absorbance of the supernatant was measured using UV–vis spectroscopy aftersettling of insoluble sorbent material by centrifugation, as outlinedabove. The same procedure was applied at pH 9.00, where a 10 mMNaHCO3buffer was pH adjusted using 1 M NaOH. At pH 6.6, a 10 mMKH2PO4buffer was pH adjusted using 1 M NaOH.