Lecture 8

Piero m armenante njit solubility of metals sulfides

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Unformatted text preview: dation with chlorine under alkaline conditions. PIERO M. ARMENANTE NJIT Solubility of Metals Sulfides as a Function of pH 10 10 10 10 10 Solubility (mg/L) 10 10 10 10 10 10 10 10 10 10 10 2 1 0 -1 -2 Ni Cu -3 -4 -5 Zn -6 -7 Cd -8 Ag Pb -9 -10 -11 -12 -13 2 4 6 8 10 12 14 pH After EPA, 1983 PIERO M. ARMENANTE NJIT Precipitation of Heavy Metals as Sulfides • Metal sulfides are typically very insoluble. Therefore metals can be precipitated by adding sulfide ions (S-2). • Metal sulfides have much lower solubilities than the corresponding metal hydroxides, thus allowing lower residual metal concentrations in the treated wastewater. PIERO M. ARMENANTE NJIT Solubility of Metals Sulfides as a Function of pH 10 10 10 10 10 Solubility (mg/L) 10 10 10 10 10 10 10 10 10 10 10 2 1 0 -1 -2 Ni Cu -3 -4 -5 Zn -6 -7 Cd -8 Ag -9 -10 -11 -12 -13 Pb Precipitation of Heavy Metals as Sulfides (continued) • Hydrogen sulfide (H2S) is a weak acid and dissociates in two steps according to: H2S ⇔ H++HS HS ⇔ H++S-2 - • Sulfide salts undergo hydrolysis, partially reforming the undissociated acid: - - S + H2O ⇔ HS + OH -2 - HS + H2O ⇔ H2S + OH PIERO M. ARMENANTE NJIT - Precipitation of Heavy Metals as Sulfides (continued) • S-2 is not the dominating species until the pH is very high (~14). Hence, by adding a sulfide salt an equilibrium is generated resulting in only the partial formation of the S-2 ions actually used in precipitation. • Sulfide precipitation is always conducted under alkaline conditions to promote sulfide ion formation. • Sulfide precipitation is rather insensitive to the presence of chelating agents. PIERO M. ARMENANTE NJIT Precipitation of Heavy Metals as Sulfides (continued) • Sulfide precipitates tend to form colloidal particles requiring the addition of coagulants to settle. However, sulfide sludges are easier to dewater than hydroxide sludges. • Sulfide sludges are more prone to oxidation resulting in resolubilization of the metals as sulfates. • Evolution of H2S is a potential hazard if the pH is not carefully maintained in the alkaline region. PIERO M. ARMENANTE NJIT Precipitation of Heavy Metals as Carbonates • Some metals (lead, cadmium, nickel) form insoluble carbonates that can be used in carbonate precipitation. • Some wastewaters already contain enough carbonates to allow precipitation to occur. Alternatively, inorganic carbonates (e.g., Na2CO3) can be added. PIERO M. ARMENANTE NJIT Precipitation of Heavy Metals as Carbonates (continued) • Carbonate precipitation takes place only if carbonate ions (CO3-2) are present. Free carbonat...
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This document was uploaded on 03/30/2014 for the course CHEM 685 at NJIT.

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