In-situ Formation Of Active Iron Species Induced By CaCO₃ For Enhancing Arsenic Removal From Water

Arsenic pollution in water is considered to be one of the most serious environmental problems in the world,the damage of its toxicity to the natural environment and human health has received increasing attention.As a result,the efficient removal of arsenic in water is of extraordinary practical significance,environmental value and social benefits.In this work,a new concept applied to arsenic removal was proposed based on the in-situ generation of new iron phases with high reactivity using synergistically calcium carbonate(CaCO₃)and variety of cheap iron-containing materials as starting samples.A series of improved low-cost and high-efficiency water arsenic removal methods such as coagulation,adsorption and zero-valent iron treatment have been developed to deal with specific problems of arsenic removal in water currently.Moreover,with relevant theoretical interpretations and interacting mechanism of arsenic removal,the main conclusions of this work are as follows:(1)Efficient removal of As(?)by active iron species from ferrous-calcium carbonate systemA new low-cost ferrous-calcium carbonate(Fe(II)-CaCO₃)coagulation system was constructed to achieve the efficient removal of As(?)based on the phenomenon that CaCO₃ induced Fe(II)to in situ generate highly active Fe(?).When Fe/As molar ratio was 2:1 and the Ca/Fe molar ratio was of 1.5:1,the removal efficiency of As(?)was as high as 99.84%,indicating the occurrence of molar equivalent chemical precipitation between iron and arsenic,not just the traditional coagulating precipitation.CaCO₃ was used to replace traditional lime(calcium hydroxide,Ca(OH)₂)and the relatively slow hydrolysis of CaCO₃ would release OH^-gradually which was conducive to regulate the generation of active iron species Fe(OH)₃ and ensure the sufficient reaction time for adequately As(?)removal,thereby avoiding the conversion of active Fe(OH)₃ to low-reactive Fe OOH.In addition,by replacing Ca(OH)₂ with CaCO₃,the submicron content of the arsenic-removing sediment was significantly reduced,the filtration speed increased by nearly 6 times,and the volume of sediment was also reduced by 2/3.The improved treating system in this work showed superior properties of large sediment size,small sediment volume and high filtration rate,which favoured the subsequent processing of arsenic removal.(2)Direct removal of As(?)without pre-oxidization by ferrous-calcium carbonate systemThe ferrous-calcium carbonate system also showed excellent removal effect on As(?).When the Fe/As molar ratio was 4.5,near 100%removal efficiency was achieved by direct precipitation without pre-oxidizing As(?)to As(?).With presence of CaCO₃ particles,the diffused Fe(II)ions adsorbed on the surface of CaCO₃ would in situ generate hydrated Fe(?)oxide without agglomeration.The OH^-released by slow hydrolysis of CaCO₃ is conducive to improving the alkalinity of the arsenic sediment.As a result,a low soluble basic iron arsenite could replace the high soluble iron arsenite thereby improving the removal efficiency of As(?).(3)Adsorption efficiency and mechanism of As(?)by Ca-Al-Fe ternary compositionA new and low-cost Ca-Al-Fe ternary composite,which is porous,amorphous and has large specific surface area,was synthesized through a facile method including mechanical activation and low-temperature heating.The ternary components have showed significant synergistic effects in the adsorption process of As(?).Amorphous Al(?)and Fe(?)composition would undergo hydrolysis during the adsorption process to form iron-aluminum complex hydroxides,which have high adsorption capacity for As(?).In addition,OH^-from calcium carbonate hydrolysis in ternary composite induced Fe(II)in-situ oxidation to generate new active Fe-Al hydroxides,which would also help to enhance the adsorption of As(?).(4)Enhanced As(?)removal by mechanically activating ZVI/CaCO₃ composite and mechanism discussionA new arsenic removal method with the improved activity of micron ZVI based on mechanical activation and CaCO₃ induction strengthening in-situ corrosion of ZVI could generate more high-oxidatively active intermediates,accelerate the oxidation of As(?)and Fe(II)to As(?)and Fe(?)respectively,then consequently enhance the removal of As(?)by ZVI/CaCO₃.Furthermore,the original passivation layer on the surface of ZVI can be peeled off under the action of mechanical activation.CaCO₃ Co-milled with ZVI can effectively prevent the agglomeration of ZVI and re-formation of passivation layer over freshly exposed ZVI surface.During the removal process of As(?)by ZVI/CaCO₃,CaCO₃ initially covering the surface of ZVI kept being detached during agitation and continuously exposed the fresh ZVI surface to the solution,which is conducive to the efficient removal of arsenic.