Removal of colloidal biogenic selenium from wastewater

• Colloidal stability of Se(0) is linked to its nanosize and ζ-potential (−20 ± 5 mV).
• High speed (4500 rpm) centrifugation achieved 91% Se(0) removal.
• Filtration through 0.45 μm filters yields a Se(0) removal efficiency of 87%.
• Aluminum sulfate (10−3 M) can sediment up to 92% of colloidal elemental selenium.
• Al–Se sediment shows better dewaterability than Fe–Se sediment.

Biogenic selenium, Se(0), has colloidal properties and thus poses solid–liquid separation problems, such as poor settling and membrane fouling. The separation of Se(0) from the bulk liquid was assessed by centrifugation, filtration, and coagulation–flocculation. Se(0) particles produced by an anaerobic granular sludge are normally distributed, ranging from 50 nm to 250 nm, with an average size of 166 ± 29 nm and a polydispersity index of 0.18. Due to its nanosize range and protein coating-associated negative zeta potential (−15 mV to −23 mV) between pH 2 and 12, biogenic Se(0) exhibits colloidal properties, hampering its removal from suspension. Centrifugation at different centrifugal speeds achieved 22 ± 3% (1500 rpm), 73 ± 2% (3000 rpm) and 91 ± 2% (4500 rpm) removal. Separation by filtration through 0.45 μm filters resulted in 87 ± 1% Se(0) removal. Ferric chloride and aluminum sulfate were used as coagulants in coagulation–flocculation experiments. Aluminum sulfate achieved the highest turbidity removal (92 ± 2%) at a dose of 10−3 M, whereas ferric chloride achieved a maximum turbidity removal efficiency of only 43 ± 4% at 2.7 × 10−4 M. Charge repression plays a minor role in particle neutralization. The sediment volume resulting from Al2(SO3)4 treatment is three times larger than that produced by FeCl3.

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