Quantification and isotherm modelling of competitive phosphate and silicate adsorption onto micro-sized granular ferric hydroxide.

Adsorption onto ferric hydroxide is a known method to reach very low residual phosphate concentrations. Silicate is omnipresent in surface and industrial waters and reduces the adsorption capacity of ferric hydroxides. The present article focusses on the influences of silicate concentration and contact time on the adsorption of phosphate to a micro-sized iron hydroxide adsorbent (µGFH) and fits adsorption data to multi-component adsorption isotherms. In Berlin drinking water (DOC of approx. 4 mg L-1) at pH 7.0, loadings of 24 mg g-1 P (with 3 mg L-1 initial PO4 3--P) and 17 mg L-1 Si (with 9 mg L-1 initial Si) were reached. In deionized water, phosphate shows a high percentage of reversible bonds to µGFH while silicate adsorption is not reversible probably due to polymerization. Depending on the initial silicate concentration, phosphate loadings are reduced by 27, 33 and 47% (for equilibrium concentrations of 1.5 mg L-1) for 9, 14 and 22 mg L-1 Si respectively. Out of eight tested multi-component adsorption models, the Extended Freundlich Model Isotherm (EFMI) describes the simultaneous adsorption of phosphate and silicate best. Thus, providing the means to predict and control phosphate removal. Longer contact times of the adsorbent with silicate prior to addition of phosphate reduce phosphate adsorption significantly. Compared to 7 days of contact with silicate (c 0 = 10 mg L-1) prior to phosphate (c 0 = 3 mg L-1) addition, 28 and 56 days reduce the µGFH capacity for phosphate by 21 and 43%, respectively.

Competition in chromate adsorption onto micro-sized granular ferric hydroxide.

Hexavalent chromium is highly toxic and elaborate technology is necessary for ensured removal during drinking water production. The present study aimed at estimating the potential of a micro-sized iron hydroxide (µGFH] adsorbent for chromate removal in competition to ions presents in drinking water. Freundlich and Langmuir models were applied to describe the adsorption behaviour. The results show a high dependency on the pH value with increasing adsorption for decreasing pH values. The adsorption capacity in deionized water (DI) at pH 7 was 5.8 mg/g Cr(VI) while it decreased to 1.9 mg/g Cr(VI) in Berlin drinking water (DW) at initial concentrations of 1.2 mg/L. Desorption experiments showed reversible adsorption indicating ion exchange and outer sphere complexes as main removal mechanisms. Competing ions present in DW were tested for interfering effects on chromate adsorption. Bicarbonate was identified as main inhibitor of chromate adsorption. Sulfate, silicate and phosphate also decreased chromate loadings, while calcium enhanced chromate adsorption. Adsorption kinetics were highly dependent on particle size and adsorbent dose. Adsorption equilibrium was reached after 60 min for particles smaller than 63 µm, while 240 min were required for particles from 125 µm to 300 µm. Adsorption kinetics in single solute systems could be modelled using the homogeneous surface diffusion model (HSDM) with a surface diffusion coefficient of 4∙10 -14 m2/s. Competitive adsorption could be modelled using simple equations dependent on time, adsorption capacity and concentrations only.