Incorporation of Functionalized Halloysite Nanotubes (HNTs) into Thin Film Nanocomposite (TFN) Nanofiltration Membranes for Heavy Metal Removal from Wastewaters

Abstract

Global water scarcity is an enormous and yet imminent challenge for our time. Industries such as machinery manufacturing, metallurgy, etc., generate increasing volumes of wastewaters containing heavy metals. These wastewaters are often directly or indirectly discharged into the environment. Heavy metals are not biodegradable; therefore, they can accumulate in the body, creating severe health problems such as cancer, nausea, or even death. It is necessary to remove heavy metals from wastewaters before their discharge to the environment. There are different methods to do that. Among them, membrane separation technology is a promising method for wastewater treatment. In particular, nanofiltration (NF) membranes, due to their rejection mechanism (size exclusion and charge repulsion), are most suitable to remove s heavy metals from wastewater. A new class of high-performance semipermeable membranes for reverse osmosis (RO), nanofiltration (NF), and forward osmosis (FO) applications are thin-film nanocomposite (TFN) membranes. In this work, novel NF TFN membranes were fabricated by incorporating the functionalized halloysite nanotubes (HNTs) with the first generation of poly(amidoamine) (PAMAM) dendrimers (G1). Also, the same membranes were used for the removal of heavy metals in FO separation. The combination of FO and NF processes represents the novel two-stage FO-NF hybrid process for heavy metal removal. The membranes showed high rejections of Na2SO4 (97%-98%) and MgCl2 (82%-90%) in NF tests. Although lower than Na2SO4, the rejections of MgCl2 are much greater than those reported in the literature. The remarkable rejections of MgCl2 are attributed to positively charged HNTs-G1 nanoparticles incorporated in the selective polyamide (PA) layer of TFN membranes. The FO experiments using MgCl2 as a draw solution revealed excellent rejections of Cu2+ and Pb2+ by TFN membranes ranging from 95% to 98%. The presence of heavy metal in the feed solution enhanced the FO performance of the membranes. In particular, the reverse flux of draw solute (MgCl2) decreased by at least 2.5 times compared to the experiments with pure water as a feed. Simultaneously, the water flux also increased. The improved FO performance in the presence of heavy metals is attributed to their adsorption by the membranes.