ReviewModification of naturally abundant resources for remediation of potentially toxic elements: A review
Graphical Abstract
Introduction
The widespread contamination of aquatic and soil systems by potentially toxic elements (PTEs) has been presenting an increasing global concern for environmental and human health (Zheng et al., 2020, Zheng et al., 2020, Anastopoulos et al., 2019). Cationic PTEs usually refer to metals and anionic PTEs for metalloids. While some PTEs such as copper (Cu), nickel (Ni), zinc (Zn), chromium (Cr), iron (Fe), vanadium (V), manganese (Mn), magnesium (Mg), molybdenum (Mo), selenium (Se), cobalt (Co) and tin (Sn) are essential nutrients in metabolic processes of both prokaryotes and eukaryotes (Marella et al., 2020), they can become toxic when exceeding tolerance levels. Other PTEs like lead (Pb), antimony (Sb), mercury (Hg), cadmium (Cd), arsenic (As) are highly toxic even at low levels of exposure, seriously harming all living organisms (Ahmad et al., 2020, Gadd, 1993). Exposure to PTEs leads to a spectrum of illnesses and diseases for both plants and animals. Their environmental occurrence, industrial production, use, toxicity and carcinogenicity have been well-documented in several reviews (Anastopoulos et al., 2019, Gu et al., 2019, Baby et al., 2019). Pb, Cd, Cr, Hg and As are priority PTEs that pose great public risk given their high degree of toxicity, bioaccumulation and widespread occurrence (Garba et al., 2020). For example, trace levels of Cd in the human body can significantly magnify the risk of kidney damage and prostate cancer (Huang et al., 2018), while Pb can lead to severe neurological disorders and proximal renal tubular damage (Inyang et al., 2016). The presence of As in most plants inhibits their principal metabolic process of photosynthesis, thereby inhibiting growth and often leading to death (Marques and Anderson, 1986).
Historically, most environmental contamination and human exposure associated with PTEs result from unregulated industrial, domestic, technological and agricultural activities including mining (Hamid et al., 2020), electroplating, photo-etching processes (Humelnicu et al., 2020), smelting, and use of agrochemicals such as pesticides and fertilizers (Ahmad et al., 2014). This has markedly increased human exposure to PTEs. For example, Cd is an impurity in phosphate fertilizers, which has led to an increased level of Cd in agricultural land worldwide (Tiller, 1992, Al Mamun et al., 2016); Pb is released to the environment through battery manufacturing and acid mine drainage (Tchounwou et al., 2012); formulations of Cu, Cr, and As (CCA) were commonly used to preserve wood in the timber industry (Beiyuan et al., 2018). The amount of generated pollutants is significant, e.g. it is reported that more than 6000 tons of Hg is emitted into the atmosphere per year (Aguila et al., 2017). As a result, there are now many derelict sites, such as the superfund sites in the USA, where soil PTE concentrations greatly exceed background concentrations (Beattie et al., 2017). According to a joint report issued by Ministry of Environmental Protection and Ministry of Land and Resources of the People’s Republic of China, 19.4% of agricultural soil sampling sites in China are contaminated, mainly by PTEs (Zhao et al., 2015), impacting food quality.
Unlike organic pollutants (Tao et al., 2020, Ma et al., 2020, Ma et al., 2021), inorganic PTEs are non-degradable and tend to bioaccumulate across trophic levels in the food chain. Prager (Prager, 1995) reported that soluble Cd(II) could be concentrated 900–1600 times in the body of shellfish. The presence of PTEs in soil can also greatly slow down the biodegradation of organic contaminants (Wuana and Okieimen, 2011). Therefore, the World Health Organization (WHO), the International Agency for Research on Cancer (LARC) and the United States Environmental Protection Agency (U.S. EPA) have established guidelines for the admissible discharge concentrations of PTEs into the environment (Gu et al., 2019, Tchounwou et al., 2012). The State Council of China has also issued a Soil Pollution Prevention and Control Action Plan, which aims to make 95% of polluted farmland soil safe for human use by 2030 through remediation and risk management measures, as specified in China’s 13th Five-Year Plan (Xu et al., 2017).
