Sol–gel derived poly(vinyl alcohol)-3-(2-aminoethylamino) propyl trimethoxysilane: Cross-linked organic–inorganic hybrid beads for the removal of Pb(II) from aqueous solution

Abstract

This manuscript reports sol–gel derived cross-linked poly(vinyl alcohol) (PVA)-3-(2-aminoethylamino) propyl trimethoxysilane (AEAPTMEOS) beads with low degree of swelling as a new adsorbent for the removal of Pb(II) from aqueous solution. Beads were prepared by condensation polymerization followed via acid-catalyzed sol–gel process in presence of non-ionic surfactant. Cross-linking was achieved by glutaraldehyde. Presence of –NH/–NH₂ groups in beads provide active sites for Pb(II) adsorption, and were responsible for high adsorption capacity. SEM/TEM studies confirmed the spherical and rough surface morphology, which was changed after Pb(II) adsorption. For the removal of Pb(II) in aqueous solution, effect of equilibrium time, temperature, pH, adsorbent dose and adsorbate concentration were investigated in batch process. Pseudo-first- and pseudo-second-order kinetics were also evaluated. The equilibrium adsorption followed Langmuir and Freundlich isotherms. Thermodynamic parameters such as ΔG°, ΔH° and ΔS° revealed endothermic and spontaneous adsorption. Monolayer adsorption capacity (Q₀) value for the developed PVA-AEAPTMEOS beads was 67.56 mg g⁻¹ at pH: 5.0, much higher as compared to other adsorbents reported in literature. Desorption studies also suggested that cross-linked PVA-AEAPTMEOS beads can be effectively utilized for the removal of Pb(II) from wastewater.

Introduction

The presence of toxic heavy metal ions in industrial effluents or drinking water is serious environmental problem [1]. The most common heavy metals (Cr, Ni, Mn, Hg, Cd, Pb, Cu and Zn) found in industrial wastewater are not biodegradable. Thus, there is necessary for their removal either by physical or chemical methods from contaminated water [2], [3], [4]. Pb(II) pollution aroused due to use of Pb in service pipes, particularly with soft water, battery industry, auto-exhaust, paints, etc. The maximum allowable concentration of Pb(II) in drinking water is 0.1 mg dm⁻³. Pb(II) contamination causes poisoning, nervous and renal breakdown, headache, brain damage, convulsions behavioral disorders and constipation [5], [6], [7]. Thus, removal of Pb(II) is important for the protection of environmental quality and public health [8]. The traditional methods, such as ion-exchange, solvent extraction, chemical precipitation, electrochemical reduction, and reverse osmosis were commonly used for the removal of heavy metal ions from wastewater [9], [10], [11], [12]. Also, neutralization and precipitation of metal hydroxide are widely used techniques. Disposal of metal containing sludge is one of the main drawbacks of this technique. Thus, economically feasible and eco-friendly methods are necessary for wastewater treatments.

A number of adsorbents were used for the removal of heavy metal ions from wastewater [13], [14], [15], [16]. During last three decades, activated carbon was frequently used as adsorbent, but it was not cost effective. Thus, developments of low cost adsorbent for the removal of heavy metal ions from wastewater, was highly attractive in recent years [17], [18]. Chitosan/PVA blended beads were extensively studied for biomedical applications and lead adsorption, due to their good mechanical and chemical properties [8], [19], [20]. The cross-linked or blended chitosan beads were highly water swollen (water content > 19%) and showed poor mechanical strength for their practical applications [19]. The organic–inorganic hybrid materials were also attractive because of their attractive mechanically properties, thermally stable inorganic backbone, the specific chemical reactivity and flexibility of the organic functional groups [21], [22], [23]. Properties of hybrid materials were depended on their structural, dynamic properties and chemical composition. Number of organic–inorganic hybrid adsorbents such as MCM-41, polymer-supported nanosized hydrous manganese dioxide (HMO), silica nanospheres, MCM-41 modified by alumina and pyrimidine-derivated mesoporous materials, etc., were reported for heavy metal ions removal from wastewater [24], [25], [26], [27], [28], [29], [30]. But suitable tailoring of organic–inorganic hybrid materials by introducing –NH/–NH₂ groups (containing lone pair of electron responsible for metal ion adsorption) as active sites may provide an efficient adsorbent with high affinity for the removal of heavy metal ions (such as Pb(II)).

Herein, we are reporting sol–gel derived cross-linked PVA-AEAPTMEOS organic–inorganic hybrid beads with low water content for the removal of Pb(II) from wastewater. Applicability of developed beads for the removal of Pb(II) from aqueous solution was tested in laboratory scale experiments. Presence of –NH/–NH₂ groups provides active sites for Pb(II) removal from aqueous solution. Pb(II) adsorption capacity was explored under different experimental conditions and compared with the reported values in literature.