A Study On The Effects Of EDDS On The Removal Of Chromium And Cadmium By Nanoscale Zero-valent Iron In Contaminated Water

The treatment of wastewater polluted by heavy metals,which exhibits great hazards to all kinds of animals,plants and human health,has got popular attention at home and abroad in recent decades.It is often reported about major environmental pollution accidents of chromate(Cr(?))and cadimium(Cd(?)),mainly deriving from electroplating,mining,manufacturing and other industries.At present,there are many treatment processes to remove heavy metals from wastewater and nanoscale zero-valent iron(nZ?)reduction process has been used widely because of its high specific surface area and strong reducing reactivity among them.However,passivation layer of iron oxyhydroxides gradually forms on the external surface of nZ? during treatment,hindering further reactivity of nZVI.Thus,the solution of preventing the formation of passive shells will determine its further engineering application.The study found that addition of environmental-friendly chelants could deal with this problem efficiently by chelating with Fe(?)and inhibiting Fe(?)precipitation on the nZVI surface and it avoids producing secondary pollution on the environment.In this paper,the nZVI particles suspension were synthesized using the borohydride reduction method and remove Cr(?)and Cd(?)with the influence of ethylene diamine disuccinic acid(EDDS),a biodegradable chelating agent.The results showed that the average size of nZVI particles was 113nm and the main composition was Fe0 with very little iron oxyhydroxides.Meanwhile,it displayed a little agglomerate in chain and the zero point charge of nZVI particles was 7.1 in the pH range from 4 to 10.The elimination of single Cr(?)and Cd(?)by nZVI particles was dependent on solution pH.At pH 5.6-7.0,addition of EDDS enhanced Cr(VI)reduction efficiency significantly because EDDS chelated with formed Cr(?)/Fe(?),inhibiting their co-precipitation on the surface of nZ?.Under alkaline conditions,EDDS facilited the reaction between nZVI/Fe(?)and dissolved oxygen(DO)in the solution,resolting in the decrease of Cr(?)reduction.Without DO,EDDS showed the same positive effect on Cr(?)reduction.Besides,EDDS significantly increased the kinetics of nZVI corrosion in deionized water.For Cd(?),the higher removal efficiency by nZVI exhibited at alkaline solution.Low EDDS enhanced the Cd(?)adsorption by nZVI at pH 5.6?7.0 and high EDDS was the opposite.However,the Cd(?)removal was severely inhibited by EDDS at pH 9.0 under various conditions.DO played an important role in the formation of iron oxyhydroxides and thus influenced the removal of Cd(?).In the system of combined pollution of Cr(?)and Cd(?),Cd(?)exerted no influence on Cr(?)removal but the co-existence of Cr(?)obviously increased Cd(?)adsorption when no EDDS was added.Cd(?)removal efficiency almost achieved 100%and Cr(?)removal improved to some extent without EDDS and DO.The addition of EDDS restrained greatly Cd(?)removal in all experimental conditions and promoted Cr(?)reduction with high concentrations under acidic,netural and alkaline system without DO.