Study On The Stabilization Mechanisms Of Iron-calcium Reinforced Solidification Of Arsenic Alkali Residue With Geopolymer Composite

Arsenic-alkali residue(AAR)is a hazardous waste discharged from antimony smelting and processing,which contains a large amount of heavy metals,mainly As and others such as Pb,Zn,Cd,Cr,and it will pose great threats to humans and the surrounding environment if not disposed properly.As a new cementitious material,geopolymer has become a promising heavy metal solidification/stabilization material for its characteristics of environmental friendliness and acid/alkali resistance.In this paper,fly ash(FA)and granulated blast furnace slag(GBFS)were used to prepare high strength geopolymer composite,and water-quenched slag(WQS)was utilized for the reinforcement of the solidification of arsenic-alkali residue.The effects of sodium silicate modulus,sodium silicate content,water cement ratio and the ratio of raw material for the compressive strength of geopolymer were studied;The synergistic effect on the solidification for arsenic with geopolymer and water quenching slag was investigated;The combined effect of Fe and Ca for As solidification and stabilization in the geopolymer composite was studied and its solidification/stabilization mechanism was clarified.The specific research contents are summarized as follows:(1)Geopolymer with high compressive strength was synthesized by FA and GBFS with the activation of water glass.The compressive strength of the geopolymer reached36.9 MPa after curing for 28 days when water glass modulus was 1.9,water glass content was 8 wt%(calculated by Na₂O),water binder ratio was 0.40,and FA/GBFS ratio was 3:2.(2)Using the highly toxic AAR discharged from antimony smelting industry as the treatment object,the effects of the addition of AAR on the compressive strength and solidification of heavy metals by geopolymer were investigated.Water quenching slag(WQS)with high iron content produced by lead-zinc smelting process was used to reinforce the solidification of arsenic alkali residue by geopolymer,and its strengthening effect on geopolymer solidification of AAR was studied.The results showed that the compressive strength of the geopolymer decreased with the increase of AAR content,and the leaching rate of As increases with the increase of AAR addition.After curing for 28 days,the compressive strength decreased from 29.87 MPa to 16.78MPa and the arsenic leaching ratio increased from 4.23%to 17.77%with the content of AAR increased from 5%to 25%(Acetic acid solution method according to HJ/T300-2007).WQS could reinforce the solidification of AAR in the geopolymer.When the content of AAR was 5%,the arsenic leaching concentration decreased from 6.13mg/L to 1.01 mg/L and 0.18 mg/L,respectively with the addition of 5%and 10%WQS under the acetic acid leaching method(HJ/T 300-2007)after curing for 28 days.(3)The effect of pH of the leaching solution on arsenic leaching was studied.The leaching concentration of arsenic of AAR decreased and then increased with the increase of initial p H of the leaching solution.However,the leaching concentration of arsenic exceeded the China standard for hazardous waste landfill regardless of the p H range.The geopolymer of FA and GBFS had a poor curing effect for AAR.When the content of AAR was 5%,the leaching concentration of As was 6.13 mg/L after curing for 28 days(Acetic acid solution method according to HJ/T 300-2007).When 10%WQS was added to geopolymer to reinforce the solidification of 5%AAR,As leaching of the geopolymer was 0.18 mg/L(Acetic acid solution method according to HJ/T 300-2007)after curing for 28 days,which could meet the standard of GB 16889-2019 for domestic waste landfill pollution control,but could not meet the arsenic leaching limit of GB 18588-2008 for hazardous waste landfill pollution control.The addition of Ca O and Fe₂O₃ significantly reduced the arsenic leaching concentration in the geopolymer.With the content of arsenic alkali residue of 5%,the addition of 20%Ca O or 6%Fe₂O₃could meet the arsenic leaching limit of domestic waste landfill(0.3 mg/L)and hazardous waste landfill(1.2 mg/L).(4)The mechanism of arsenic solidification and stabilization was mainly through the following six pathways:1)The adsorption of As by iron oxide in the geopolymer at low p H;2)Formation of stable Fe-As minerals in the geopolymer at low p H;3)The formation of Ca-As minerals at high p H;4)As adsorption by ferric hydroxide in the geopolymer at high p H;5)The oxidation of highly toxic As(III)to less toxic As(V)by Fe;6)As solidification by physical encapsulation within the three-dimensional network of geopolymer.