Synthesis and applications of new modified magnetic sporopollenin and graphene oxide-based materials for removal of selected heavy metals from industrial wastewater

Abstract

The presence of heavy metal ions in aqueous environment even at low concentrations is a serious concern and the polluting industries must conform to strict environmental limits and regulations. Conventional techniques such as evaporation, nanofiltration, precipitation and electrocoagulation have been used to remove heavy metals from wastewaters. However, these techniques are tedious, expensive or ineffective. Thus, adsorption is considered as a promising technique for heavy metals removal from wastewater and the search for cost effective, environmentally friendly and sustainable materials for this application has been intensified. In this study two new magnetic sporopollenin-based materials and one new magnetic graphene oxide-based material were synthesized for the removal of three selected heavy metals namely Pb(II), Ni(II) and Cd(II) from industrial wastewater samples. The first magnetic sporopollenin-based materials were synthesised from magnetite (M), sporopollenin (Sp), and 3- aminopropyltrimethoxysilane (APTMS) to give a ternary composite, MSp@SiO2-NH2, which was then modified with silica-coated graphene oxide (GO@SiO2) to finally form a quinary composite, GO@SiO2-MSp@SiO2-NH2. The other magnetic sporopollenin-based material was synthesised from magnetite (M), Sp, 3-Chloropropyltrimethoxysilane (CPTMS), and tetrakis(4-hydroxyphenyl) porphyrin (THPP) to give a quaternary composite, MSp@SiO2-THPP. The magnetic graphene oxide-calix-4-arene based material was synthesized from magnetite (M), GO, silica, calix-4-arene (Calix) and APTMS to give a quinary composite, MGO@SiO2-Calix@SiO2-NH2. These newly synthesized materials were applied for the first time as adsorbents for removal of three heavy metals namely Pb(II), Ni(II) and Cd(II) from aqueous solution of industrial wastewater samples. The newly synthesized materials were characterized using Fourier transform infrared spectroscopy, thermogravimetric analysis, field emission-scanning electron microscopy, energy dispersive X-Ray analysis and vibrating sample magnetometry. The effect of important adsorption parameters such as solution pH, temperature, contact time, adsorbent dose, initial concentration and co-existing ions were studied and optimized. Evaluation of the adsorption performance of the materials at optimum conditions using batch adsorption technique reveals that the heavy metal ions removal efficiencies of the adsorbents were in the order Pb2+> Ni2+> Cd2+ and maximum adsorption capacity (qmax) of GO@SiO2-MSp@SiO2-NH2 for Pb(II), Ni(II) and Cd(II) were 323, 278 and 256 mg/g, respectively. The qmax values of MSp@SiO2 -THPP for Pb(II), Ni(II) and Cd(II) were 454, 435 and 416 mg/g, respectively and the qmax values of MGO@SiO-Calix@SiO2NH2 for Pb(II), Ni(II) and Cd(II) were 256, 243 and 222 mg/g, respectively. MSp@SiO2-THPP was found to offer the highest qmax values probably due to the strong affinity of porphyrins for the metal ions. The initial and final concentrations of the metal ions in the wastewater samples were analyzed using flame atomic absorption spectroscopy. The adsorption behaviors of the respective metal ions on the adsorbents were studied using Langmuir, Freundlich, Dubinin-Radushkevich (DRK) and Temkin isotherms models. The experimental data and values of coefficient of determination (R2) showed that the adsorption fitted the Langmuir and DRK models better for all the materials and the divalent cations, suggesting chemisorption through monolayer coverage. According to thermodynamic studies, the adsorption processes are endothermic and spontaneous. Furthermore, kinetics studies reveal that the adsorption processes followed a pseudo second order rate model. The findings show that the synthesized materials are excellent adsorbents for the removal of the heavy metals from wastewater samples and could be reused for up to 10 cycles without significant deterioration of the signal response. Analytical ecoscale analysis confirmed the greenness of these developed methods.