| Rapid developments in the economy and society have extensively accelerated the pace of urbanization.Unfortunately,urbanization has also left heavy metal pollution in sites previously occupied by heavy industry enterprises and the soil around mining areas.Soil contaminated by Cd and Pb is widely distributed in all parts of the country and poses serious threats to human life and the ecological environment.In addition,the disordered storage of solid waste will also cause harm to the soil environment,the low utilization rate of resources has become an important bottleneck restricting the sustainable development of the environment.The core solution of these problems may lie in the concept of "treating waste with waste." The development of new environmental materials is an important means to utilize bulk industrial solid wastes.Geopolymers,as amorphous aluminosilicate materials activated by alkali,have favorable engineering characteristics,such as high strength,high temperature resistance,fire resistance,and acid and alkali resistance.Coal bottom ash(CBA)is rich in Al and Si,has strong chemical activity after high-temperature treatment,and,thus,is beneficial to the synthesis of geopolymer environmental materials.Relevant research shows that geopolymers have positive effects on the reduction of leaching toxicity of heavy metals and municipal solid waste incineration of fly ash.In the current paper,CBA was selected as the research object.The CBA-based geopolymer was synthesized by mechanical and alkaline activation.Polypropylene and coconut fibers were added to the geopolymer to solidify/stabilize the soil contaminated by Cd and Pb.The barrier effects of the fiber-bearing geopolymer on heavy metal sealing and structural strengthening were then evaluated on the basis of heavy metal migration and transformation,macromechanical properties,and solidified soil block micro structures.Finally,the mechanism of heavy metal sealing and structural strengthening were assessed and provides a new method for resource regeneration and sustainable development.The main conclusions are as follows:(1)The total amount of SiO2 and Al2O3 in CBA accounts for 82.91%of the material components;of these components,Al is present at a relatively higher proportion compared with Si.The Si/Al ratio is 2.61.The solid structure of the particle surface is evident,and a large number of sub-nanometer fragments are observed.The specific surface area of the CBAis 3.3316 m2/g,and the material shows strong chemical activity.Specifically,it is easily activated in an alkaline environment.The mineral phase in the soil mainly includes quartz,sanidine,and illite.The soil may be considered cohesive soil with high liquid and plastic limits.It also has strong water holding capacity,good chemical stability,and highly stable characteristics and structures that are difficult to modify.The polypropylene and coconut fibers are rod shaped.Polypropylene fibers have a smooth surface and strong deformation resistance,while coconut fibers have high roughness,which can improve the friction between the fibers and soil.(2)The duration of mechanical grinding has a great influence on the particle size and pore structure of the CBA.Activation at 550 rpm for 25 min could effectively improve the particle size distribution of the material and save energy consumption.The compressive strength and Si/Al,Na/Al,and W/B ratios of the CBA-based geopolymer meet the Box-Behnken design mathematical model.The optimum Si/Al,Na/Al,and W/B ratios are 2.666,0.687,and 2.422,respectively,and the maximum unconfined compressive strength is 30.74 MPa.Analysis of the mineral composition of the geopolymer reveals no evident diffraction peak.The diffraction curve has a hump-like protrusion at diffraction angles between 20° and 35°,and the particle surface shows a dense amorphous structure with strong integrity.When the geopolymer is formed for 3-7 days,its macromechanical strength could exceed 70%of the strength observed at 28 days.Increases in curing time can promote the development of the geopolymer structure and generate AlOx/Al,Na12[Al12Si12O48]·18H2O,and other phases,thus forming an amorphous system with a three-dimensional network structure.(3)The heavy metal adsorption capacity of the soil shows the order Pb(Ⅱ)>Cd(Ⅱ),and the adsorption curve obtained corresponds to the Langmuir fitting model.Infiltration of heavy metal solution seriously damages the particle distribution and skeleton structure of the soil.The hydraulic conductivity of soil could reach 1.47×07-5.03×10-7 cm/s,and the dispersion coefficient is 1.2×10-8-1.3 ×10-7 m2/s.After solidification of the contaminated soil by the fiber-bearing geopolymer barrier,the hydraulic conductivity of the contaminated soil meets China’s Standard for Pollution Control on the Landfill Site of Municipal Solid Waste.The concentration of heavy metals in the exudate of the lower layer also meets the country’s Integrated Wastewater Discharge Standard,and the leaching toxicity decreases by 33.58%-46.20%.VS2DTI simulations indicate that,under extreme conditions,heavy metals detected in pore water located 0.60 m below the solidified soil could meet the national standard after 10 years and that the dispersion coefficient may decrease by 26-132 times.Fiber addition exerts side effects on the curing effect of solidified soil blocks but has a limited effect on the curing effect of the geopolymer barrier.(4)The unconfined compressive strength of contaminated soil decreases by 11.9%-23.0%,and the maximum deviator stress is 28.0%-29.5%lower than that of remolded soil.The maximum compressive strength of the contaminated soil solidified by the fiber-bearing geopolymer barrier could reach 660 kPa.Under high confining pressures,the deviator stress first increases and then decreases sharply,thus showing evident brittle failure.After freeze-thaw cycling,the elastic modulus decreases slightly from 4.8 MPa to 3.3 MPa.Addition of the fibers plays an important role in bridging and reinforcing the soil particles and remarkably improves the anti-shearing properties of the soil.The maximum strength of the soil structure increases by 170.4%,and the effect of the soil mixed with coconut fibers is better than that of soil mixed with polypropylene fibers.The macromechanical parameters and proportion of illite show a negative correlation with the leaching concentration.By contrast,the specific surface area is positively correlated with the leaching concentration.An expression of the structural parameters is established on the basis of soil microstructures,and this expression could be written as θ=0.290x1+0.247x2+0.279x3+0.205x4-0.203x5.Moreover,the empirical relationship expression between the structural and macromechanical parameters is determined to be θ=0.067P-1.141c+1.890φ+29.610.The fitting effect between empirical relationship expression and test value is better of structural parameters is good.(5)After contamination of the soil by heavy metals,compounds suspected to be composed of Cd and Pb can be detected on the particle and pore surface.After solidification of the contaminated soil by the geopolymer barrier,the peak values of Si and Al at characteristic points increase by 6-10 times and the soil density increases considerably,thus forming a dense and integral amorphous cluster structure.Addition of polypropylene fibers promotes the attachment of a large amount of soil fragments to the fiber surface,and coconut fiber fragments are evenly dispersed in the soil structure to alleviate the concentration of external forces.After wrapping of the soil particles with the geopolymer barrier,a multilayer geopolymer space-skeleton barrier structure with skeleton-supporting effects and a geopolymer-fiber barrier network-structured system that could effectively improve the structural bearing capacity of the soil and block the migration and diffusion of heavy metals are formed. |
PDF Full Text Request