Water is a vital resource for the sustenance of life and it forms the basis of the blue economy. Water quality is often threatened by pollution such as nutrient pollution (eutrophication) and microbial contamination. This causes a deterioration of water quality, infections and in worse cases, death of both humans and animals. Various methods have been used in the removal of microbiological contaminants in water such as physical treatments (filtration), biological processes, chlorination, ultra violet (UV) disinfection and ozonation. However, these methods are not effective in removing nutrients such as phosphates from the same wastewater. Adsorption is one of the methods used in the removal of phosphate from wastewater. The use of low-cost materials makes adsorption a cheap and easy method to use. Mollusc shells are by-products generated during shell processing and the sustainable use of these by-products is key to the actualization of a blue economy within the shellfish processing industry. Can these ‘waste shells’ and low-cost material, be used in the removal of nutrients (specifically phosphate) and water-borne bacteria? This study aims to investigate the phosphate removal capacity and antimicrobial activity of mollusc shell waste powder in both its non-heat treated and heat-treated forms, against four bacterial strains which affects both humans and fish.
Materials and methods
Waste shells were cleaned and oven dried at 105℃ for 2 h, sieved to their respective particle size and calcined for 2 h. Two Gram-negative bacteria, Escherichia coli Pseudomonas stutzeri, and two Gram-positive bacteria, Staphylococcus aureus and Bacillus subtilis, which are widely distributed in the environment (such as in water and soil), were selected as the indicator organisms. The indicator bacteria were prepared following the National Committee for Clinical Laboratory Standards (NCCLS) protocol. The antibacterial activity of the non-heat treated and heat treated mollusc shell waste powder were determined using the well diffusion method at a concentration of 25 mg. Batch experiments were conducted to determine the phosphate removal efficiency of the shells. The shell samples were characterized using physico-chemical methods such as Scanning Electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRD) analysis.
Non-heat treated mollusc shells showed no antibacterial activity against all the bacteria strains. However, the heat treated mollusc shells had good antibacterial activity with activity highest for B. subtilis and P. stutzeri, followed by E. coli and S. aureus. Furthermore, the results of the batch experimental study showed that the non-heat treated shells had a lower phosphate removal efficiency when compared with the heat-treated shells. The result of the characterization study showed that heat treatment of the shell powder resulted in a change of structure, morphology and elemental composition all of which contributed to its increased phosphate removal capacity and antibacterial activity.
Heat treated mollusc shells used in this study exhibited antibacterial activity against all the bacteria strains, thus demonstrating a broad spectrum of activity for both gram-negative and gram-positive bacteria. Furthermore, the heat-treated shells show a higher phosphate removal capacity when compared with the non-heat treated shells. Research is currently ongoing on the use of the shells for the simultaneous removal of phosphate and water-borne bacteria from both synthetic and industrial wastewater.
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