Sorption-desorption studies on alumina pretreated with acids: I. The anionic dye orange II

Brockmann alumina was subjected to chemical pretreatment with HNO₃, H₂SO₄ and H₃PO₄ of various concentrations, and samples of surface phase pH 3.5 − 8.5 (HNO₃Al₂O₃(n), H₂SO₄-treated Al₂O₃(s)) and pH 4.0 − 8.0 (H₃PO₄-treated Al₂O₃(p)) were prepared. Sorption-desorption behaviour of the anionic dye Orange II (C.I.15510) with change in pH on these substrates was studied. Quantitative sorption is shown at pH ⩽ 4.0 (Al₂O₃(n)), and a maximum is observed at pH 5.0 (Al₂O₃(s)) and pH 3.0 (Al₂O₃(p)). Variation in the amount of sorption with time (10 min – 72 h), temperature (30 – 60 °C) and regeneration of the substrates with aqueous inorganic electrolytes is also reported. Desorption efficacy of the anions is in the order PO₄³⁻ >SO₄²⁻ >NO₃⁻. The acid treatment, and hence the specifically adsorbed anions (NO₃⁻, SO₄²⁻, PO₄³⁻), appears to lower the isoelectric pH of alumina (pH 8.0). The results show the involvement of anion-exchange properties of the alumina.