A multi-level porous MgO/biochar composite for the highly efficient adsorption of Pb(II) and Cd(II) from water

Abstract

A MgO/biochar composite (MBC) with a wide range of pore sizes was prepared by a MgCl2-NaOH co-impregnation method using lavender stalks as the biochar source, which can effectively remove Pb(II) and Cd(II) from wastewater. The co-impregnation treatment resulted in a specific surface area of the MBC that was approximately 54 times greater than that of the biochar derived from untreated stalks. The ion-exchange capacity of MBC was increased by the incorporation of MgO nanoparticles, which increased the alkali metal ion (Mg2+) content. These specific structures and compositions gave the MBC a high adsorption capacity for Pb(II) and Cd(II). The adsorption data followed a quasi second-order kinetic model. For Cd(II) and Pb(II), the maximum adsorption capacities of MBC-700 (treated at 700 ℃ for 2 h) reached 520 mg/g and 808 mg/g, respectively. The primary adsorption mechanisms were ion exchange, precipitation, electrostatic attraction and surface complexation. Furthermore, metallic lead was recovered by using the reducing properties of the biochar at high temperatures. This study provides a reference for developing inexpensive and efficient heavy metal adsorbents and the low-carbonization utilization of biomass waste.

1. Introduction

In recent years, the rapid growth of global economy and population has led to expanding industrialization, causing the problem of heavy metal pollution to become increasingly serious. An increasing quantity of heavy metals is flowing into the water bodies on which human beings depend, resulting in many rivers and groundwater reserves with heavy metal levels exceeding safety standards[1−2]. Heavy metal ions are characterized by their stability and persistence, as they cannot be degraded or destroyed in nature. They can enter the human body through the food chain, thus posing a serious threat to human health. Among these heavy metal ions, lead (Pb(II)) and cadmium (Cd(II)) are not only categorized as human carcinogens, but are also the most common contaminants in many industrial applications[3]. The presence of excess cadmium in water or food ingested by the human body increases the likelihood of developing diabetes and kidney disease. Similarly, excessive exposure to and absorption of Pb(II) by human body can result in chronic poisoning, with Pb(II) damaging the body’s central nervous system, kidneys and hematopoietic system, as well as the digestive system, ultimately leading to death[4]. Therefore, developing cost-effective ways to remove Pb(II) and cadmium from wastewater or contaminated surface water is essential for pollution control and environmental remediation.

Currently, the main treatment methods for removing heavy metals from contaminated waters are chemical precipitation, electrochemical and redox methods, ion exchange, solvent extraction, adsorption and membrane separation. Among these techniques, the adsorption method is widely used for the removal of heavy metal ions due to its simple operation, high removal rate, low cost of raw materials and adaptability to various environments[5]. Many adsorbents have been reported for the removal of heavy metals by adsorption，such as activated carbon[6], g-C3N4[7], and graphene oxide[8]. However, most of these materials have low removal rates of heavy metal ions and are expensive and can cause secondary pollution to the environment. Therefore, there is an urgent need to explore novel materials to remove heavy metals from wastewater.

Biochar is a carbon-rich solid material with well-developed porosity, surface functional groups, and also various minerals[9]. In recent decades, agricultural and forestry wastes have been widely used as precursors for the preparation of adsorbents for biochar materials due to their low cost and easy accessibility. Biochar adsorbents derived from agroforestry wastes are a promising alternative for heavy metal removal.