The influence of metal ions on the adsorption of phosphonates onto goethite

Mono- and polyphosphonates, which contain R3CP(O)(OH)(2) functional groups, are used in an increasing variety of industrial and household applications including cooling water systems, oil production, textile production, and detergents. The main pathway of phosphonate removal in the environment is via adsorption onto surfaces. This study examines the effect of Ca, Cu, Zn, and Fe-III on the adsorption of six phosphonates onto the iron (hydr)oxide goethite. HEDP, NTMP, EDTMP, and DTPMP are commonly used, while AMP and IDMP have been identified as breakdown products of NTMP. When the molar concentration of Ca, Cu, Zn, and Fe-III is equal to the concentration of the phosphonate, the effects on phosphonate adsorption are either negligible or slight. This can be explained by dissociation of the metal-phosphonate complex and separate adsorption of the metal ion and phosphonate onto different surface sites. When Ca and Zn concentrations are added in excess of the phosphonate concentration, considerable increases in adsorption are observed, presumably through ternary surface complex formation and adsorption onto precipitated (hydr)oxides of Zn. Excess Ca concentrations can double the maximum surface coverage of NTMP. Phosphonate adsorption at low pH deposits negative charge on (hydr)oxide surfaces and hence increases Cu adsorption through favorable long-range electrostatic interactions. The results are discussed with respect to the removal of phosphonates in natural systems. The presence of di- and trivalent metals is not expected to diminish the removal of phosphonate during wastewater treatment or in natural waters contrary to aminocarboxylates such as EDTA where most metals result in a reduction of EDTA adsorption. Optimal phosphonate removal can be expected in waters containing mM Ca and employing addition of iron salts for flocculation or phosphate elimination.