An investigation into the use of chitosan for the removal of soluble silver from industrial wastewater

Chitosan was examined as a means of removing soluble silver from industrial waste streams. Stirred-batch and column methods were used to remove free (hydrated) silver ion as well as the ammonia, thiocyanate, thiosulfate, and cyanide complexes of silver in simulated wastewater at an initial concentration of 50 ppm and in a pH range of 2-10. An actual sample of X-ray film development rinse water was also tested. Batch method results show 80-95% silver bound at pH 4-8 for Ag+ and AS(NH3)(2)(+), while 92% and 75% of Ag(S2O3)(2)(3-) and Ag(SCN)(3)(2-), respectively, were bound at pH 2. Using a column containing 0.500 g of chitosan, 160 bed volumes of Ag+, 875 bed volumes of Ag(NH3)(2)(+), 715 bed volumes of AS(S2O3)(2)(3-), and 190 bed volumes of AS(SCN)(3)(2-) solution were treated before silver ion concentration in the effluent reached 5 ppm. Chitosan did not significantly bind AS(CN)(2)(-) at any pH tested. Chitosan treated 450 bed volumes of 40 ppm X-ray rinse water before effluent silver concentration reached 5 ppm. Capacity experiment results using the column method indicated 42 mg of silver bound per gram of chitosan. Four commercial resins, Amberlite IRA-67, IRA 458, IRC-718, and Duolite GT-73, were also tested as silver binding agents.