Preparation And Assessment Of Potential Ecological Risks Of Arsenic Solidification With Slag Adhesives

Arsenic is a widely distributed element in the environment that can pose a threat to the ecological environment and human health.Efficient disposal and sustainable utilization of arsenic-containing waste residues is a great challenge.Cement-based materials are widely used for solidification/stabilization in China,but their secondary pollution and poor environmental stability pose great risks and hidden dangers to ecological security.Therefore,there is an urgent need to develop a green and environmentally friendly solidification/stabilization material to address the ecological security issues faced.This study focuses on high concentrations of arsenic-containing waste residues,with the development of green slag cement materials as a goal.It summarizes and analyzes existing papers on the treatment and disposal of arsenic containing waste residue,develops a green slag cementitious material（BCM）to repair high concentration arsenic containing waste residue.The reaction mechanism and phase transition in the solidification process of arsenic-containing waste residues in green slag cement materials are also thoroughly elaborated.The environmental stability and potential ecological risks of arsenic-containing solidified materials under different environmental erosion conditions were evaluated by simulating the dynamic behavior of arsenic-containing solidified bodies and the release process under semi-dynamical conditions.The main conclusions were as follows:（1）The unconfined compressive strength（UCS）and heavy metal solidification effect of slag and red mud were studied under different dosages,water cement ratio,and alkali activator dosages.Experimental results show that when slag and red mud are combined in a ratio of 80:20 with a water-to-cement ratio of 0.35,an alkali activator modulus of 1 and a dose of 10m L,they can effectively stimulate the depolymerization and re-polymerization of inert cement materials.After 28 days of curing at room temperature,the compressive strength of BCM can reach 67.8MPa.Arsenic solidification experiments were conducted under optimal conditions,reducing the arsenic concentration from 480.75 mg/L to 56.88 mg/L,effectively alleviating the environmental pressure caused by the large amount of solid waste stored.（2）Mechanical ball milling combined with alkali activation was used to solidify arsenic-containing waste residues,and the effect of different ball milling process parameters on the leaching concentration of heavy metals was studied.The results show that the optimal solidification conditions are maintained with a ball milling time of 240minutes,a ball milling frequency of 40 Hz,and a 5:1 ball-to-matter ratio.Arsenic solidification experiments were performed by adding prepared alkali activators.The arsenic concentration decreased from 480.75 mg/L to 1.38 mg/L,which was lower than the standard limit of solid waste toxicity leaching（5 mg/L）,and the harm to the ecological environment was significantly reduced.（3）Innovatively,it has been proposed to increase the solidification efficiency and environmental stability of arsenic by adding activation agents Ca O and Na Cl.The experimental results show that the addition of Ca O provides an alkaline environment for the synthesis of hydrated products and improves the production efficiency of high sulfur hydrated calcium sulphoaluminate（AFt）,single sulfur hydrated calcium sulphoaluminate（AFm）and iron（chloride）ettringite（Fe（Cl）-AFt）compounds.The formation of Fe（Cl）-AFt and hydrated calcium silicate（C-S-H）improves the early compressive strength of green slag cementitious materials,improves the solidification efficiency of heavy metals,and reduces the leaching concentration of arsenic from 480.75mg/L to 0.309 mg/L,with a solidification efficiency of 99.9%.This comprehensively improves the environmental stability of solidified bodies under natural conditions,effectively addresses ecological safety and vulnerability issues,and enables harmless disposal.（4）The phase transition and the mechanism of arsenic fixation by BCM have been investigated using analytical methods such as X-ray diffraction（XRD）,Fourier transform infrared absorption spectroscopy（FTIR）and scanning electron microscopy（SEM）.The results indicate that the Ca²⁺generated during the hydration process can form calcium arsenate precipitation with free arsenate ions(As O₄^3-)or hydrogen arsenate ions(HAs O₄^2-)in the pores,achieving rapid solidification and stabilization of arsenic.During the formation of the hydration product,As O₄^3-or HAs O₄^2-in the pore fluid will replace some of the oxygen-containing anion groups in the hydration product,resulting in a more efficient,safe,and stable arsenic fixation.At the same time,Ca²⁺and As O₄^3-/HAs O₄^2-can also generate different types of calcium arsenate products,which can rapidly convert the target arsenic form to a low mobility form.By altering the binding form of arsenic in the medium,the risk of toxic release of arsenic can be reduced to safe levels through precipitation,chelation,and adsorption.（5）Risk assessment of arsenic fixation of green slag cementitious materials under various environmental conditions was carried out by using risk assessment code（RAC）and potential ecological risk index（PERI）.The results showed that the risk assessment index showed that MF=3.55%and RAC value was 1.00,indicating low risk.The potential ecological risk assessment showed that the solidified body had potential slight ecological harm to the environment under different erosion environments under the selection value of ClassⅡconstruction land.The toxicity of the leached solution was found to be below the standard limit by means of dynamical and semi-dynamic simulation tests.In semi-dynamic extraction experiments,landfill arsenic had the highest leaching toxicity for the same leaching time and continued to increase with leaching time.The reason is that the solidified body is corroded and impregnated by the strong acid solution at the landfill site,and the reaction between the acid solution and the alkaline curing agent in the solidified body leads to unstable curing.The arsenic adsorbed on the surface of Ca（OH）₂ and C-S-H is dissolved and leached in large quantities under the strong acid environment,which speeds up the leaching rate of arsenic,but it is still within the safe range.Under simulated seawater and acid rain conditions,arsenic leaching toxicity was gradually reduced and environmental stability and potential ecological safety issues were ensured.