Solidification/Stabilization Of Chromium-Bearing Electroplating Sludge With Alkali-Activated Cementitious Materials

Electroplating sludge(ES)is one of hazardous solid wastes containing a great amount of toxic heavy metals such as Cr,Cu,Ni,Pb,and Zn,especially Cr(VI),which has high solubility,strong oxidation and biological toxicity.ES that is not adequately treated and stacked randomly,will cause serious risk to surrounding soil,water sources and have detrimental effect on human health and environment.The safe disposal and utilization of chromium-bearing ES is one of the most urgent problems in China,which is of great significance to environment protection and resources conservation.In this dissertation,alkali-activated cementitious materials,alkali-activated slag ce-ment and fly ash geopolymer cement were applied to solidify/stabilize the chromium-breaing ES.Mechanical properties,chromium leachability and drying shrinkage of so-lidified samples were studied.Microstructural changes of solidified samples were as-sessed by means of XRD,FTIR,SEM and EDS analysis.Factors affecting ES solidifi-cation/stabilization(S/S)and related mechanisms were also discussed.The main re-search results are shown in the following:1.Strength and drying shrinkage(1)Compressive strength of alkali-activated slag will increase first and then decrease with the increasing liquid-solid ratio,water glass modulus and dosage.The strength of binder decreased with the increase of ES amount and increased with prolonged curing time.When liquid-solid ratio was 0.35,water glass dosage was 10 wt%,modulus was 1.3,ES amount was as high as 18 wt%,compressive strengths of 3d,28d and 90d sam-ples reached 33.9 MPa,79.1 MPa and 82.4 MPa,respectively.(2)Compressive strength of fly ash geopolymer decreases with the increase of liquid-solid ratio,water glass modulus and the amount of ES,while improves with the increase of water glass amount.When curing time is prolonged,the strength will gradually in-crease with a stable trend at 90d.When liquid-solid ratio was 0.30,water glass dosage was 10%,modulus was 1.5 and ES amount was 18 wt%,compressive strengths of 3d,28d and 90d samples were 6.9 MPa,11.4 MPa and 12.8 MPa,respectively.(3)When adding ES,the strength of alkali-activated slag cement is higher than that of fly ash geopolymer.The strength of fly ash geopolymer was promoted by slag addi-tion,25 wt%of the slag improves its 3d strength by 20%.(4)Drying shrinkage of alkali-activated slag and fly ash geopolymer were obvious and the former had higher shrinkage.With the increase of wet curing time,dry shrink-age was obviously reduced.2.Chromium leaching concentration(1)Chromium leaching concentration of alkali-activated slag decreased slightly with the increase of liquid-solid ratio,water glass dosage,modulus and curing time,and slightly increased with the increase of ES amount.In general,chromium leaching con-centration of the sample is lower,and the Cr(?)leaching concentrations of 3d,28d and 90d of solidified samples are all below 0.1 Omg/L when ES amount is less than 18 wt%.(2)Fly ash geopolymer leaching concentration of Cr(?)and total-Cr increased with the increase of liquid-solid ratio and ES amount.Water glass modulus had no significant effect on Cr(?)and total-Cr leaching concentration and it decreased when water glass dosage and curing time was improved.Cr(?)leaching concentration of 18 wt%ES samples at 3d,28d and 90d were 0.90mg/L,0.67mg/L and 0.10mg/L,respectively.It is significantly higher than that of alkali-activated slag.3.S/S mechanism of chromiumS/S mechanism can be attributed to the following three methods:(a)encapsulation,adsorption and blocking of diffusion channels of Cr(?)by hydration products;(b)solid solution of Cr(?)in hydration products;(c)reduction to Cr(?),decrease its mobility in alkaline environments.For fly ash geopolymer,it is mainly the former two methods;for alkali-activated slag,it is a combined effect of the three methods.In summary,S/S of chromium-bearing ES by alkali-activated cementitious materials,especially alkali-activated slag,can obtain better results.With higher compressive strength and chromium leaching concentration below the V-level of Environmental Quality Standards for Surface Water,it is worth further study and expected to be used as an alternative resource in building materials.