Adsorption behavior of crystal violet onto opal and reuse feasibility of opal-dye sludge for binding heavy metals from aqueous solutions

The potential of opal for removing crystal violet (CV) dye and reuse feasibility of opal-CV sludge for binding heavy metals from aqueous solutions were investigated. The adsorbent was characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-Ray Diffraction (XRD) and the point of zero charge (pHpzc) analysis. The thermal treatment of opal at temperatures of 300, 400, 500, 700 °C increased the pHpzc value from 6.6 to 7.4 and lead to a decrease in the adsorption of CV dye, demonstrating the significance of the functional groups spread over the surface of opal. The equilibrium adsorption data fitted well to the Freundlich model and the kinetic data conformed to the pseudo-second-order model. Intra-particle diffusion was not the sole rate-controlling factor. The maximum adsorption capacity of CV was found to be 101.13 mg g−1 at 30 °C. The activation energy (Ea) of 13.85 kJ mol−1 indicated that the adsorption of CV onto opal was a physisorption. Thermodynamic parameters (ΔGΘ, ΔHΘ, and ΔSΘ) were evaluated and revealed that the adsorption process was spontaneous and endothermic in nature. Moreover, the opal-CV sludge was reutilized after calcination at 250 °C (O-CV-250) for the removal of heavy metals from aqueous solution. The O-CV-250 had a maximum adsorption capacity of Pb(II) of 20.92 mg·g−1 while has almost no adsorption of Ni(II) and Cr(VI). Enrichment of Pb(II) was conducted and almost 65% of the Pb(II) was collected. This work presents an effective approach for the treatment of dye wastewater and reutilization of dye sludge to enrich heavy metals.

► The effects of silanol groups spread over the surface of opal were evaluated.
► The maximum adsorption capacity of crystal violet (CV) was 101.13 mg g−1 at 30 °C.
► The opal-CV sludge is calcined to fix organic carbon and avoid second pollution.
► The calcined opal-CV sludge displays definite binding capacity for heavy metals.
► The opal-CV sludge reflected the selective adsorption performance of Pb(II).