Silicate sieves absorb hexavalent chromium waste
The remediation of hexavalent chromium from wastewater effluents is no mean feat, but researchers at Universidad de Guanajuato, Mexico, have produced novel titanium-modified mesoporous silica compounds that could enable the development of more efficient, low cost processes.
Chromium is a common element found in two main oxidation states - trivalent (Cr3+) and hexavalent (Cr6+). Naturally occurring chromium is in the trivalent state in rocks, soil, plants and volcanic emissions. Hexavalent chromium is a carcinogenic substance, often found in wastewater from tanneries, dye and pigment factories, and the electroplating industry. Many of these sectors have no effective method of removing chromic waste.
To avoid contaminating soil and drinking water, ion exchange resins are commonly used for this purpose, but they are expensive and resin deterioration is a frequent problem. Attempts to customise these resins for adsorbing specific ions have only achieved limited success, therefore, recent research in this field has focused on developing ion-specific mesoporous sieves.
Dr Irene Cano Rodriguez, Principal Researcher at Universidad de Guanajuato, says, ‘Most chromium wastes are in the form of Cr3+, which can be oxidised, as a result of environmental conditions, to its more hazardous form, Cr6+. The advantage of these novel [mesoporous] materials is their ability to selectively adsorb and desorb Cr6+ ions. We can easily use them in several adsorption-desorption cycles and reuse makes for low cost technology.'
Prepared by chemical grafting and incipient wetness impregnation, the silicate sieves reach adsorption equilibrium after one hour and are claimed to be capable of removing over eight per cent of their dry weight of Cr6+ in multi-ion aqueous solutions - more than current processes. Chemical selectivity is achieved by the presence of titanium dioxide - a chemically stable compound with a high affinity for chromium ions - in the pores of the silicate material.
‘Hexavalent chromium adsorption occurs as ions bind to the inner surfaces of the silicates through several mechanisms such as physical adsorption, complexation, ion exchange and surface microprecipitation. The quantity of Cr6+ removed is dependant on the adsorption capacity,' Cano Rodriguez explains.
The next step is to optimise and scale up the remediation technology for possible use in water reservoirs and aqua filters. The team at Universidad de Guanajuato is currently seeking funding for future development and is searching for new applications for these materials.
Universidad de Guanajuato