Study On Characteristics Of Chromium-Contaminated Construction Waste From Chromic Salts Enterprises And Its Disposal Process

In recent years, there have been various environmental pollutions caused by chromium,including severely polluted residues, soil, construction waste, groundwater and surface water,which extensively threatens the ecological environment and human health. It shall attain moreattention on chromium pollutions due to its high contamination concentration and diffusivity,which may expand the scope of pollution, and thus it is one of the urgent environmentalproblems waiting for solutions. At present, there has been no report on disposal ofchromium-contaminated construction waste, and due to the large number, it was necessary tostudy the disposal process.Generally, chemical cleaning is the most effective way removing metal water-solublespecies, including hexavalent chrome. It can thoroughly remove contaminants fromcontaminated construction waste with low cost and best effects, and besides, it’s easy tocontrol the disposal process. Thus, it is suitable for the disposal of chromium-contaminatedconstruction waste. In this research, to choose appropriate washing agents, the characteristics,contents of total Cr and Cr(VI) of the construction waste were studied. Also, the processparameters have been studied, and finally, it determined the disposal process and verified theprocess feasibility.The main results of research were shown as follows:(1) Chromium-contaminated construction waste is alkaline. The content of total Cr andCr(VI) was6714.67mg/kg and6387.96mg/kg, respectively. The main fractionation ofchromium in sample was water-soluble, and the secondary is exchangeable and acid-soluble.The washing agent selected shall be able to remove both fractionations. As indicated by theresults, when the grain diameter was less than0.87mm, the content and desorption of total Crand Cr(VI) in sample were maximum, and thus the samples shall be crushed to0.87mm.(2) After washing sample once, the removal rate of total Cr and Cr(VI) was75%, buttheir contents were still high,1662.25mg/kg and1431.40mg/kg, which are harmful to theenvironment and need further disposal with agents. After cleaning with different washing agents, citric acid has the best removal effect on total Cr and Cr(VI), over99%, and theremained Cr(VI) in sample was less than10mg/kg.(3) Water-soluble Cr and reducible Cr in construction waste were mainly Cr(VI). Theremoval effects of high concentrated citric acid, hydrochloric acid and acetic acid were better.Cr(VI) can be dissolved and reduced into Cr(III) of other form, significantly reducingwater-soluble Cr and reducible Cr. Especially, the effect of citric acid was best. EDTA, citricacid, oxalic acid, hydrochloric acid and acetic can reduce the pH of construction waste, andunder acidic conditions, it can better remove exchangeable and acid-soluble Cr. The mainoxidizable Cr in construction waste was Cr(III). EDTA and citric acid can significantlyremove oxidizable Cr(III) by complexation. In conclusion, Citric acid is a good candidate ofsecondary washing for it can better remove. water-soluble, extractable and acid-soluble,reducible, and oxidizable fractionation Cr. Moreover, citric acid is organic and it is harmlessto the environment.(4) The washing process was divided into two steps, firstly washing with water andsecondly with0.3mol/L citric acid, the washing time was10min, and the solid-to-liquid ratiowas1:10. As verified by the experiments, with this process, the concentrations of total Cr andCr(VI) in leaching solution were below target limits after washing. This technology wasfeasible, and this study put forward the washing process of “water+citric acid”.