Evidence for Surface Precipitation of Phosphate on Goethite

Abstract

Recent studies have suggested that the interaction between phosphate and goethite includes ternary adsorption/ surface precipitation as well as surface complexformation. The ternary adsorption/surface precipitation process envisioned involves the dissolution of the goethite crystal and subsequent adsorption of iron on the surface-bound phosphate. Further evidence to support the suggested process is needed. The process was investigated using two approaches. First, the sorption of iron spiked into a slurry of phosphated goethite and the effect of the iron sorption on phosphate uptake kinetics were investigated to determine whether iron would be adsorbed on the phosphated surface and whether it would enhance phosphate adsorption. Lead was also spiked into solution for comparison. Second, changes in the xi-potential of phosphated goethite were monitored with time. Adsorption of iron on the surface of phosphated goethite should increase the xi-potential of the goethite. Iron spiked into a phosphated goethite slurry was adsorbed on the solid with a concurrent adsorption of phosphate. The iron adsorption did not change the slow phosphate adsorption kinetics. Lead spiked into the solution was also sorbed but to a lesser extent than iron and with a lower apparent P:Pb mole ratio. Lead addition also changed the phosphate adsorption kinetics. With time, the xi-potential of phosphated goethite became more positive, returning almost to the potential of unphosphated goethite at low surface coverages. The slow increase in xi-potential over time indicates that long-term reactions are occurring on the goethite surface, most likely involving the dissolution of goethite to release iron and the subsequent reaction between the iron and surface-bound phosphate. These results provide strong support for the surface precipitation model, and are inconsistent with models envisioning the reaction between phosphate and the goethite surface as involving only monolayer surface complex formation.

Molecular Insights into Ternary Surface Complexation of Arsenite and Cadmium on TiO2

“…This charge neutralization signals the formation of the ternary surface complex. 12 One possibility for ternary surface complex formation is that the Cd−As(III) aqueous complex may initially form in solution and then adsorb on the TiO 2 surface as an assembly. To test such a premise, we used surface-enhanced Raman scattering (SERS) analysis because it provides an ultrasensitive probe of molecular structure.…”

“…11−14 However, no clear boundaries exist distinguishing ternary surface complexes and surface precipitation, and these two terms sometimes are even used interchangeably. 12 For example, surface precipitation was used to describe elevated As(V) removal in the presence of Zn 11 and Cu 15 on goethite. On the other hand, more recent reports use ternary surface complexation to describe the co-adsorption of cations and anions on a surface where the newly formed cation−anion solid phase does not grow beyond monolayer coverage.…”

Co-adsorption of phosphate and zinc(II) on the surface of ferrihydrite

“…Decreased adsorption may arise from the competition for coordination sites at the metal oxide surface, or the formation of a stable non-adsorbing cation-oxyanion complex in solution (Benjamin and Leckle, 1982;Theis and West, 1986). Promoted adsorption could be attributed to the reduced electrostatic repulsion during the co-adsorption of oppositely charged contaminants (Diaz-Barrientos et al, 1990;Collins et al, 1999) as well as the formation of surface precipitates (Vaca-Escobar et al, 2012) and surface ternary complexes (Ler and Stanforth, 2003;Jiang et al, 2013;Elzinga and Kretzschmar, 2013). The different adsorption behaviors probably can be attributed to the physicochemical properties of the adsorbents and the cation-anion pairs, solutions pH, and surface adsorption coverage, etc.…”

“…In the present study, phosphate was chosen as oxyanion because it has a relatively high affinity to iron (oxyhydr)oxides and often reported to form oxyanion-bridged ternary complexes on geosorbents (Ler and Stanforth, 2003); Zn(II) was selected as heavy metal cation due to its relatively weak affinity to geosorbents (Zhou et al, 2002). One therefore may expect the formation of phosphate-bridged ternary complexes on Fh.…”

Phosphate adsorption from sewage sludge filtrate using zinc–aluminum layered double hydroxides

“…More recently some researchers studied the feasibility of phosphate adsorption from phosphate-rich streams. The various adsorbents used include industrial materials and byproducts (i.e., iron oxide tailings [5], fly ash [6], blast furnace slag [7] and red mud [8]), natural or synthetic minerals (i.e., goethite [9,10], dolomite [11] and alunite [12]), metal oxide/hydroxide (i.e., aluminum oxide, iron oxide, zirconium oxide [13] and zirconium hydroxide [14]) and other materials (i.e., ion exchange resin [15]). …”