Removal of heavy metals from aqueous solutions by novel melamine-formaldehyde-polyaminopolycarboxylic acid chelating adsorbents
The contamination of water recourses by heavy metals is a serious worldwide environmental problem. Industrial activities, mining and coal combustion are typical contamination sources. Removal of these metals from wastewater effluents is crucial as this contamination is non-biodegradable and highly toxic. Extensive research has been carried out to introduce new materials which alleviate these metals from wastewater effluents before their discharge into water bodies such as rivers and lakes. Conventional methods to remove heavy metals from wastewater include chemical precipitation, ion-exchange and chelation-adsorption. Adsorption is an important and developing research area because of the new material types available according to the application. Furthermore, it is standard process to place the adsorbent in a column and pump the wastewater through in a continuous system. It is also a cost-effective process. Chelating adsorbents are typically characterised by functional groups containing 0, N, S, and P donor atoms which coordinate to different heavy metal ions. It is necessary that the adsorbent has a high capacity and that the kinetics of adsorption is sufficiently fast. Polyaminepolycarboxylic (PAPC) acids are strong chelating agents and form stable chelates with different types of metals: transition, lanthanides and actinides. In spite of its exceptional chelating power, many of the PAPC compounds - such as DTPA (8-coordinations), CDTA (6-coordinations) and NTA (4-coordinations) - have not been thoroughly studied for use as active sites in adsorbent materials for heavy metal remediation from contaminated water effluents. Furthermore, the effect of the number of coordination groups on the adsorption behaviour has not been investigated. Use of these strong chelating agents (PAPC) for heavy metal removal by a polymeric adsorbent is presented in this study, with discussion of the chelation mechanism and affinity. The PAPC chelating agents were anchored on melamineformaldehyde (MF) gel. Although MF gel has suitable chemical and physical properties allowing the production of an adsorbent for heavy metal removal, it has not been studied. MF gel is porous and its matrix has a suitable platform to functionalize with some chelating compounds. PAPC-modified melamine-formaldehyde matrix is easy to produce compared to conventional chelating resins based on styrene/divinylbenzene. In this work, melamine-formaldehyde-polyaminepolycarboxylic acid (MF-PAPC) chelating adsorbents were synthesised by anchoring polyaminepolycarboxylic acids (PAPC) to melamine by the reaction of the carboxylic group of PAPC with a primary amine group of melamine forming a covalent amide bond during MF matrix formation. A series of samples of these adsorbents were prepared by varying water content, acidity of water and temperature as parameters to control the properties of the product. Samples of MF-DTPA, MF-NTA and MF-CDTA were chemically characterized using IR, elemental analysis, TPD-MS, 13C-NMR and 15N-NMR. Physical characterisation was carried out using BET, FE-SEM, and XRD techniques. Elemental analysis and BET results were used to select optimum samples for adsorption experiments. Selected MF-PAPC adsorbent samples are hydrophilic, amorphous and rigid. The content of PAPC in the dry adsorbent samples ranges from 1.08 to 2.28 mmole g⁻¹. The MF-PAPC adsorbents have reasonable surface areas (ranges from 159 to 179 m² g⁻¹) and a mesoporous structure (average pore diameter: 19 - 130 Å). The adsorption performance of MF-PAPC adsorbents was investigated against environmentally problematic divalent metal ions, namely, Cu(II), Co(II), Cd(II) and Zn(II). The adsorption behaviour of these adsorbents was characterised using mixture solutions of these four ions. The effects of different controlling parameters (solution initial pH, temperature, metal ions initial concentration and contact time) on adsorption were considered. Experimental data was fitted to the selected kinetic and isotherm models to suggest the best models to represent the adsorption process on MF-PAPC adsorbents. The thermodynamic parameters (adsorption free energy, enthalpy and entropy) were also calculated and a mechanism of adsorption is suggested according to the evaluation of the results. It was found that MF-PAPC adsorbents follow reversible first order and pseudo second order models to represent the adsorption kinetics. The Langmuir isotherm model gives the best representation of the adsorption processes. These findings indicate the chemical and reversible nature of the adsorption process. Thermodynamically, the adsorption was found to be spontaneous and exothermic. The entropy change shows that adsorption is not favourable. The results indicate that chelation and ion exchange are the mechanisms of adsorption with chelation the dominant type especially at lower temperatures and higher initial pH values. The PAPC type controls the affinity order of the four heavy metals. MF-PAPC adsorbents are distinguished by chelation-adsorption. The adsorption can be universal, or selective according to the PAPC type. Moreover, the selectivity order is different and depends on the PAPC type. MF-PAPC adsorbents can be used for metal-separation applications due to the higher affinity towards transition elements, lanthanides and actinides with respect to alkali and alkaline earth metals. The elution of the adsorbed metal ions was successfully accomplished using a solution of EDTA due to its high chelation power. The MF-DTPA adsorbent was used in a packed column for removal of the Cu(II) ion in a continuous up-flow system. The parameters of the study were: Bed height, flow rate and initial concentration. The Thomas model was used to fit the kinetic data. The BDST model was used to examine the possibility of scaling-up the laboratory set-up to industrial scale. The capacity of dsorption was found to be sensitive to bed height (positive: due to mass transfer), initial concentration (positive: due to concentration driving force) and flow rate (negative: due to contact time). It was found that the adsorption zone moves up the column at a constant speed for different bed heights. Hence, the process can be scaled-up for practical use using a BDST model.