leachability of the solidified/stabilized soil met the China criterion for groundwater quality of Class IV prescribed in GB/T 14848—2017.The steel slag-based binder had many advantages over the traditional binders such as Portland cement and quicklime on engineering,environmental and economic performance.(7)The long-term effectiveness of solidified/stabilized soil and time-dependent migration characteristics of heavy metals from the subgrade materials to off-site soil environment was evaluated by filed post-implementation monitoring and numerical simulation.The results indicated that,with the monitoring extending to 600 d,the resilient modulus of solidified/stabilized soil shown a continuous increase,but the leachability of solidified/stabilized soil exhibited a continuous decrease.The migration distance of target heavy metals from the on-site solidified/stabilized soil to off-site soil was less than 5 cm at 600 d monitoring.After 50-year diffusion,the migration distances of Zn from the untreated contaminated soil and solidified/stabilized soil to off-site adjacent soil were18.9 cm and 3.2 cm,respectively.