# Testing of Natural Sorbents for the Assessment of Heavy Metal Ions' Adsorption
## 1 Introduction  
Modern intensive industrial production is accompanied by the formation of various man-made wastes. These wastes need to be neutralized and disposed. Wastes containing compounds of toxic chemical elements--heavy metals--are particularly dangerous. Heavy metals are pollutants which can be accumulated in the environment for a long time. Heavy metals are stable to biological and chemical degradation [1; 2; 3; 4; 5].  
Nowadays, high research interest in the detection and sorption of heavy metal ions has been observed, including the development of novel sensors [6; 7] because at relatively low concentrations, heavy metals are dangerous for soil, plants, living organisms and human health [8; 9]. That is why technologies aimed at limiting the mobility of heavy metals are increasingly used [10]. Special functional substances--natural and synthetic sorbents--reduce the mobility of toxic elements by immobilizing heavy metal ions. Sorbents fix the mobile forms of heavy metals and significantly reduce their invasion into the biomass of plants, animals, fish and the human body.  
Currently, the sorption-oriented approach is widely used. It acts as a tool for restoring disturbed land after any anthropogenic activity. This is due to the fact that natural soil remediation from heavy metals is very slow. For instance, the half-lives of lead, copper and cadmium are equal to 740-5900, 310-1500 and 13-110 years, respectively [11].  
However, some of the traditional methods for the deactivation of heavy metals are extremely expensive. In this regard, they are used to isolate certain small contaminatedareas [12]. Therefore, researchers from different countries are currently studying the effectiveness of different sorbents. The core aim is the decontamination of heavy metals in mining and other industries. Natural sorbents are interesting due to their low cost, efficiency and availability of large reserves. They have the functional ability to act not only as sorbents, but also as ameliorants [13; 14; 15].  
Organic raw materials of plant origin, such as peat, coal, sapropels, diatomite, vermiculite, industrial wood residue and agricultural waste present the greatest practical interest for the development of sorbent-ameliorants. The issue of bioremediation, using microorganisms in situ to decompose pollutants, is also relevant. In recent years, this technology has attracted significant attention from scientists and biotechnologists focused on practical research [16; 17].  
As a natural sorbent, diatomic modified with polyhydroxyl-aluminum was researched, particularly its increasing adsorption capacity to Pb\({}^{2+}\) and Cd\({}^{2+}\). The results showed that diatomic modified with polyhydroxyl-aluminum significantly improved the adsorption of Pb\({}^{2+}\) and Cd\({}^{2+}\) by 23.79% and 27.36%, respectively [18].  
There was research about peat's (from the Vale do Ribeira in Brazil) adsorption capacity of lead (Pb) and cadmium (Cd). The obtained results demonstrated that peat could improve the characteristics of soil contaminated with heavy metals. The researchers concluded that soil-peat mixtures were capable of minimizing the potentially toxic metal contamination. In addition, these are affordable and cheap materials [19].  
Numerous studies confirm that adsorbents such as clay, coal, peat moss, zeolite and chitosan are affordable and effective for the remediation of land contaminated with heavy metals. However, despite the significant adsorption characteristics and low cost, there is a lack of a comprehensive studies of the sorption abilities of natural adsorbents [20; 21], except for a few studies conducted a long time ago [22; 23; 24; 25]. This article is an attempt to compare the sorption abilities of common natural substances, including peat, diatomite, vermiculite and their mixtures, in different proportions and physical shapes.