Adsorption in Aqueous Media, Removal of Arsenic and Fluoride

- Alumina has a neutral surface at pH 9
– At lower pH, it is positively charged – An ion adsorption: AlOH + Lz—-> AlL(z-1) + OH-
– Below pH 3, alumina starts to be soluble
– Capacity is a function of:
– ion polarity
– ion equilibrium solubility
– competitive adsorption from other ions
– pH of the solution

Method of regeneration:
– Non-aqueous media: heat or pressure swing regeneration
– Aqueous media: chemical regeneration (pH adjustment)

Regeneration in Aqueous media:
– Backwash with raw water: Upflow – 10 min
– removal of solids accumulated in the bed
– expansion of the bed
– Regeneration: 2 steps – 1 % NaOH solution
– Upflow
– Rinsing with raw water
– Downflow
– Neutralization: H2SO4 solution – pH: 2.5

When the fluoride or arsenic level drops below the limit, the water becomes usable. The pH at the inlet is adjusted to 5.5 when the outlet pH gets below 7.5.

Waste treatment:
Waste water: 4% of the total throughput
– from backwash: only raw water – discharged to existing storm water disposal systems
– from regeneration and neutralization: water with high levels of fluoride or arsenic ions – must comply with local wastewater discharge standards – Concentrate waste in lined evaporation ponds and further dispose of it.

Arsenic removal: Design guidelines
– pH: for Arsenate (As(V)): about 6 – 6.2 for Arsenite (As(III)): 8-9, Arsenate adsorbes better than arsenite, Arsenite should be first oxidized into arsenate then be treated
– Bed size: minimum of 1 ft3 per gpm of water; minimum depth: 3 ft
– Flowrate: minimum 2.5 to 3 gpm/ft2
– Temperature: 40oC rather than 25oC

Capacity for As (V): Capacity is reduced in presence of chloride and sulfate at pH 6, 25oC, SO42-=7.5 meq/l, Cl-=7.5 meq/l:
q=16.2 C0.43
q and C in mg/g
As (V) outlet content: down to 0.02 ppm

Fluoride removal: design guidelines
– Flowrate: 12 bed volume/hr to obtain low levels (down to 0.2 ppm)
– Capacity for Fluoride: Capacity is reduced in presence of bicarbonate ions.

Typical capacity is 1.2-1.5% Wt.
– Fluoride outlet content: typically 1 ppm down to 0.2 ppm


 

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