SOLAR UV RADIATION ASSISTED AS(V) REMOVAL (SORAS) FROM NATURALLY CONTAMINATED GROUND WATERS BY MEANS OF A SEMI-CIRCULAR SECTION TUBULAR PHOTO-REACTOR

A semi-circular section tubular photo-reactor has been constructed, characterized and applied to the treatment of ground waters contaminated with As(V) by means of the solar oxidation and removal of arsenic (SORAS) technique, using a ferrous salt and sodium citrate as basic chemicals. The solar radiation concentrator was built with recyclable materials: Ne lamp glass tubes and 6” PVC tubes (intended for use as sewage drainage) covered by aluminum foil. The reactor concentrates solar radiation up to 2.8 times its natural intensity.

Simultaneous batch experiments without agitation were carried out under UV solar radiation on synthetic water (2 liter volume of distilled water spiked with an As(V) salt, 1000 μgL^-1), using the photo-reactor, a Ne glass tube alone and a 2 liter PET bottle. Fe-citrate complex floccules were visually perceptible at 40, 50 and 90 min respectively under a range of 50-70 Wm^-2 UVA integral (290-390nm) radiation intensities, indicating that removal rate was higher in case of the photo-reactor. As(V) removal efficiencies above 98% were accomplished in all cases.

Batch irradiation experiments followed by controlled agitation (shear rate=30-33s^-1; 20 min agitation period) showed that the photo-reactor accelerates the formation of sedimentable floccules (D_p>0.5mm). Elapsed times necessary for such formation in the photo-reactor, the Ne tube and the PET bottle were 15, 25 and 60 min respectively.

Continuous flow experiments on groundwater (spiked with 1000 μgL^-1 As(V)),using a photo-reactor of 1 m² irradiation area under a hydraulic retention time of 15 min (equal to irradiation time), showed an immediate formation of floccules of good settleability when the solution is subjected to 30 min agitation (33s^-1). An As(V) removal efficiency of 98.36% was obtained achieving an As(V) concentration of 16.5 μgL^-1 in decanted waters. These results mean the photo-reactor is able to treat approximately 130 Lm^-2 within a 6-h period of UVA irradiation intensity of 50-70 Wm^-2. This treatment capacity would be adequate to supply safe water to small families living in the Bolivian “Altiplano”, certain regions of northern Chile and northern Argentina where the climatic and solar radiation intensities are similar and whose ground waters are affected by natural contamination of arsenic.