CHROMATE SORPTION ON METAL IONS DOPED GOETHITE (α-FeOOH)

Abstract

Goethite (α-FeOOH), being a model adsorbent, has a very highly reactive surface. Therefore, it is considered to be an efficient adsorbent in soil systems and plays an important role in controlling the mobility of trace contaminants, like Pb, Ni, Cu, Cr, As, Cd, Co, Zn, and Cu etc. Being the most abundant iron oxide mineral in almost all the soil types, many researchers have focused, recently, on the sorption properties of metal doped goethite, not only due to its greater resemblance with natural goethite, but also for its possible use as an adsorbent in water purification technology. In line with this trend of research, the present study reports the characterization of Pb, Ni and Cu-doped goethite and their adsorption properties for chromate with stepwise comparison to pure goethite. Further, characterization and chromate adsorption properties of microcrystalline and amorphous phases of copper/iron mixed oxides, are also a part of this study. All the solid samples, used in the present investigation, are prepared according to a well known procedure reported in the literature. The amounts (%) of Pb 2+ , Ni 2+ , Cu 2+ ions doped in goethite samples were 0.38, 0.25 and 0.32, while the microcrystalline goethite and amorphous phases of copper/iron mixed oxides contained 6.27 and 11.31% Cu 2+ ions respectively. All these solids are characterized for Surface Area, pH of Point of Zero Charge (pH pzc ), XRD, TG-DTA, TEM/SEM and FTIR analyses. XRD and TEM/SEM analyses of all the goethite samples suggest the solids to be crystalline and doping of the metal ions have only slightly affected the unit cell parameters of the crystals. TG-DTA analyses reveal that all the goethite samples exhibit similar thermal behavior with a distinct peak for the degradation of doped metal hydroxides, present in the solid matrix.. Furthermore, an increase in the concentration of Cu in the precipitate results in the formation microcryastalline goethite and amorphous phase of copper/iron mixed oxide with a marginal increase in the surface area and pH pzc of the resultant solid phases. xDissolution study of goethite in KNO 3 and 303K suggest that goethite and its metal doped counterparts are quite stable in the pH range 4-7. In all the solid samples dissolution is maximum at the lowest pH of 3 and decreases with the increasing pH. Dissolution of pure goethite is inhibited by increasing the concentration of chromate, while in the metal-doped goethites, it has no effect on their dissolution. The amorphous and microcrystalline mixed oxides of copper/iron also observed to follow the dissolution behavior of metal doped goethites. Adsorption studies of chromate suggest that doping of metal ions increases the chromate adsorption capacity of goethite from 62.8 to 72.2% for Pb, from 61.4 to 82.92% for Ni and from 64.4 to 84.5 for Cu doping in it. The adsorption studies carried out at pH 3, 5 and 7 suggest that the adsorption of chromate is maximum at low pH values and decreases with increasing pH by all the solid samples used here. The effect of temperature is, however, different for different samples. In case of Pb and Cu-doped goethite, an increase in temperature decrease the adsorption capacity of the solid, while for all the other samples adsorption capacity is observed to increase. The amorphous copper/iron mixed oxide sample has been found to be the best adsorbent for chromate of all the solids used in the present investigation. The values of isosteric heat of adsorption, calculated from the Clausius Clapyron Equation, are positive for Ni doped goethite and negative for Cu and Pb-doped goethite. In case of amorphous phases of copper/iron mixed oxides these values are also found to be positive. The values of isosteric heat of adsorption for chromate adsorption by all the solids are consistent with the effect of temperature determining the endothermic/exothermic nature of the surface reactions. Langmuir equation is applicable to the data under all the experimental conditions. The constants of this model calculated for the adsorption of chromate by all the xisolids coincide well with the adsorption capacity of the respective solids obtained from the experimental data. TEM/SEM analyses suggest that doping of foreign elements in the iron oxide structure, not only increases the sizes of the solid particles, but also increase their surface roughness. Theses changes in the solid particles result in an increase in the surface area, H + /OH - ions adsorption capacity and hence increase the net surface positive charge. These changes collectively increase the adsorption capacity of the doped solids. FTIR analyses suggest that all the goethite samples show bands at 636, 793 and 894cm -1 which are the characteristic bands of goethite and are due to OH bending vibrations. Similarly, the goethite samples show bands at 1383 and 833cm -1 for NO 3 - moiety, which either disappear or present with reduced intensity after chromate adsorption. Thus, all the experimental findings suggest that chromate is adsorbed by all the solid samples through innersphere complexation at pH 5 and 7 by replacing OH - , NO 3 - and CO 3 2- anions from the surfaces. However, at the lowest pH of 3, some outersphere complexes of chromate are also formed on the surface of solids.