A new carbon nanotube modified by nano CaO2 for removal of chromate and phosphate from aqueous solutions

Abstract

Chromate and phosphate simultaneously exist in several types of waste streams (e.g., leather tannery). Herein, we developed a new carbon nanotube (CNT) modified by the nano calcium peroxide particles in-situ formed for the treatment. The CNTs with different ratios were prepared and tested for the performance. The CNT (CNT-3) prepared by 0.5-M CaCl2 and 0.5-M H2O2 demonstrated the best performance. It had a total -O-O- group content of 5.24 ± 0.68 mmol/g, and was uniformly covered by synthesized particles with an average diameter of 20–50 nm. At a dosage of 200 mg/L, the removal efficiency of Cr(VI) and phosphate was achieved with the percentage of 81 and 99%, respectively, at a contact time of 120 min. The maximum phosphate adsorption capacity was 126.74 mg-P/g, much higher than that of many adsorbents such as the reported Ca-based, Zr-based and La-based adsorbents. Commonly existing anions (namely SO42-, CO32–, SiO32-, NO3–, and Cl-) showed negligible effects on the simultaneous removal of oxyanions. However, the presence of arsenite greatly decreased the treatment efficiencies for both oxyanions; that of fluoride had no effect on the reduction of Cr(VI), but decreased the adsorption of phosphate. Acidic condition (e.g., pH 3.0) was beneficial for the Cr(VI) removal, but caused the negative effect on the phosphate removal. The chemical and physical properties of CNT-3 were studied via several analytical approaches (e.g., BET, Zeta potential, FESEM-EDX, XRD and XPS). Cr(VI) was removed via chemical reduction to Cr(III) by H2O2 released from CaO2 on CNT-3; phosphate was adsorbed via chemical precipitation (forming hydroxyapatite (Ca10(PO4)6(OH)2)) due to the presence of calcium ion and electrostatic interaction. This research indicates CNT-3 has a great potential for treatment of wastewater containing chromate and/or phosphate oxyanions.