Synthesis and characterization of fish scales (Oreochromis niloticus) derived biochar and magnetic composites and their application in removal of anionic indigo carmine and cationic crystal violet dyes from synthetic and industrial wastewater

Abstract

The global and regional increase in demand for textile products have led to increased quantities of dye wastewater in the environment, which is a potential source of carcinogenic compounds. This situation not only hampers photosynthetic activities of benthic plants but also affects aesthetic quality of water sources. Apart from many available wood biomass, fish scales of Tilapia (Oreochromis niloticus) are also abundant wastes in the environment that can be processed and applied as materials in abstraction of indigo carmine (IC) and crystal violet (CV)dyes from wastewater.Nevertheless, physical and chemical characteristics of raw fish scales and their biochars are not known. The main objective of this study was to synthesize and characterize fish scale biochars(FSB) and magnetic composites (FSB@Fe3O4) at varying temperatures and apply them in remediation of industrial dye effluents. Pulverized raw fish scales (RFS) were pyrolysed over temperature of 200 oC - 800 oC and magnetic composites,FSB@400 oC-Fe3O4, FSB@600 oC-Fe3O4 and FSB@800 oC-Fe3O4, were synthesized by chemical co-precipitation method. The adsorbents were characterizedfor peak optical absorbance, functional groups, magnetic strength, surface morphology, particle size, elemental compositions, surface charge, surface area, thermal stability and crystallinity. The potential of pulverized RFS, FSB and FSB@Fe3O4 compared to activated charcoal (AC) for abstraction of IC and CV dyes from aqueous solutions was determined as a function of initial dye concentration, contact time, dye solution pH, adsorbent dosage and temperature. Peak optical absorbance was 252-320 nm with FSB@600 oC posting the lowest peak of all the magnetic composites studied. The adsorbents contained CO32-, PO43-, Fe-O and -OH as confirmed by EDX and XRD analysis. TEM results displayed spotted ring-like patterns on the surface of FSB and catenation of identical spherically shaped flowers was observed on FSB@Fe3O4. Magnetization values for VSM analysis ranged between 8 to 10 emu/g, sufficient for adsorbent recovery with a magnet. The pHpzc ranged between 3.80-8.42, while TGA showed that a rise in temperature led to a rise and a decline in weight loss for RFS, FSB and FSB@Fe3O4, correspondingly. BET surface areas were 94.05 and 102.67 m2/g for FSB@600 oCand FSB@600 oC-Fe3O4, respectively. Adsorption results demonstrated that the quantity (q) of the dyes adsorbed by FSB and FSB@Fe3O4 increased with initial dye concentration, material load and solution temperature. Lower quantities of IC and higher quantities of CV were recorded at higher pH levels. Statistics fittedpseudo-second-order kinetic models while the Langmuir isotherm presented the best fit of all the models tested. The optimum pH 2 and pH 8-10 were favourable for adsorption of IC and CV, respectively. Wastewater remediation data revealed that for location A, FSB@600 oC significantly reduced IC and CV by 72.00% and 96.96%, correspondingly. Reusability results showed that the adsorbents conserved their initial adsorption capacities for the abstraction of IC for five consecutive cycles without significant depletion. However, substantial decrease in adsorption capacities at the fifth regeneration cycle was observed for CV. Fish scale biomass can be applied in the synthesis of adsorbents that can be adopted as easily accessible materials for remediation of industrial dye wastewater.