Exhaustion-Regeneration Behaviour of Carboxylic Resins. The Cr(III)/Al(III)/Fe(III) system

It is not an easy task to regenerate chromium(III) ions from weak electrolyte carboxylate resins using conventional chemicals (e.g., mineral acids, saline solutions, alone or in combination). In areas of environmental control, referring to some specific cases of industrial effluents (e.g., tanneries), where metal complexing agents are present, additional difficulties in separation and recovery of the metals may arise by use of carboxylate form resins. Referring to the Cr(III)/Al(III)/Fe(III) ionic system in a solution simulating a typical spent chrome tanning bath, the exhaustion-regeneration behaviour of a commercial acrylic-based macroporous carboxylic resin (i.e., Purolite C106) is reported. The exhaustion behaviour of the resin is straightforward, whereas regeneration by using chemicals, such as sulphuric acid, sodium hydroxide, sodium sulphate, is difficult. The adoption of hydrogen peroxide in alkaline media, during the regeneration step, seems very promising. The general behaviour is interpreted in terms of ionic speciation and complex formation. A mechanistic explanation is provided in support of the experimental findings.