Lecture 8

5 and 85 the effluent concentration is comparable to

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Unformatted text preview: strial Wastewaters • Cadmium can also be removed by precipitation as carbonate. The pH required in this case is between 7.5 and 8.5. The effluent concentration is comparable to that obtained through hydroxide precipitation at high pH • Cyanides interfere with any of these processes and must be removed prior to cadmium precipitation PIERO M. ARMENANTE NJIT Chromium in Industrial Wastewaters Chromium is present in the wastewaters of a number of industries producing: • steel manufacturing • chrome plated products • tannery products • dye stuff • paints PIERO M. ARMENANTE NJIT Precipitation of Chromium from Industrial Wastewaters Chromium is typically precipitated in two steps: • hexavalent chromium (Cr+6) is reduced to trivalent chromium (Cr+3). Compounds such as FeSO4, Na2S2O5 (sodium bisulfite) or SO2 (sulfur dioxide) are used as reducing agents. The reaction is conducted at low pH (< 3) • trivalent chromium is precipitated as Cr(OH)3. Lime is typically used for the precipitation reaction. The effluent concentration is 0.2 ppm at pH 7.5 PIERO M. ARMENANTE NJIT Precipitation of Chromium from Industrial Wastewaters The reactions involved in chromium precipitation are: Reduction reaction (at pH < 3): H2Cr2O7 + 3 SO2 → Cr2 (SO4 )3 + H2O (i.e., 1.85 ppm SO2/ppm Cr) or, alternatively: H2Cr2O7 + 6 FeSO4 + H2SO4 → Cr2 (SO4 )3 + 3 Fe2 (SO4 )3 + 7 H2O Precipitation reaction (at pH of 8 to 9): Cr2 (SO4 )3 + 3 Ca(OH )2 → 2 Cr (OH )3 ↓ + 3 CaSO4 (i.e., 2.13 ppm Ca(OH)2/ppm Cr) PIERO M. ARMENANTE NJIT Copper in Industrial Wastewaters Copper is present in the wastewaters of a number of industries producing: • Chemicals using copper salts • Chemicals using copper catalyst • Metal processing products • Metal plated products PIERO M. ARMENANTE NJIT Precipitation of Copper from Industrial Wastewaters • Copper can be removed by precipitation as hydroxide at pH ranging from 9 to 10.3 (solubility: 0.01 ppm as cupric oxide) • Copper can be removed by precipitation as sulfide at pH 8.5. The resulting effluent concentration is 0.01 - 0.02 ppm • The presence of cyanide or ammonia may interfere with copper precipitation. In such a case activated carbon can be used to remove copper cyanide PIERO M. ARMENANTE NJIT Lead in Industrial Wastewaters Lead is present in the wastewaters of a number of industries producing: • batteries • pigments • printing products PIERO M. ARMENANTE NJIT Precipitation of Lead from Industrial Wastewaters • Lead can be removed by precipitation as hydroxide (lime) at pH 11.5. The effluent concentration is...
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This document was uploaded on 03/30/2014 for the course CHEM 685 at NJIT.

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