Removal Efficiency And Mechanism Of Hexavalent Chromium In Water Using Sodium Borohydride Enhanced By Fly Ash Extract

The removal of Cr???by traditional chemical reduction and precipitation method has the advantages of simple equipment,easy operation,good removal effect,recoverable chromium slag and so on,but there are many problems such as large amount of acid,too much slag production and settlement difficulty,excessive dosage of reducing agent and final pH confinement straitly.In this paper,the related work was carried out around the above problems.In addition,combined with the comprehensive utilization of solid waste fly ash?FA?,based on the evaluation of influence factors and characteristics of elemental leaching from FA,various types of fly ash leaching solution?FAEm?was prepared from FA to enhance Cr???removal efficiency and mechanism by NaBH₄.Through the comparative study of Cr???removal efficiency among NaBH₄ and other traditional reducing agents,the advantages of FAE-NaBH₄ for Cr???removal were expounded,which had practical significance to provide theoretical and technical support for the reduction of Cr???pollutant and resource utilization of FA waste.The direct application of FA in water treatment has the problems of multi-element leaching and large amount of sludge production.In order to solve the two issues,based on the influence of different conditions on the leaching of various elements from FA,the Fe,Al and Si-rich FAE was prepared by strong acid method?1.2 mol/L HCl?assisted by ultrasonic.In addition,the leaching characteristics of multi-element was explored by scanning electron microscope?SEM?and X-ray diffraction?XRD?combined with the characteristic of the HA adsorption onto FA.The results exhibited that under neutral leaching conditions,with the time increasing,Ca rapidly dissolved in water,then the leaching amount decreased,but rose again.It was considered that the elements represented by Al and Si increased at intial time and then decreased.In addition,the leaching amount of elements represented by S and Na rose first and then flattened out.In pH 2.0 aqueous solution,the elements of FA showed the maximum leaching amount,except for the elements of Cr,Mo,As and Al.However,in aqueous solution of pH 12.0 showed only the maximum leaching of Al element,and Fe element exhibited a small amount of leaching at initial pH in the range of 2.0 to 12.0.Under the condition of strong acid,it was beneficial to the simultaneous leaching of Al,Fe and Si.Free calcium oxide?f-CaO?as a main component of FA,it could inhibit the leaching of elements represented by Fe,Si and Al.The high content of f-CaO could react with the components such as of Al,Si and SO₄^2-in FA to form crystalline hydration products,including portlandite,ettringite?AFt?,gismondine and calcite,which effectively inhibited the leaching of Fe,Si,Al and other elements.High concentration of HA can cover the surface of FA to form sedimentary layer,resulting in the inhibition of some elements leaching and the formation of hydration products.In view of the feasibility and practicability of the technology with respect to enhanced NaBH₄ for the removal of Cr???,a comparative study between FAE-NaBH₄ and the traditional reduction-precipitation method was investigated.The effects of pH,solid-liquid ratio and FAE on Cr???reduction and TCr removal were investigated.The results exhibited that the hydrolysis or autoionization of the reducing agent in the aqueous solution affected the reduction of Cr???and the subsequent removal of TCr.Na HSO₃ and Na₂S₂O₅ could produce H⁺ through their autoionization.NaBH₄ and Na₂SO₃ could be self-hydrolyzed while reducing Cr???,resulting in H⁺ deficiency environment.FeSO₄ may produce H+ through its own hydrolysis in promoting Cr???reduction and increasing the amount of Cr???formation.In addition,the oxidation product Fe???also had an adsorption effect on Cr???.The combination of low dosage NaBH₄ and FAE has great potential for the removal of Cr???pollutants.Under the condition of pH 3.0,the concentration of Cr???was 100 mg/L,which was treated by FAE combined with low dosage?100 mg/L?NaBH₄,the reduction-precipitation process could be completed within 20 minutes with and afford a total Cr removal of 96.32%.It was significantly higher than that of other reducing agents.In order to solve the problems of the traditional chemical reduction and precipitation for Cr???treatment,NaBH₄ was used as a reducing agent,the influence factors of Cr???removal using NaBH₄ enhanced by FAE were carried out.The enhanced removal mechanism was explored by means of the series of design experiments,and the combination of the infrared spectrum,SEM,XRD and atomic force microscope?AFM?.It proved that the reduction and removal capability of NaBH₄ for Cr???could be improved by adding FAE.Compared with NaBH₄ alone,the TCr removal for 100 mg/L Cr???solution with pH 3.0 was increased from 65.93% to 99.81% by using NaBH₄ combined with FAE_>2.0.The mechanism of Cr???reduction by NaBH₄ enhanced by FAE was similar to that of acid catalysis,and there was no direct catalysis of Fe or Al for NaBH₄ hydrolysis.The mechanism depended on the interaction between NaBH₄ and Fe???or Al???,which were the main components of FAE.The production of OH-from the hydrolysis of NaBH₄,which promoted the hydrolysis of Fe???or Al???,then H⁺ produced.That is to say,the formation of buffer system slowed down the increase of pH caused by excessive BH₄-hydrolysis,and the reduction ability of NaBH₄ catalyzed by acid was improved.Fe-Al-Si in FAE can accelerate the settling velocity and precipitation rate of Cr???particles by adsorption and coprecipitation,and at the same time,it can also have a weak adsorption effect on Cr???,thus promoting the removal of TCr.At low dose,the Fe???-enhanced NaBH₄ for Cr???removal was lower than Al???.One reason was that Fe???could form nanometer zero-valent iron?nZVI?,which had a negative effect on the hydrolysis of Fe???in the FAE under the pH less than 3.0,and the other was that the Fe???itself had a weaker hydrolysis capacity than Al???with a pH greater than 3.0.In the case of H⁺ deficiency?pH?3.0?,the generation of H₂ and the reduction of Cr???by NaBH₄ was mutual inhibition and synergy,showing that,at an initial pH of 3.0,especially at high temperature,H2 generation had a certain degree of inhibition of Cr???recduction.Nevertheless,as the initial pH was greater than 4.0,Cr???reduction and H₂ generation exhibited the synergistic effect.In the presence of Fe-Al-Si complex,the competition between Cr???reduction and H₂ generation could be alleviated.The resultant H₂ yield could be increased from 32.04% to 80.70%,and TCr removal was enhanced from 46.72% to 98.96% at 30 ?,which exhibited synergistic effect.