Biosorption of toxic metal ions (Cr⁺⁶, Cd²⁺) and nutrients (PO₄^3-) from aqueous solution by diatom biomass

Abstract

This paper evaluates diatom biomass as a biosorbent for removing Cr⁺⁶, Cd²⁺, and PO₄^3- ions from water. The diatom was characterized by X-ray Diffraction (XRD), Fourier Transform Infrared (FTIR), and Scanning Electron Microscopy (SEM-EDS) for its crystallinity, functional groups, and morphology. A batch sorption study was conducted to evaluate the parameters influencing Cr⁺⁶, Cd²⁺, and PO₄^3- ions adsorption, and the mechanisms were explored. The FTIR spectra revealed Si-O, O-H, N-H, and C-O as the main functional groups present on the surface of the adsorbent. The SEM showed a rough and irregular-shaped morphology, while the EDS indicated that the diatom biomass is an aluminosilicate material. The rate-limiting steps for Cr⁺⁶ and Cd²⁺ were pseudo-first order, and pseudo-second order sorption favored PO₄^3- based on their R² values. Moreover, the dominant adsorption model that best described the equilibrium data was the Freundlich isotherm. The maximum adsorption capacities obtained for Cr⁺⁶ was 5.66 (mg/g), and Cd²⁺ was 5.27 (mg/g) at 313 K while PO₄^3- was 19.13 (mg/g) at 298 K. The thermodynamic data revealed that the reaction was endothermic for Cd²⁺ and exothermic for Cr⁺⁶ and PO₄^3-, respectively. Diatom biomass was observed to be a promising bio-sorbent for removing Cr⁶⁺, Cd²⁺ and PO₄^2- from wastewater.

Keywords:

• Adsorption
• surface water quality
• wastewater
• biosorbent
• water treatment

Acknowledgments

The authors thank Dr Tatenda Dalu for technical support in collecting and identifying the diatoms, Dr Remi at ithemba labs for XRD analysis and Miranda Waldron at the University of Cape Town for SEM and SEM-EDS analysis.

Authors’ contributions

Conceptualization, W.B.A and W.M.G.; methodology, N.M. and W.B.A.; software N.M. and W.B.A.; validation, N.T.T., W.B.A. and W.M.G.; formal analysis, N.M.; investigation, N.M.; resources, W.M.G., N.M., W.B.A; writing—original draft preparation, N.M.; writing—review and editing, W.B.A., N.T.T., R.M., O.U.I. and W.M.G.; supervision, W.B.A., N.T.T. and W.M.G.; project administration, W.M.G.; funding acquisition, N.M., W.B.A. and W.M.G.

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in the paper.

Additional information

Funding

This work was supported financially by the Department of Water and Sanitation (DWS), South Africa, Prof Gitari DHET research incentive funds, Directorate of Research and Innovation, University of Venda, South Africa and USAID PEER Cycle 6 Award (no: AID-OAA-A-11-00012) and Department of Science and Technology (no: DST/CON 0227/2015).