Immobilization Efficiency And Mechanism Of Hexavalent Chromium By Alkali-activated Slag Binder

Hexavalent chromium is an extremely toxic heavy metal with high solubility and mobility.It can cause serious harm to human health as the Cr?VI?migrates to the environment.The safe disposal of Cr?VI?contained wastes has great positive significance to the environment and society.Cement-based immobilization technologies have been widely used for the treatment of heavy metal contained wastes due to their low cost,versatility,physical and chemical barriers.The alkali-activated slag binder?AAS?presents some superior properties,including high mechanical strength,low porosity and high resistances to chemical attack.In addition,it seems that the slag has some reducing substance which can convert Cr?VI?to Cr?III?,making the AAS be more appropriate in the process of Cr?VI?immobilization.There have only been a few previous studies regarding the immobilization of Cr?VI?by AAS.Especially,the exact immobilization mechanism was not completely understood.Enhancing the investigation of Cr?VI?immobilization efficiency and mechanism of Cr?VI?by AAS can provide a new technical approach for the safe disposal of hazardous pollutants containing Cr?VI?.In this paper,using the highly soluble Na₂CrO₄ as the object,the main research contents were to:study the effect of Na₂CrO₄ on the slag hydration and the reaction kinetics of Na₂CrO₄-slag paste;discuss the effect of main preparation parameters such as alkali constituent type,waterglass modulus,Na₂O equivalent and water to binder ratio on the immobilization of Cr?VI?by AAS;analyse the effect of Na₂CrO₄ addition on the hydration products and microstructure of AAS;emphatically investigate the the main immobilization mechanism of Cr?VI?by AAS.compare the immobilization efficiency and mechanism of Cr?VI?by alkali-activated slag binder,Portland cement and calcium aluminate cement;explore the technique to improve the immobilization of Cr?VI?in AAS paste.The main results can be summarized as follows:?1?The hydration of slag could be accelerated by the addition of neutral Na₂CrO₄.Firstly,the Na₂CrO₄ could react with Ca²⁺and[AlO₄]^5-released from slag to form the CrO₄-U phase,consumed the concentrations of Ca²⁺and[AlO₄]^5-in pore solution and then accelerated the dissolution of slag;Secondly,a lot of OH^-ions was released in the process of CrO₄-U phase formation,which increased pH values of pore solution and then accelerated the decomposition of slag and the formation of hydration products.In the absence of alkali activator,under standard curing condition for 28 days,the treatable amount of Cr?VI?should be limited to 0.5%to meet the TCLP limit of 5mg/L though the reaction between slag and Na₂CrO₄.?2?The type and nature of alkali constitunent would significantly affect the immobilization of Cr?VI?in AAS.By comparison,the immobilization efficiency of Cr?VI?by NaOH activated slag binder was slightly higher than that by waterglass activated slag binder,but far higher than that by Na₂CO₃ activated slag binder and Na₂SO₄ activated slag binder.The immobilization efficiency of Cr?VI?by Na₂SO₄ activated slag binder was the lowest.NaOH and water glass could promote the hydration of slag,and make the interaction between slag and Na₂CrO₄ more intense,which could enhance the immobilization of Cr?VI?by AAS.Although the Na₂CO₃ and Na₂SO₄ could promote the hydration of slag,the immobilization of Cr?VI?was decreased with the addition of Na₂CO₃ and Na₂SO₄ due to the competition between of CrO₄^2-and CO₃^2-/SO₄^2-in hydrates,which weakened the chemical fixation of Cr?VI?and then decrease the immobilization efficiency of Cr?VI?.In the range of 3%⁶%,increasing the Na₂O equivalent could improve the activity of slag,thus dereasing the leaching concentration of Cr?VI?.In the range of 0.25⁰.4,a higher water to binder ratio leaded to a large amount of free water,which increased the interactional area between the Cr?VI?and the S^2-,resulting in a higher reduction efficiency and then causing a decrease in the leachability of Cr.?3?The immobilization mechanisms of Cr?VI?by AAS included physical encapsulation,chemical fixation and reduction.CrO₄^2-could participate in the hydration process of AAS,it could react with Na⁺,Ca²⁺and[AlO₄]^5-to form the CrO₄-U phase fixing a part of Cr?VI?.Meanwhile,the hydrotalcite,one of the hydration products,could capture a part of Cr?VI?though the isomorphous replacement.The sulfur in slag was mainly present in the form of S^2-providing a reduction environment,where the Cr?VI?was converted to Cr?III?,reducing its toxicity.Moreover,the low porosity,expecially the low capillary porosity,could physically encapsulate the Cr?VI?in the AAS pastes.The above three excellent properties were the basis of alkali-activated slag binder to effectively immobilize Cr?VI?.?4?Gypsum as the retarding component of Portland cement gave a negative effect on Cr?VI?immobilization.With the adequate amounts of gypsum,the ettringite was formed due to the hydration of C₃A.However,in the presence of Na₂CrO₄,the C₃A would react with Na₂CrO₄ to form the CrO₄-U phase with the release of OH^-,increasing the pH of pore solution.The ettringite could not be formed in the highly alkaline conditions.The CrO₄^2-and the SO₄^2-could react simultaneously with C₃A to form solid solution of U phase,which decreased the chemical binding of Cr?VI?.?5?The hydration process of calcium aluminate was seriously affected by Na₂CrO₄.Under the standard curing condition,the main hydrates of calcium aluminate included CAH₁₀,C₂AH₈,C₃AH₆ and AH₃.While in the presence of Na₂CrO₄,the formation of CAH₁₀ and C₂AH₈ was inhibited.The Na₂CrO₄ could react with CA and CA₂ to form CrO₄-U phase,CrO₄-C₃AH₆ and AH₃.The densities of CrO₄-U phase,CrO₄-C₃AH₆ and AH₃ are much larger than that of CAH₁₀ and C₂AH₈.Therefore,the decreased amounts of CAH₁₀ and C₂AH₈ leaded to the decrease in solid volume and the increase in pore volumn,thus the compressive strength was greatly reduced.The immobilization of Cr?VI?by calcium aluminate was achieved by physical encapsulation,chemical fixation in CrO₄-U phase and CrO₄-C₃AH₆.?6?The inclusion of calcined layered double hydroxides at 500 ^oC?CLDHs?significantly improved the immobilization efficiency of Cr?VI?in AAS pastes.The LDHs structure was rebuilt in the immobilization process,combining a lot of CrO₄^2-and water.The addition of CLDHs also accelerated the hydration of slag.Moreover,the nano CLDHS could exhibited the nano filler effect in the pore.These properties significantly decreased the leachability of Cr?VI?.?7?The addition of FeCl₂·4H₂O and Na₂S·9H₂O could convert Cr?VI?to Cr?III?,and then decrease the Cr?VI?leachability.However,the FeCl₂·4H₂O would react with NaOH to form NaCl and Fe?OH?₂,weakening the activation for slag and then seriously hindering the setting and hardening properties of AAS pastes.The addition of Na₂S·9H₂O also impaired the compressive strengths of AAS pastes,however,the impairment degree was lower than that brought by FeCl₂·4H₂O.By comparison,the Na₂S·9H₂O was more suitable in NaOH activated slag immobilization system.