Trivalent Chromium Ion Adsorption on Various Types of Wastewater Sludge

This research aimed at evaluating the adsorption of trivalent chromium present in high concentrations [about $100–170\mathrm{mg}\mathrm{Cr}\left(\mathrm{III}\right){\mathrm{L}}^{-1}$ ] in effluents on various types of wastewater sludges, namely, primary sludge (PWS), secondary sludge (SWS), mixed sludge $(\mathrm{primary}+\mathrm{secondary})$ (MWS), physicochemical treatment sludge (CWS), and agro-processing industry sludge (AWS). Adsorption tests of chromium were carried out at $25\pm 1°\mathrm{C}$ in $500\mathrm{mL}$ Erlenmeyer flasks at various adsorbent concentrations (2, 5, 10, 15, 20, and $30\mathrm{g}{\mathrm{L}}^1$ ) for all types of sludges studied (PS, SWS, MWS, CWS, and AWS). A synthetic chromium nitrate $\left[\mathrm{Cr}\right({\mathrm{NO}}_3{)}_3\cdot 9{\mathrm{H}}_2\mathrm{O}]$ solution adjusted to ${\mathrm{pH}}_{\mathrm{i}}=3.2$ at $112\mathrm{mg}{\mathrm{L}}^{-1}$ was used. The results revealed that CWS had the best adsorption capacity for chromium, followed by SWS, and adsorption capacity of the different wastewater sludges was in the following order: $\mathrm{CWS}>\mathrm{SWS}⩾\mathrm{MWS}>\mathrm{PWS}>\mathrm{AWS}$ . Kinetic adsorption studies also showed that almost complete removal of chromium in solution was reached in the first $2\mathrm{h}$ of reaction with all types of sludges. Finally, the adsorption tests of chromium on tannery effluent $\left(\right[\mathrm{Cr}{]}_i=143\mathrm{mg}{\mathrm{L}}^{-1})$ confirmed the effectiveness of SWS for the removal of chromium. A removal yield of 40.8% of chromium was observed following $2.0\mathrm{h}$ of adsorption on $5.0\mathrm{g}{\mathrm{L}}^{-1}$ of SWS at a ${\mathrm{pH}}_i=3.94$ . The quantity of chromium adsorbed on sludge during these tests corresponded to a load of $11.6\mathrm{mg}\mathrm{Cr}{\mathrm{g}}^{-1}$ dry weight.