Conductive Fly Ash-based Geopolymer Solidification Of Heavy Metals And Their Photocatalytic Properties

Fly ash is fine ash collected from coal-fired flue gas.As one of the largest industrial wastes in China,the large amount of fly ash is not only occupying land resources,but also causing huge pollution to the environment.In order to promote the recycling of solid waste resources,it is imperative to use fly ash with high added value.In this paper,fly ash as raw material,Na₂SiO₃·9H₂O and KOH as basic activators,through the hydration reaction to produce alkali-activated fly-ash geopolymer?AFG?;Through the incorporation of heavy metal salts,the solidification efficiency and electrical conductivity of fly ash-based geopolymers on heavy metal ions were investigated and characterized by means of XRD,FESEM,NMR,XPS,etc.Meanwhile,response surface methodology?Response Surface Methodology,RSM?was used to evaluate the photocatalytic degradation of dye.The fly ash-based geopolymer was prepared and the effect of curing age on its mechanical properties was studied.The results showed that the compressive strengths of the test block at 3 d,7 d,and 28 d were 50.2 MPa,56.1 MPa,and 58.8 MPa,respectively.With the extension of age,the mechanical properties show a rapid increase in the early period and a slow change in the later period.The results of XRD indicated that no new crystal phase was produced when the fly ash-based geopolymer was prepared by alkali activator.SEM results showed that the fly ash particles produced a dense amorphous phase under the action of the alkali activator.Through the incorporation of heavy metal salts,the performance of fly ash-based geopolymers for solidification of heavy metal ions was evaluated.The results showed that the solidification efficiency of Cu²⁺and Ni²⁺in each sample was above 99.9%,indicating that the fly ash-based geopolymers were effective for Cu²⁺and Ni²⁺has a good curing effect.Elemental scan analysis and XPS results showed that heavy metals were uniformly dispersed in the cured body,and the valence state of Cu²⁺and Ni²⁺did not change in the cured body.In addition,with the incorporation of heavy metal ions,the conductive properties of fly ash-based geopolymers have increased dramatically,when the amount of Cu²⁺and Ni²⁺in AFG is 2.5wt%(abbreviated as 2.5Cu²⁺-AFG and2.5Ni²⁺-AFG)The conductivity of the material reached 6.3×10^-22 S/m and 2.7×10^-22 S/m,respectively,which was 73.3 and 31.4 times higher than that of the AFG.The 3 d compressive strengths of 2.5Cu²⁺-AFG and 2.5Ni²⁺-AFG are 17 Mpa and 7.5 Mpa,respectively,which can still meet the requirements of photocatalyst particle strength.Response surface design?RSM?was used to evaluate the effects of photocatalyst dosage,heavy metal ion content,and dye concentration on the photocatalytic degradation rate of the basic blue solution.Response surface model analysis results show that the main factors affecting the photocatalytic degradation activity of Cu²⁺-AFG are:dye concentration>Cu²⁺content>catalyst dosage;the main factors influencing the photocatalytic degradation activity of Ni²⁺-AFG are:Ni²⁺content>dye concentration>catalyst dosage,through regression model analysis,determined that Cu²⁺-AFG and Ni²⁺-AFG were the best experimental conditions for degrading basic blue color:the amount of Cu²⁺-AFG catalyst was 0.09 g,the dye concentration was18.63 mg/L,and the Cu²⁺content was 2.50wt%,the degradation rate of the basic blue dye was 93.27%,while the basic methylene blue dye degradation rate obtained from the verification experiment was 91.94%;the amount of Ni²⁺-AFG catalyst was 0.12 g,the dye concentration was 18.90 mg/L,and the Cu²⁺content was 2.50wt%,the degradation rate of the basic blue dye is 75.31%,and the basic methylene blue dye degradation rate obtained in the verification experiment is 74.86%.The verification experimental values in the two groups are basically consistent with the predicted values,and the regression model is reliable.The conductivity of the catalyst has a positive correlation with the degradation rate of the dye,which is attributed to the incorporation of heavy metal ions to improve the conductivity of the catalyst,promote the photoelectron electron mobility,and greatly enhance the photocatalytic activity of the catalyst.