To mitigate the adverse effect of PTEs, various technologies for wastewater treatment (chemical precipitation, chemical coagulation, membrane filtration) and soil remediation (phytoremediation, soil washing, electrokinetics, immobilization) have been studied and applied with varying degrees of success (Gu et al., 2019, Carolin et al., 2017, Yin et al., 2019, Fu and Wang, 2011). However, these processes are usually subject to long treatment periods and high cost due to high energy consumptions, large chemical requirements and removal and transport. Adsorption-based functional materials are often preferred in wastewater treatment due to advantages including low energy consumption, simplicity of operation, and in the case of waste water, production of high-quality treated effluents (Garba et al., 2020, Yin et al., 2020, Burakov et al., 2018a). Adsorbent materials can trap trace contaminants from industrial, agricultural and domestic wastewater via physical/chemical confinement and can transform PTEs into less soluble species. PTEs cannot be easily eliminated from contaminated soils (Anemana et al., 2020). In-situ immobilization was developed as a promising approach for the treatment of soil contaminated by PTEs (Guo et al., 2006). This technology aims to reduce the mobility and biological effectiveness of PTEs in soils through addition of exogenous amendments to convert PTEs to less-soluble forms, thereby reducing bioavailability and toxicity (Hamid et al., 2020, Jun et al., 2020). For example, the amounts of dissolved Cd can be decreased by introducing additional cation binding sites in soils, leading to a subsequent reduction in uptake by plants (Al Mamun et al., 2016). Both water and soil remediation processes can reduce PTEs bioavailability. Despite these advantages, large-scale application requires appropriate adsorbent materials that can achieve high-rates of removal and low-cost simultaneously.
To this end, lab-created nanostructured materials and commercial adsorbents has been intensively studied for PTEs remediation, and several excellent reviews are available (Anastopoulos et al., 2019, Gu et al., 2019, Uddin, 2017, Beni and Esmaeili, 2020, Nasir et al., 2019). The abundance, cost and adsorption capacity of those materials are presented in Fig. 1. Although nanostructured materials generally show attractive adsorption capacities and rapid adsorption kinetics (Ahmad et al., 2020, Xiong et al., 2018, Peng et al., 2014, Ihsanullah et al., 2016, Ansari et al., 2017, Ding et al., 2016), their practical application has been hindered by the high cost of precursors, harsh reaction conditions and complex preparation methods (Zheng et al., 2020, Zheng et al., 2020). Commercially available adsorbent materials like silica gel and activated carbon have been used successfully for pollutant removal at large scale, but their use has been limited by their typically high price (Renu et al., 2017, Bashir et al., 2019). This drawback has led to the search for more readily available and less expensive adsorbents. Naturally abundant resources are defined as a range of inexpensive materials that are available across the world. In many cases, they are ideal renewable base materials to trap PTEs (Anastopoulos et al., 2019, Inyang et al., 2016, Pandey, 2017). They may also serve as important sources of plant macronutrients and improve the physical and chemical conditions of soil (Al Mamun et al., 2016). For example, humic acids can stimulate plant growth and enhance soil nutrient storage (Anemana et al., 2020), and biochar can release plant nutrients such as P, K and Ca into soil (Amoah-Antwi et al., 2020, Hossain et al., 2020). Nevertheless, binding affinities of naturally abundant materials towards PTEs are generally weak (Han et al., 2021). Examples of these materials are illustrated in Fig. 1. Development of effective modification methods to create strong ionic or covalent bonds between naturally abundant resources and PTEs may overcome these limitations and this has become a priority research area.
