**2. Some of the heavy metals**

#### **2.1 Mercury**

Mercury is a naturally occurring metal that comes in a variety of shapes and sizes. Metallic mercury is a lustrous, silver-white liquid with no odor. Mercury forms inorganic mercury compounds or salts, which are usually white powders or crystals when it reacts with other elements like chlorine, sulfur, or oxygen [24]. Mercury enters the brain quickly, causing tremors, sadness, and behavioral issues [25]. Thousands of years have passed since the beginning of time. It's mostly used for precious metal extraction. Mercury (Hg) is used in thermometers, barometers, manometers, sphygmomanometers, float valves, mercury switches, mercury relays, and fluorescent lights, as well as in the paint industry [26]. In New Jersey, USA, levels of mercury above 0.5 ppm were found, a level of human health concern for those who consume fish regularly. 48.8% of the sampled population of 36,422 lakes in the USA had mercury tissue concentrations that exceeded 0.3 ppm [27]. For many years, mercury, which is on the US EPA's priority pollutants list, has received a lot of attention [28].

Among the various methods developed over the years for mercury removal, adsorption has substantial promise due to its simplicity and low cost, as well as the adsorption method's efficacy in purifying water [29]. We have observed substantial progress in Hg removal in adsorptive separation technology and materials science over the last 4 years [30]. The Hg(II) removal efficiencies increased in the order of ACCl impregnated ACBr-impregnated ACI-impregnated AC, indicating that the introduced functional groups' electron-donating ability may help increase the adsorption capacities of the adsorbents [30]. There are several promising advantages for the removal of aqueous Hg(II), such as rapid separation, easy elution of analytes, and reusable adsorbent [31]. Existing adsorbent materials have largely been limited in their effectiveness and efficiency for the removal of Hg(II) from aqueous solutions due to challenges such as low surface area and improper distribution of thio/thiol groups, resulting in low capacity and moderate affinity for Hg(II), sulfur leaching, and poor stability over a wide pH range [31].

## **2.2 Cadmium**

Cadmium is a silver-white metal with a density of 8.7 g/mL and an atomic weight of 112.41 g/mol. Its main industrial applications are governed by its strong electrical conductivity, good chemical resistance, and low melting point [32]. Cadmium is released from a wide range of sources, including galvanized pipeline corrosion, erosion of natural deposits, discharge from metal refineries, runoff from waste batteries, and mining, smelting, and refining of nonferrous metals [33]. Cd impacts the brain by disrupting particular membrane function, primarily in the hippocampus, according to reports. Neurotoxicity is known to be higher in newborns than in adults, which could be owing to the absence of blood-brain barrier maturation in newborns. Cd accumulates greater in the choroid's plexus region of the brain in newborns. In prenatal exposure to Cd inhibits the acetylcholine esterase (AchE), Na+ /K+ -ATPase pump, which lowers neuronal activity in pups, according to in-vivo investigations [34]. Cadmium is one of the most toxic heavy metals, and this has piqued the attention of environmentalists. The primary sources of cadmium discharge into the environment through waste streams include electroplating, smelting, alloy production, pigments, plastic, battery, mining, and refining activities [35].

Compared to virgin biochar, biochar treated with MnO2 has more hydroxyl groups, a bigger surface area, and a higher pore volume. They also verified that Cd(II) complexation with hydroxyl groups produces Cd▬O or Cd▬OH species, which is the major mechanism for Cd elimination [24]. The batch and column desorption experiments were carried out under the same conditions. One gram of adsorbent was used to treat 50 milliliters of sample water containing 20 mg of Cd(II). It was left in contact with the adsorbent for 24 hours. Then, as detailed in a previous paper [36] investigated the effects of pyrolysis temperature on biochar Pb removal ability [24].

### **2.3 Chromium**

Chromium is an element that can be found as a liquid, solid, or gas in rocks, animals, plants, and soil. Chromium(VI) compounds are carcinogens and toxins, but chromium(III) is a required vitamin. Long-term exposure can affect the liver, kidneys, circulatory system, and nerves, as well as cause skin irritation. High levels of breathing can cause nasal irritation, nose ulcers, runny nose, and breathing problems such as asthma, cough, shortness of breath, or wheezing [37]. The use of cellulose ion exchangers for water and wastewater treatment was investigated. The ability of three cellulose ion exchangers to remove proteins, azo dyes, chromate, and heavy metal ions was investigated [38].

As a result, the elimination of Pb(II) and Cr(VI) has been recognized as a crucial environmental issue. To remediate Pb(II) and Cr(VI), mineral adsorption, coagulation, chemical precipitation, ion exchange, biosorption, chemical reduction, and membrane separation have all been developed [20].
