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 d etergents. The main pathway of phosphonate removal in the environment is vi a adsorption onto surfaces. This study examines the effect of Ca, Cu, Zn, a nd Fe-III on the adsorption of six phosphonates onto the iron (hydr)oxide g oethite. HEDP, NTMP, EDTMP, and DTPMP are commonly used, while AMP and IDMP have been identified as breakdown products of NTMP. When the molar concent ration of Ca, Cu, Zn, and Fe-III is equal to the concentration of the phosp honate, the effects on phosphonate adsorption are either negligible or slig ht. This can be explained by dissociation of the metal-phosphonate complex and separate adsorption of the metal ion and phosphonate onto different sur face sites. When Ca and Zn concentrations are added in excess of the phosph onate concentration, considerable increases in adsorption are observed, pre sumably through ternary surface complex formation and adsorption onto preci pitated (hydr)oxides of Zn. Excess Ca concentrations can double the maximum surface coverage of NTMP. Phosphonate adsorption at low pH deposits negati ve 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 presen ce of di- and trivalent metals is not expected to diminish the removal of p hosphonate during wastewater treatment or in natural waters contrary to ami nocarboxylates such as EDTA where most metals result in a reduction of EDTA adsorption. Optimal phosphonate removal can be expected in waters containi ng mM Ca and employing addition of iron salts for flocculation or phosphate elimination.