If you have been processing large quantities of material and hence have a large amount of effluent, then a precipitation-sedimentation method is your only viable option. You have stated that the As is present in V oxidation state only. I would therefore suggest that Magnesium ions in the presence of ammonium chloride and ammonia are your best option. At pH11 the As(V) is present predominantly as HAsO4 two minus ion, if Mg ions together with NH4Cl and NH3 are added you get a ppt of MgNH4AsO4. The ppt will be extremely finely divided and therefore filtration will not be easy, in addition the traditional flocculation with Al won't work either, so sedimentation probably only way. If the source of effluent is sufficiently close to the sea or a tidal estuary then the concentration of magnesium chloride in sea water is high enough to achieve the solubility product. This leaves you needing to find a source of ammonium chloride. Fortunately the pH of the effluent may be high enough for generation of a high enough ammonia concentration in situ from the added ammonium chloride. please note that any As(III) will remain in solution. Sorry I can't be of more help, but hope this is enough to get you started.

I would suggest that you can add NaHS to the solution and you can get yellow As2S5 if the the As in V oxidation state. If the As is present in lower oxidation state, you can oxidate them by HClO to V oxidation state firstly,then add NaHS.

Though different methods are used to remove As (V) from different substances but it looks that As(V) can be best removed from the effluent containing arsenates ,SO4 (-2), C l(-) through adsorption at pH 7.0 using adsorbents like IRON CHITOSAN Flakes (ICF) [22.5 mg/g of As (V)] and IRON CHITOSAN GRANULES (ICG)[(2.24 mg/g for As(V)] in two sorption-desorption cycles .This reduces As(V) conc. to appxo. 10 μ g/ litre. The spent sorbent can be regenerated by 0.1M Na OH. SO4 (-2) and C l (-) remain in the effluent. Of course, you have to change the pH to7.0 from11.0.

Note: CHITOSAN is deacetylated form of CHITIN; is a polysaccharide obtained

from crab and shrimp shells.[Turk.J.Med.Sci.,30(2000)341-48.