**1. Introduction**

With the international scale of rapid industrialization and urbanization, many developing and emerging economies have exploited local natural resources. These activities are energy-intensive, associated with significant interventions in the natural ecosystem including the water balance, which leads to emissions of pollutants to water, soil, and air. Metal pollution is one of the most serious and frequently encountered problems. For example, the River Ganga in India, which is considered sacred by Indian society, has serious pollution from Mn, Cr, Pb, Cd, and other potentially toxic elements (PTEs) [1]. Pollutants, such as Cd, Cu and Pb, in the Red River in Vietnam, are also much higher than the local discharge standards, and the maximum enrichment factor for Cd is 19.3 [2]. China's rapid industrialization has also led to a severe deterioration in water quality in the country's lakes and rivers. More than 80 percent of Chinese rivers and lakes, including seven key river systems, are contaminated with different types and to different degrees of PTEs with As, Cd, Hg, and Pb being the most frequently detected in these rivers [3–5]. Soils in China also suffer from high degrees of contamination. A report of the national survey of soil contamination of China, which was published in 2014, showed that the exceedance of environmental standards for Cd, Hg, As, Cu, Pb, Cr, Zn, and Ni in soil samples reached 7.0%, 1.6%, 2.7%, 2.1%, 1.5%, 1.1%, 0.9%, and 4.8%, respectively [6].

The persistence of PTE pollution has the potential to impact on both the human and wider ecological environment because of their long residence time and the potentially toxic impact through biological amplification [7,8]. PTE enrichment in the human body through the food chain eventually destroys the normal function of proteins and enzymes in the body, and high concentrations can form more toxic compounds, which do grea<sup>t</sup> harm to organisms [9,10]. Acute toxicity mainly affects the normal function of a particular organ and can damage or destroy the reproductive organs with intergenerational influence on child health [4,11].

Many techniques including adsorption, precipitation, biological treatment, ion exchange, and membrane separation are used to deal directly with PTE pollution [12–14]. Among them, adsorption, which refers to the adsorption of metals by means of intermolecular force or electron transfer and electron pair bonding [15], is widely used in pollution control because it is cheap and easy to apply and operate and systems are often reusable. Adsorption processes often refer generically to removal of target pollutants, which may also include other mechanisms such as ion exchange (the exchange of aqueous pollutant ions with available surface ions on the solid phase) and precipitation (where solution conditions exceed solubility conditions for specific species). Many kinds of adsorbent materials have been applied for the removal of soluble pollutant metals such as activated carbon, and modified complex materials such as metal ferrite doped carbon [16], and metal organic framework systems [17]. Capacities can be quite high, for example, 200–300 mg/g for Hg on modified carbon and similar range for, for example, U and Th on metal organic framework materials. However, applications are often limited due to its relatively high synthesis costs. Naturally derived clay minerals like kaolin, zeolite, sepiolite, bentonite, and perlite have also been utilized as alternative low-cost adsorbents for remediation of metal polluted environments. Whilst adsorption capacities for, for example, sepiolite may be an order of magnitude lower than synthetic systems [18] (there are many examples of studies of these materials for the treatment of metals by adsorption that show useful level of performance [19,20]. The focus for the future development of adsorption based systems should be on identifying adsorbent materials that are cheap and effective adsorbents in the context of the treatment scenarios (for example, at point source or to deal with diffuse pollution), to improve the effectiveness of these treatments and to ensure that no secondary pollution is produced [21–24]. Secondary tasks include the need for good solid/water separation and the regeneration of adsorbent. In this review, we focus on the potential of one mineral system in the context of a local demand for treatment, potentially supported by locally derived materials, which fits with the circular economy principles of resource use and efficiency. There are a number of serious regional pollution problems, within the Central Southern Chinese province of Hunan. It is rich in mineral resources, which have been extensively exploited over recent decades, compromising its significant contribution to food production from a strong agricultural sector. In addition to extensive base metal deposits, one of the regions' other significant mineral resources is sepiolite, a clay mineral with already widespread industrial and manufacturing applications, located in Xiangtan City, Hunan Province. We focus our review to consider work describing mechanisms for the removal of locally relevant metal pollutants, the modification and regeneration steps from the view of economic efficiency and resource sustainability. We identified a number of published studies of this topic, presented in Chinese literature and academic repositories. Our review, therefore, also provides wider international access to relevant data, which will be of benefit to similar locations worldwide.

### **2. Metal Pollution in Hunan, China**

It is well known that there are abundant reserves of non-ferrous metals in Hunan Province, and most ores for mining, mineral processing, and smelting of non-ferrous and rare earth elements are located in the Xiangjiang Valley. The Xiangjiang River, which cuts across Hunan Province, is a main water resource for drinking water, process water, and the irrigation of crops. Because

### *Int. J. Environ. Res. Public Health* **2018**, *15*, 1653

of prolonged mining and smelting activities for non-ferrous metals, wastewater has been discharged to the surrounding environment and the Xiangjiang Valley is the most infamous polluted area in central China [25]. Water, soils, and crops in Xiangjiang River basin are heavily contaminated by Cd, Hg, Pb, as well as As [26,27]. The "12th Five-Year" Plan for Comprehensive Prevention and Control of Heavy Metal Pollution indicates that the five major PTE pollutants in China are identified as Pb, Hg, Cd, Cr, and As [28]. In 2015, the state statistics for "the discharge of major pollutants in regional wastewater", Hg, Cd, and As in Hunan province account for 20.3%, 37.9%, and 32.6% of the total emissions, respectively [29]. The "11th Five-Year Plan" Xiangjiang River basin water pollution prevention and control plan of Hunan province reported that the pollution in Xiangjiang is predominantly caused by Hg, Cd, Pb, As, Zn, and others [30]. The monitoring data for the main pollution indicators in the Xiangjiang are shown in Table 1 with Figure 1 summarizing the geographical distribution of these sources/effects.

**Figure 1.** Geographical distribution of the main PTE pollution sources in Xiangjiang Valley. Data from references in Table 1.


