# Characteristics of Removal of Lead, Cadmium and Chromium from Soil Using Biosorbent and Biochar
## 1 Introduction  
Heavy metals such as chromium (Cr), lead (Pb), cadmium (Cd), mercury (Hg), arsenic (As), copper (Cu), zinc (Zn), and nickel (Ni) are considered major hazardous trace elements due to their high toxicity, long residence time in the environment, and persistent bioavailability. Their presence in soil can result from various sources, such as weathering of parent rocks, mining, vehicular emissions, smelting, the use of chemical fertilizers, and industrial, agricultural, and commercial activities [1, 2]. Heavy metal contamination in soil is persistent and can have detrimental effects on soil fertility, microbial activity, biodiversity, crop yields, and thus human health [3]. Soil contamination with heavy metals has garnered widespread interest globally, making the monitoring and control of these pollutants a priority. Chronic exposure to heavy metals can lead to serious health issues such as lung cancer, bone fractures, kidney dysfunction, and impairments of the immune, nervous, and hormonal systems [4]. Since the 20 th century, rapid economic and social development has increased the demand for land resources due to human activities. Industrial production, fossil fuel combustion, metal extraction, the use of pesticides and fertilizers in agriculture, and wastewater discharge have contributed to soil degradation. The consequences of these activities include soil erosion, desertification, salinization, pollution, and a decline in soil fertility. Heavy metalcontamination, characterized by long-lasting persistence, irreversibility, low mobility, severe toxicity, complex chemical properties, and ecological reactions, has become the most serious soil pollution problem [5]. The fundamental approach to protecting agricultural land and products is to trace the sources of heavy metal contamination and develop and implement appropriate mitigation and prevention measures. However, due to the complexity of heavy metal sources in soil and the influence of numerous parameters on their migration and accumulation, as well as the significant spatial variability of soil properties, these challenges are substantial. To mitigate the risks associated with heavy metals in contaminated soils, significant efforts have been devoted to developing effective remediation technologies such as soil washing, excavation of contaminated soil, and physicochemical, chemical, phytoremediation, and bioremediation methods. Physicochemical remediation involves changing the soil's pH, redox conditions, or ionic composition by adding appropriate amendments to achieve heavy metal immobilization. Phytoremediation involves enhancing plants' tolerance to heavy metals and their ability to remove them. Bioremediation utilizes microorganisms to absorb and precipitate heavy metals or reduce their toxicity through redox reactions [6; 7]. One of the efficient and eco-friendly approaches to reclamation is the use of biochar, produced through the pyrolysis of biomass or plant or animal manure at temperatures ranging from approximately 200 to 700 \({}^{\circ}\)C under limited oxygen conditions [8; 9; 10]. Biochar improves soil properties, such as increasing pH, water retention, and promoting the decomposition of organic compounds, and can effectively reduce the mobility and bioavailability of heavy metals in contaminated soils, thus contributing to enhanced plant growth [11; 12]. Biochar, a solid product of the pyrolysis process, is characterized by a high carbon content (65-90%). It has a fine-grained and porous structure and features oxygen-containing functional groups and an aromatic surface [13]. Due to the extensive microporous structures on its surface and the resulting large specific surface area, biochar is a practical adsorbent [14]. Wood is a widely used material in many areas, such as agriculture, construction, manufacturing, and consumption. Sawdust is mainly composed of cellulose, and also contains soluble sugars, acids, resins, oils, waxes, and other organic substances that affect its physicochemical properties.