Fenton's oxidation of different based synthetic tannins (syntans)

A class of chemicals synthesized for tanning or retanning purposes in the leather tanning industry is collectively termed synthetic tannins, or syntans (ST). These include an extended set of chemically different products, i.e. phenol-, naphthalene-, melamine-based syntans, and acrylic resins. Differing from the vegetable tannins, the common syntans are known persistent in biological process. The literature on tannins is focused on individual chemical species, e.g. tannic or gallic acid, thus providing only scanty information on tannin complex mixtures. The present study evaluates Fenton's oxidation (FO) as the most basic advanced oxidation process one for treatment of four types of syntans (ST1, ST2, ST3 and ST4) in different origin (phenol-, formaldehyde-, cresol-based). To achieve high COD removal, and enhance the biodegradability by increasing BOD(5)/COD ratio, a 0.5-1 concentration ratio of H(2)O(2)/FeSO(4) was applied to synthetic solutions containing of around 300 mg/l of COD equivalent. FO process was performed at pH 3.0 and 40-45 degrees C which is original temperature applied in re-tanning process by mixing samples for 2 h at 30 rpm. Than the samples were left settling for 30 min, subsequently the pH of the supernatants was increased over 8.0 to precipitate iron ion and to decompose any residual H(0)O(2). After 2 h their supernatants were filtered through 0,45 um Millipore filter paper for COD, BOD(5), UV(254), and toxicity measurements using 24 h born Daphnia magna. Although COD equivalents of the syntans were in the order of 1, BOD(5)/COD ratio varied from 0.098 to 0.153. All syntans displayed 100% immobilization before FO treatment. The highest COD removal was obtained for all types of syntans at 600/750 (mg/mg) ratio of H(2)O(2)/FeSO(4). When H(2)O(2) concentration was kept constant at 600 mg/l and FeSO(4) was varied form 750 to 400 mg/l, 600/600 (mg/mg) ratio of H(2)O(2)/FeSO(4) COD removal of ST1 (cresol-based), ST3 (phenol-based) and ST4 (condensed phenol-based) increased further. The biodegradability of the syntans increased was in the ranking of ST3 > ST1 > ST2 > ST4 at 600/750 (mg/mg) ratio of H(2)O(2)/FeSO(4). UV(264) removal of ST4 was in accord with less COD removal and biodegradability increase at 600/750 (mg/mg) ratio. FO treated ST1 samples displayed 100% immobilization even at 50% dilution independently from residual H(2)O(2) concentration while the immobilization decreased after FO treatment of ST2, ST3 and ST4 samples even at the higher residual H(2)O(2) concentration. COD removal ratio.