A Comparison of Water Treatment Sludge and Red Mud as Adsorbents of As and Se in Aqueous Solution and Their Capacity for Desorption and Regeneration

The adsorption of As(III), As(V), Se(IV) and Se(VI) by seawater neutralized red mud and alum water treatment sludge was investigated and compared using the batch adsorption technique. For water treatment sludge, adsorption of As(V), Se(IV) and Se(VI), at equimolar concentrations of added metalloid, declined with increasing pH. The decline was rapid above pH 4.0 for Se(VI), above pH 5.0 for Se(IV) and above pH 6.0 for As(V). Adsorption of As(III) increased with increasing pH up to pH 9.0 and then declined. For red mud, adsorption of As(V), Se(IV) and Se(VI) showed a maximum at about pH 5.0 and for As(III) adsorption remained relatively constant over the pH range 2.0–10.0 after which it declined. Water treatment sludge removed 50 % or more of solution As(V) between pH 2.0 and 10.8, Se(IV) between 2.0 and 8.9, Se(VI) between 2.0 and 5.8 and As(III) between 8.4 and 10.9. By contrast, red mud showed less than 25 % adsorption of added Se(VI) and As(III) over the entire pH range tested (2.0–12.0) and reached 50 % or more for As(V) only over the pH range 4.0–6.9 and for Se(IV) between pH 4.3 and 5.6. At pH 5.0, adsorption of As(III) and Se(IV) was better described by the Langmuir than Freundlich equation but the reverse was the case for As(V) and Se(VI). Kinetic data for adsorption of all four oxyanions onto both adsorbents correlated well with a pseudo-second-order kinetic model suggesting the process involved was chemisorption. NaOH was more effective at removing adsorbed metals from both adsorbents than HNO₃. Water treatment sludge maintained its As(III) and Se(IV) adsorption capability at greater than 70 % of that added over eight successive cycles of adsorption/regeneration using 0.5 M NaOH as a regenerating agent. By contrast, for red mud, As(V) adsorption capacity declined very rapidly after three adsorption/desorption cycles and that for Se(IV) it decreased progressively with increasing numbers of cycles. It was concluded that water treatment sludge is a suitable material to develop as a low-cost adsorbent for removal of heavy metal oxyanions from wastewater streams.