Applicability of Phosphonic Acid Complexes as a Means of Cadmium Removal from Aqueous Solutions

Abstract

The phosphoric acid based chelating agents are water treatment chemicals that are effectively utilized in many industrial processes as dispersants, bleaching agents, or scale and corrosion inhibitors. These chelating agents can be used in heavy metal removal since they bound to heavy metal ions to form stable and insoluble compounds.In the present study the removal of cadmium from aqueous solutions by precipitation with 1-hydroxyethane-(1,1-diphosphonic acid (HEDP) was investigated. The effects of Cd²⁺:HEDP molar ratio, reaction pH, precipitation duration and initial Cd²⁺ concentration on the performance of Cd²⁺-HEDP precipitation systemwere investigated. Experimental results showed that the maximum Cd²⁺ removal efficiency could be realized at Cd²⁺:HEDP molar ratio of 2:1 and at the reaction pHs higher than 5.0. It was observed that HEDP could effectively reduce 88.5 mg/L Cd²⁺ concentration in aqueous solutions to 0.2 mg/L for 10 days precipitation duration at Cd²⁺:HEDP molar ratio of 2:1 and at pH 6.0. The Cd²⁺ removal efficiency could reach over 99% also in relatively shorter precipitation periods of 3 hours. It was found that Cd²⁺ removal was inhibited at low Cd²⁺ concentration (8.85 mg/L) whereas longer precipitation duration (10 days) was needed for high Cd²⁺ concentration (177 mg/L) presumably due to kinetic limitations.