Biodegradation of metal-[S,S]-EDDS complexes.
The [S,S]-stereoisomer of ethylenediaminedisuccinic acid (EDDS), a biodegradable strong metal chelant, has substituted traditional chelants in a number of consumer products. However biodegradability of metal-EDDS complexes has remained largely undocumented. In the present study, activated sludge fed with EDDS as sole C and N source, was shown to readily biodegrade 1 mM pulses of Ca-, Cr(III)-, Fe(III)-, Pb-, Al-, Cd-, Mg-, Na-, or ZnEDDS (the latter only after extensive lag phase). On the other hand, the Cu-, Ni-, Co-, and Hg-complexes remained essentially undegraded. Only in the case of HgEDDS was lack of biodegradation due to metal toxicity. Speciation analysis revealed free HEDDS3- concentration was higher than 10(-5.4) M for all readily biodegradable metal-EDDS complexes and smaller than 10(-9.0) M for all recalcitrant complexes at pseudo-steady-state (i.e. after initial rise of aquo metal concentration at onset of biodegradation). The rate of metal-EDDS degradation may be modeled with a Monod expression with HEDDS3- as substrate (half-saturation constant ca. 10(-6) M). This model explains the drastic effect of additional metal ligands, e.g. phosphate or iron, on biodegradation rate of several recalcitrant metal-EDDS complexes. Continuously fed aerated biofilters removed 10 mM Pb- or ZnEDDS at a rate of ca. 0.4 mM h-1.