Abstract
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 PO43−–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 (c0 = 10 mg L−1) prior to phosphate (c0 = 3 mg L−1) addition, 28 and 56 days reduce the μGFH capacity for phosphate by 21 and 43%, respectively.
Highlights
Phosphate loads in up to 20% of European surface waters have to be drastically reduced to reach the good ecological status as demanded by the European water framework directive.[1]
Critical phosphate concentrations for slowly owing or dammed waters are set as 0.1 mg LÀ1 total phosphorus (TP).[2]
The mGFH was wet-sieved to the desired grain fractions and air-dried at room temperature
Summary
Phosphate loads in up to 20% of European surface waters have to be drastically reduced to reach the good ecological status as demanded by the European water framework directive.[1] Critical phosphate concentrations for slowly owing or dammed waters are set as 0.1 mg LÀ1 total phosphorus (TP).[2] In Germany, the majority of surface waters exceed this value, especially shallow and polymictic lakes. Depending on the lake type, even lower values of 0.02–0.06 mg LÀ1 have been suggested in order to limit algae growth and to reach a good status.[3]. Low residual phosphate concentrations are achieved with high doses of occulants. The advanced treatment leads to an increased input of salts, increased sludge volumes and the need for large sedimentation tanks. A more simple and Competitive adsorption of phosphate onto iron hydroxides was widely studied.[6,8,9,10,11] While sulfate and chloride were shown to have no effects[12] and bivalent cations (e.g. Ca2+, Mg2+) can have bene cial surface complexation effects on phosphate, bicarbonate and silicate proved to have adverse impacts.[13]
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