Lecture 8

A caustic is often added to raise the ph high ph also

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Unformatted text preview: e ions are present only if the pH is high. A caustic is often added to raise the pH. • High pH’ also promote the precipitation of the s metals as hydroxides. Hence, carbonate precipitation is often a co-precipitation. PIERO M. ARMENANTE NJIT Precipitation of Heavy Metals as Carbonates (continued) • pH values above 10 promote the formation of metal hydroxy complexes that can increase the metal solubility and reduce the precipitation effectiveness. • Carbonate precipitates settle and can be dewatered more easily than the corresponding hydroxide precipitates. PIERO M. ARMENANTE NJIT Arsenic in Industrial Wastewaters Arsenic is present in the wastewaters of a number of industries producing: • metallurgical products • glassware and ceramic • tannery products • dye stuff • pesticides • synthetic chemicals • petroleum refinery products PIERO M. ARMENANTE NJIT Precipitation of Arsenic from Industrial Wastewaters • Arsenic can be removed by precipitation as sulfide, through the addition of sodium sulfide or hydrogen sulfide to the wastewater. The effluent concentration is 0.05 ppm • Arsenic can also be removed by coprecipitation with FeCl3 when a Fe(OH)3 floc is formed. The effluent concentration is 0.005 ppm PIERO M. ARMENANTE NJIT Barium in Industrial Wastewaters Barium is present in the wastewaters of a number of industries producing: • metallurgical products • glassware and ceramic • dye stuff • explosives • rubber products PIERO M. ARMENANTE NJIT Precipitation of Barium from Industrial Wastewaters • Barium can be removed by precipitation as sulfate, by adding any sulfate ion source • The solubility of barium sulfate is 1.4 ppm • Even lower residual barium concentrations (0.5 ppm) can be obtained using an excess of sulfate ions Note: 1 ppm = 1 mg/L PIERO M. ARMENANTE NJIT Cadmium in Industrial Wastewaters Cadmium is present in the wastewaters of a number of industries producing: • metallurgical products • ceramics • electroplated products • photographic products • pigments • textiles • synthetic chemicals PIERO M. ARMENANTE NJIT Precipitation of Cadmium from Industrial Wastewaters • Cadmium can be removed by precipitation as hydroxide at pH ranging from 8 (solubility: 1 ppm) to 11 (solubility: 0.05 ppm) • Cadmium can be removed by precipitation as sulfide. The effluent concentration is 0.05 ppm • Cadmium can also be removed by coprecipitation at pH 6.5 with FeCl3 when a Fe(OH)3 floc is formed. The effluent concentration is 0.008 ppm PIERO M. ARMENANTE NJIT Precipitation of Cadmium from Indu...
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This document was uploaded on 03/30/2014 for the course CHEM 685 at NJIT.

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