Synthesis and adsorption properties of gelatin-conjugated hematite (α-Fe₂O₃) nanoparticles for lead removal from wastewater

Highlights

• Gelatin-conjugated HT NPs is synthesized by a novel precipitation method at low temperature.

• Phosphate ions as inhibitors play a crucial role in phase transformation from ferrihydrite to hematite.

• HT NPs exhibited ultrafast absorption kinetics and high adsorption capacity.

• Easy collection of HT NPs by their magnetic properties result in high reusability.

Abstract

Gelatin-conjugated hematite nanoparticles (HT NPs) are prepared through the solid-state phase transformation in the presence of phosphate. Their adsorption capacity and kinetics are investigated for Pb removal in wastewater. The gelatin-conjugated HT NPs with a size of 4–6 nm exhibit an excellent Pb removal performance, with a high adsorption capacity of 169.49 mg g⁻¹ and a fast equilibrium adsorption kinetics, attributed to the large number of active sites and highly negative charge on the surface of HT NPs. Moreover, the magnetic property of HT NPs enables to selectively collect NPs in the wastewater by using a permanent magnet, leading to its high reusability.

Introduction

Water pollution is the most serious environmental concerns, which can pose a direct risk to living creatures who may consume contaminated water. Heavy metals are regarded as one of the most hazardous water contaminants because they are nondegradable and accumulate in the body, causing serious issues such as brain disorders, anemia, kidney failure, and cancer (Xu et al., 2018, Riaz et al., 2020, Elgaml et al., 2019, Zeng et al., 2019). In particular, lead, one of ten substances which falls under the restriction of hazardous substances (RoHS) directive, is one of the most problematic materials in the world (Deubzer, 2019). Lead pollution is caused by the industrial manufacturing of materials such as metal plating, ammunition, ceramic, glass, and batteries. Recently, perovskite solar cells have taken the spotlight in the solar industry, owing to their unprecedented efficiency improvements. However, lead is a major component of perovskite materials (Park et al., 2020).

The Pb contaminated wastewater should be purified to less than 15 ppb as per the limitation of permissible levels and limits for lead in drinking or wastewater, provided by the World Health Organization (WHO) and Environmental Protection Agency (EPA). To purify Pb-contaminated wastewater, various methods such as chemical precipitation (Sun et al., 2020, Fu and Wang, 2011, Chen et al., 2018), reverse osmosis (Zhang et al., 2018, Li et al., 2017), ion exchange (Chen et al., 2017, Bashir et al., 2019, Dąbrowski et al., 2004), and surface adsorption (Hong et al., 2019, Baby et al., 2019, Maleki et al., 2016) have been investigated. The surface adsorption method, which uses metal oxide adsorbents, is the most efficient way to remove Pb ions because the process is economic, nontoxic, and simple. Recently, novel adsorbents such as doped metal oxides (Chen et al., 2019, Zou et al., 2018), nanostructured metal oxides (You et al., 2016, Sun et al., 2019), and the convergence of organic and metal oxides (Yilmaz et al., 2019) have been designed to enhance adsorption capacity and kinetics. For example, Co-doped Fe₂O₃ nanoparticles exhibited an improvement in adsorption capacity of 136 mg g⁻¹ with an equilibrium time of 30 min (Chen et al., 2019). You et al (You et al., 2016). reported a hierarchically ordered Ca₂SiO₄ nanostructure that exhibited a high adsorption capacity of 458 mg g⁻¹ with an equilibrium time of 2 h. It was reported more recently that polymer-grafted Fe₃O₄ nanoparticles can provide larger active sites by the surface functional groups, which exhibit a high Pb adsorption capacity of 129.65 mg g⁻¹ with an equilibrium time of 108 min (Yilmaz et al., 2019). However, the recollection of metal oxide adsorbents is still challenging as well as adsorption performance.

In this study, we synthesized gelatin-conjugated hematite nanoparticles (HT NPs) with a small size of 4–6 nm by a novel and simple precipitation method, and investigated their adsorption capacity and kinetics for Pb removal. These gelatin-conjugated HT NPs exhibited simultaneous improvement of the adsorption capacity and kinetics. We found that the highly negative charge of HT NPs strengthens its reactivity with Pb, and the conjugated gelatin on the surface HT NPs provides a large number of active sites, such as amino and carboxyl groups, yielding a high purification performance, with a high adsorption capacity of 169.49 mg g⁻¹ and a fast equilibrium time of 10 min