Chromium removal from aqueous media by ferritisation and its stabilization

IV SUMMARY PhD Thesis CHROMIUM REMOVAL FROM AQUEOUS MEDIA BY FERRTTISATION AND ITS STABILIZATION Mehmet ERDEM Fırat University Graduate School of Natural and Applied Sciences Department of Chemical Engineering 2001, Page: 156 Chromium is found in aqueous system in the form of +3 and +6 oxidation states. Since the hexavalent chromium which is among particular pollutants is toxic an carcinogenic, the allowable concentration of it was restricted as 0.05 mg/1 in drinking waters. The wastewater from mining and metallurgy, metal plating, chrome tanning, ceramic, glass, paper mill industry and cooling tower at the power plant may contain chromium species. While Cr(III) can be removed from wastewaters, it is necessary that Cr(VI) is firstly reduced by using reducing agents such as sulphur dioxide, metal sulphides and ferrous sulphate to remove Cr(VI) from wastewater. The reduced chromium is then precipitated as chromium(III) hydroxide using an alkali agent. The seconder waste forming in the heavy metal removal processes may be solubilized when they contact with acidic waters and complexing agents. For that reason, before the seconder wastes are discharged to the environment, these wastes must be stabilized by an appropriate process. There are several methods to stabilize the wastes of heavy metals removal processes and one of them is to convert such waters into in the form of spinel ferritecompounds. As ferritisation can be thermally carried out at higher temperatures, it may be also performed in aqueous solutions. To do this, heavy metal ions are precipitated together with iron(II) ions in the form of hydroxides and then crystalline ferrite is formed. Precipitates of ferrite with closest packed and less volume are stable wastes and these wastes can be easily sperated by filtration and/or magnetic seperation. In this study, firstly, removing conditions of Cr(III) ions obtained from reduction of Cr(VI) by using iron(II) sulphate and sodium sulphite by means of ferritisation were investigated. Then, the solubilization properties of the ferrites obtained were investigated in the presence of mineral acid and some complexing agents such as citric acid, ascorbic acid, oxalic acid, tartaric acid, acetic acid, EDTA, humic acid sodium salt and ammonia. In order to compare the environmental stabilization characteristics of these secondary chromium removing wastes, these tests were also applied to the chromium hydroxide precipitate and iron-chromium mixed precipitate obtained by conventional precipitation methods. Also, the products obtained by heating of iron- chromium mixed precipitate at various temperatures were subjected to these tests in order to see the effects of heating on the its stabilization. On the other hand, extraction tests suggested by USEPA for the determination of pollution potentials of solid wastes were applied to waste models containing chromium obtained in this study te see their pollution effect. In the experiments, the most suitable Fe2+/Cr3+ ratio for ferritisation of 100 mg/1 Cr(III) solution, prepared by reducing Cr(VI) by sodium sulphite, was determined as 16 at the conditions of mixture pH, 9; temperature, 50 °C; stirring speed, 300 rpm and the reaction time, 60 min. In the experiment carried out by considering the iron ions in the solutions obtained by reducing Cr(VI) with iron(II) sulphate, the most suitable Fe2+/Cr3" ratio was found to be 18 at the conditions mentioned above. Mineralogical analyses showed the formation of an inverse ferrite having chromite structure together with iron(III) oxides. It was determined that the chromium is completely removed while the amount of iron remaining in the solution was found to be quite low. By using USEPA extraction tests, it was determined that the amount of chromium solubilized from both model waste containing ferrite and hydroxide-based waste of conventional process were below the 5 mg/1 which is the maximum allowable concentration. However, the amount of soluble chromium determined by one ofVI USEPA tests (TCLP test) for chromium hydroxide precipitated conventionally from the solution prepared by reduction with sodium sulphite was over the limit. On the other hand, as a result of solubilisation tests conducted at different pHs and concentrations in the presence of various complexing agents, it was found that the amounts of chromium and iron solubilised from all model solid wastes tested in this study reached significantly high concentrations. The amounts of metals solubilised increased by increasing acidity, concentration of complexing agents and contact time. The solubilities of model solid wastes obtained by ferritisation methods in the acidic media containing complexing agents were much lower than those of wastes obtained by conventional precipitation methods. As a general result of solubilisation characteristics of model solid wastes generated from chromium removal precesses, it can be stated that converting chromium and iron into spinel or individual oxides by either dry or wet methods is important in view of environmental stability. In addition, the results of this study suggested that the extraction procedures should be modified by making a procedure reflecting natural interactions, for instance, by adding appropriate complexing agent to the extraction media. Key words: Ferrite, chromium removal, stabilization, solubilisation, solid waste.