Functionalized Geopolymers Derived From Clay And Rice Husk Ash For Removal Of Selected Heavy Metals And Methylene Blue From Aqueous Solution

ABSTRACT

With the onset of industrialization, humanity has witnessed various ecological issues in the society and disturbance of ecosystem. Heavy metals and methylene blue are very toxic substances known to cause detrimental effects to human health when ingested even at low concentrations. Several techniques are available for removal of heavy metals, and dyes from the wastewater such as chemical precipitation, ion exchange, adsorption, membrane among others. Among these technologies adsorption is preferable for wastewater treatment due to its simplicity in design, high efficiency and availability of materials involved. Geopolymers are inorganic polymers and have been in use in construction industries as alternatives to ordinary portland cement but very little information is available on their use in environmental pollution management. This study aimed at synthesizing and functionalizing amorphous geopolymers from common clay and rice husk ash for use as adsorbents in removal of heavy metal ions and methylene blue from contaminated water. There is adequate literature indicating that clay minerals contain both silicates and alumina while rice husk ash contains substantial amounts of silica. This work therefore reports synthesis of geopolymers GP-1, GP-2 and GP-3 using clays and rice husk ash that were functionalized using citric acid and EDTA for use in adsorption of heavy metals and colour. Characterization of the geopolymers was done using FT-IR, EDS, XRD and SEM. Batch experiments using Pb (II), Cd (II), Zn (II) ions and methylene blue (MB) were done. FT-IR indicated presence of Al-O and Si-O bonds which are the finger prints of geopolymers. XRD analysis showed presence of amorphous phase between 18-36° (2θ) an indication that geopolymerization occurred. SEM analysis revealed the formation of a heterogeneous matrix which consists of a dense continuous gel with microcracks and voids on geopolymer adsorbents. The mean percentage uptakes of 99.32 ± 0.04, 99.74 ± 0.01, 91.33 ± 0.06 and 91.99 ± 0.57 of Pb (II), Cd (II), Zn (II) and MB were achieved respectively. Various equilibrium models were employed and from the correlation coefficients (R2 > 0.94), the data was found to fit best in Langmuir Freundlich model (Sips). The highest adsorption capacities of Pb (II), Cd (II), Zn (II) and MB were 326.5, 175.5, 169.9 and 20.74 mg/g respectively, attained using GP-3E. Increased adsorption of Pb (II), Cd (II), Zn (II) ions and MB was observed upon functionalization as well as when Si/Al ratio increased. Kinetic studies showed that a pseudo- second order model was more suitable than the first order in explaining the adsorption mechanism. This indicated that the adsorption transient behaviour used valence forces or exchangeable electrons and that chemisorption was more pronounced in the rate determining step. Thermodynamic studies revealed that the adsorption process was endothermic and physisorption since values of ΔH° obtained were < 40 kJ/mol. From the adsorption data, it’s evident that synthesized geopolymers are potential adsorbents for removal of heavy metals and MB and may be employed in wastewater management.