SOLUBILIZATION OF FE(III) FROM HUMIC-FE COMPLEXES, HUMIC FE-OXIDE MIXTURES AND FROM POORLY ORDERED FE-OXIDE BY ORGANIC-ACIDS - CONSEQUENCES FOR P-ADSORPTION

The effect of organic acids on the solubility of Fe from humic Fe complexes, poorly ordered Fe-oxides and humic/Fe-oxide mixtures was investigated at pH 4 and 7. After reaction, the suspensions were filtered through a 20 000 D membrane and Fe and humic concentrations were determined in the filtrate. Among four organic acids, citric acid had the strongest effect on Fe-solubility and solubilized up to 25 % of total Fe from the humic complexes at pH 4. Humic substances were solubilized by citrate too, the proportions being higher at pH 7 than at pH 4. Malic and tartaric acid solubilized substantial but lower amounts of Fe from humic complexes than citric acid, whereas phthalic acid did not solubilize Fe and humics. Citrate added to poorly ordered Fe-oxide and its mixtures with humics, aged for 65 days, increased 20 000 D filtrable Fe. the effect being higher for humic/Fe-oxide complexes. Adding citrate to humic/Fe-oxide mixtures and to Fe-oxide strongly decreased the P sorption, the extent being higher for the former adsorbent.