Heavy metals cannot be biodegraded and they remain in the environment until being removed. Thus, the removal of heavy metals from contaminated water is of special concern for the protection of human and aquatic lives. Studies on polymer inclusion membranes (PIMs) started more than 50 years ago and have shown outstanding separation performance of metal ions. The potential and capabilities of PIMs have made it more favorable than ion exchange and liquid-liquid extraction process. To achieve efficient transport of metal ions, different types of extractant with compatible base polymer have been successfully used along with suitable targeted metal ions. However, selectivity of metal ion is only limited to one type of metal ion based on the extractant used in PIMs. The present review describes the current literature on heavy metal removal using PIMs for the past 3 years. The compatibility of extractant with base polymer and plasticizer is discussed. Most of PIM studies used cellulose triacetate (CTA) and polyvinyl chloride (PVC) as the base polymer, and only a few studies have used other base polymers. These new base polymers have shown better PIMs in terms of stability and separation performance compared to the CTA- and PVC-based PIMs. Moreover, a new invention of dual PIM separation system has allowed simultaneous separation of multiple metal ions. Such improvement in PIM technology can speed up commercialization process and make it viable for large scale and industrial use especially in hydrometallurgy and wastewater treatment.

Introduction

Industrial applications do not only use heavy metals in their manufacturing process but also produce heavy metals in their discharge wastewater and effluents. Without adequate treatment, these heavy metals will be released into water bodies and cause severe damage to the environment and ecosystem. Ion exchange [1], adsorption [2], and liquid-liquid extraction [3] are among methods used to separate heavy metal compounds from wastewater. However, these methods are often inefficient and costly. For example, liquid-liquid extraction has been widely used in separating heavy metals based on their relative solubility in water and organic solvents. Liquid-liquid extraction is highly risky due to toxicity and flammable nature of organic solvents, especially when they are used at industrial scale [4]. The evolution in membrane development and widely acceptance of membrane technology has developed new technology for treatment of heavy metals in wastewater. Over the last four decades, liquid membranes notably polymer inclusion membranes (PIMs) have been proven to be a better alternative than ion exchange and liquid-liquid extraction methods in extracting and recovering various metal ions [5, 6•]. This is because PIMs can simultaneously conduct extraction and back extraction at the same time unlike liquid-liquid extraction where back extraction only happens after the extraction process takes place. Although PIMs are initially used as sensing component in ion-selective electrodes (ISEs) and optodes, research on PIM-based separation especially in removing heavy metals has become very popular due to their excellent stability and versatility compared to other liquid membrane types especially supported liquid membranes (SLMs) [7••].