SYNTHESIS, METAL-BINDING PROPERTIES AND DETOXIFYING ABILITY OF SULPHONATED HUMIC ACIDS

摘要

The complexing properties of humic substances are of primary importance for their application as detoxifying agents and microfertilizers. The promising approach for enhancing solubility of metal-humics complexes is incorporation of SO_3H-groups into the structure of humics. The water soluble sulphonated humic materials can be used as flushing agents for heavy metal polluted sites and as chelating agents for production of microfertilizers. The objectives of this study were: 1) to synthesize sulphonated humic materials; 2) to evaluate solubility and stability of their complexes with Fe(Ⅲ) and Hg(Ⅱ); 3) to assess potential toxicity and detoxifying properties of the sulphoderivatives with respect to Hg(Ⅱ) and Cu(Ⅱ). Humic acids (HA) from leonardite and peat were used for modification. Concentrated sulphuric acid and chlorosulphonic acid were used as sulphonating agents. Chlorosulphonic acid was found to be much more efficient sulphonating agent causing introduction of 1.6 and 0.4 mmol SO_3H per g of humic material from leonardite and peat, respectively. The binding properties of the sulphonated HA were studied in relation to Hg(Ⅱ) and Fe(Ⅲ). The solubility of metal-humic complexes was estimated as an amount of strong metal-binding sites (SBS) in the humic material. The sulphonated materials were characterized by twofold increase in the amount of SBS: 2-4 mmol/g versus 1-2 mmol/g in the parent material. Stability constants were determined using the ligand exchange technique. For Hg(Ⅱ) complexes, the log K values accounted for 13.1 and 12.4 L/mol SBS for leonardite HA and their sulphoderivative, respectively. For Fe(Ⅲ) complexes, the log K values accounted for 12.0 and 11.5 for the parent and chlorsulphonated coal HA, respectively, and 12.4 and 11.4 - for the parent and chlorsulphonated peat HA. Assessment of the toxicity of the obtained derivatives and of their detoxifying properties was performed using algological tests. The sulphoderivatives did not exhibit adverse effects onto algae. Both parent and sulphonated humic materials displayed high detoxifying properties with respect to Hg(Ⅱ) and Cu(Ⅱ), leonardite HA were more effective than peat HA. The obtained results allow to consider sulphonation as a promising tool for obtaining soluble metal-HA complexes.