Significant efforts have been dedicated to improving adsorbent materials performance by modifying naturally abundant resources with promising functional materials over recent years. A few review articles have focused on the effect of operational parameters such as pH, temperature, adsorption time, initial concentration, co-existing ions, adsorbent dose in wastewater treatment, together with limited physical analysis including isothermal, kinetics and thermodynamics(Anastopoulos et al., 2019; Uddin, 2017; Beni and Esmaeili, 2020; Nasir et al., 2019). But review articles critically analyzing mechanisms of adsorption pathways and adsorption sites are scarce. Moreover, although reducing mobility of PTEs using adsorbent materials is a promising approach for soil remediation, current reviews of literature on immobilization of PTEs in soil are limited (Amoah-Antwi et al., 2020, Palansooriya et al., 2020, Shakoor et al., 2016). Thus, a review to comprehensively cover the mechanisms of PTEs immobilization from both soil and water is highly desirable. This will provide detailed scientific and technical information around remediation approaches for PTEs, especially in soils. The current review paper aims to summarize relevant research findings in the modification of naturally abundant materials and underlying mechanisms for the mitigation of PTEs bioavailability and toxicity based on surface functional groups and Hard-Soft Acid-Base theory. We will (1) review and compare representative naturally abundant resources and effective modification approaches; (2) outline and elucidate the adsorption mechanisms with a focus on surface functional groups; and (3) present challenges and perspectives of emerging naturally abundant adsorbent materials such as lignite for PTEs mitigation.
Section snippets
Adsorbent materials
Naturally abundant adsorbent resources can be broadly categorized into clay materials (e.g. bentonite and zeolite), biomass and biomass derived products such as biochar. Clay minerals are naturally occurring alumino-silicates formed from weathering of primary minerals and mainly consist of water, alumina and silica (Burakov et al., 2018a, Han et al., 2021, Abegunde et al., 2020). Biomass is renewable organic material that comes from plants or animals (Chi et al., 2020, Zhang et al., 2017).
Adsorption pathways
The pathways by which PTEs are adsorbed can be divided into chemisorption (ion-exchange, surface complexation, precipitation, hydrogen bonding, coordination and cation-π interactions) and physisorption (electrostatic interaction and non-specific van der Waals forces). Physisorption is weakly specific and reversible whereas chemisorption is selective and not readily reversible (Burakov et al., 2018). While electrostatic forces aid in driving the physical movement of PTEs at a certain distance
Adsorption sites
From a mechanistic point of view, high surface area and pore volume of adsorbent materials are highly desirable. Micropores tend to promote physical adsorption by increasing surface area, whilst mesopores can enhance contaminant diffusion to accelerate adsorption kinetics (Hsieh and Teng, 2000). Surface area and porosity, however, are not the only requirements for an adsorbent material to achieve PTE adsorption. Strong binding between PTE ions and adsorbent materials is preferred especially for
Emerging naturally abundant resources and modification methods
Although numerous adsorbent materials have been investigated, low-cost and effective options are still scarce. Lignite, also known as brown coal, is formed from peat at temperatures lower than 100 °C under reduction conditions (Anemana et al., 2020). It is abundant in nature and deposited at shallow depths of the earth surface, and thus less expensive to be mined than other high-rank coals and clay materials (Amoah-Antwi et al., 2020, Han et al., 2021). Depending on the region from where
Conclusion and future research
As one of the most toxic inorganic contaminants, PTE occurs widely in soils and water systems. Naturally abundant resources, endowed with well‐tailored modification methods, are important adsorbent materials for PTE adsorption. Although bentonite and zeolite exhibit relatively weak affinity with PTEs, this can be improved by coating inorganic metal oxide and organic modifiers. The performance of biochar and biomass can be improved by applying treatment by acid, base and oxidizing chemicals
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by Cooperative Research Centres Projects (CRC-P) ‘Optimising Nitrogen Recovery from Livestock Waste for Multiple Production and Environmental Benefits", Victoria-Jiangsu Innovation and Technology R&D Fund “Development of a novel carbon-based Trichoderma bio-organic fertilizer and establishment of integrated agricultural technologies for improving arable land quality using the developed novel bio-organic fertilizer”, National Natural Science Foundation of China
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