Thermosensitive polymer Enhanced Filtration (TEF) process: An innovative process for heavy metals removal and recovery from industrial wastewaters

Highlights

• Al, Ni, Cd, Ca sorption occurs on low LCST statistical copolymers in various conditions.

• In presence of Al³⁺, the metal–copolymer complexes form aggregates easily separated by conventional filtration process.

• 100% metals recovery at pH 1, sorption capacity maintained at least over 4 sorption–desorption cycles.

• pH 1 enables LCST decrease, leading to aggregates formation to separate the regenerated polymer.

• Proposal of an innovative process, the Thermosensitive polymer Enhanced Filtration process.

Abstract

This study reports on a new process for the removal and recovery of metal elements from wastewaters, associating a sorption step on a thermosensitive copolymer bearing phosphonic acid moieties, namely the poly(N-n-propylacrylamide-stat-hydrolyzed (dimethoxyphosphoryl)methyl 2-methylacrylate) (P(NnPAAm-stat-_hMAPC1)), with a separation step by filtration. The phosphonic acid groups enable high sorption capacities for multi-component solutions (Ca²⁺, Ni²⁺, Cd²⁺, Al³⁺), while the thermosensitive behavior of the polymer allows, for temperatures higher than its LCST (25.6 °C), to get copolymer–cations non soluble particles. In the presence of aluminum, which was shown to have simultaneous coagulation and neutralization actions, these particles form aggregates of mean diameter 27–63 μm easily removed by conventional filtration (pore diameter >10 μm), leading to a lower energetic separation process compared with the well-known Polymer Enhanced Ultrafiltration (PEUF) Process. To recover metal ions and regenerate the thermosensitive copolymer sorbent, an acidic (H₂SO₄) regeneration step was studied. A decrease of the regeneration solution pH from 4 to 1 led to 100% desorption when the pH was set to 1. Such low pH conditions simultaneously allowed (i) a decrease of the copolymer LCST down to 19.5 °C, and (ii) to reach the isoelectric point of copolymer particles to finally lead to the formation of regenerated copolymer particles which could be recovered by filtration and reused in additional sorption–filtration cycles without any loss of sorption capacities.

Introduction

In the last decades, heavy metal pollution contained in aqueous effluents never stopped increasing due to the considerable development of industries like mine extraction, batteries production or textile industry. Such an increase of the metallic pollution is of special concern since heavy metals are persistent, tend to accumulate in the environment and can lead to serious diseases as lung and kidney problems, cancers or severe damage in the central nervous system [4]. Facing this problem, many works have been conducted in order to develop new methods for heavy metals removal from wastewaters. Most recent ones are either based on separation as lime precipitation [4], [2] or coagulation–flocculation [1], electrodialysis [18], electrocoagulation [9], [13] or electrodeposition [10], or on sorption involving adsorption (Bhattacharyya and Sen Gupta 2008) [11], [12], ion exchange, [23], [8], [5] or complexation [8], [5], [15], [16]. In this last field, the most common treatment methods consist in (i) coupling sorption on water-soluble polymers with ultrafiltration in a process called Polymer Enhanced Ultrafiltration (PEUF) [17] or (ii) sorbing onto insoluble polymeric resins in a process named Solid Phase Extraction (SPE) (Beauvais and Alexandratos 1998). In the PEUF process, the hydrophilicity of water-soluble polymeric sorbents enables a direct contact of the functional groups with the metal elements present in water [21]. However, the soluble polymer–metal complexes need to be removed from the treated effluent by an energy consuming ultrafiltration separation (pore diameters 10–100 nm), which limits the use of PEUF processes. In the case of SPE, water-insoluble materials can be easily removed from the treated water after metal sorption but the insolubility of the material implies slow sorption kinetics and low sorption capacities since the metallic cations must diffuse through the material prior its sorption [14]. In this context, the innovative process reported in the present paper consists in combining advantages of both PEUF and STE water treatment processes by using thermosensitive polymeric sorbents coupled with a conventional filtration separation step. Such polymeric materials are of great interest for two reasons. First, at temperatures lower than the Lower Critical Solution Temperature (LCST), the thermosensitive polymeric sorbent is soluble in water leading to a favored sorption of the metallic cations in solution compared with SPE. Secondly, above the LCST, the polymeric sorbent becomes non-soluble giving the possibility to use an easy handling and low energy-consuming filtration separation rather than the ultrafiltration.

New copolymer materials synthetized to develop this original process were poly(N-n-propylacrylamide-stat-hydrolyzed (dimethoxyphosphoryl)methyl 2-methylacrylate) P(NnPAAm-stat-_hMAPC1) statistical thermosensitive copolymers. In previous works, high sorption properties have been demonstrated for the removal of aluminum, nickel and cadmium from wastewaters [5], [6], [7]. In this contribution, we now focus on the separation and the regeneration steps following the sorption of metal ions frequently encountered in real industrial wastewaters, to design a low energy-consuming process enabling the simultaneous recovery of the metal elements and the polymeric sorbent.