# Removal of Nitrate Ions Using Thermally and Chemically Modified Bioadsorbents
)1 Process Design and Biomass Utilization Research Group (IDAB), Chemical Engineering Department, University of Cartagena, Avenida del Consulado St.  
in water due to their high solubility in this medium and reach the human water supply, and they are difficult to detect because they are odorless and tasteless, but are potentially toxic. The WHO established the maximum acceptable daily intake level of NO\({}_{3}\) as 0.3-0.7 mg/kg. For human water supplies, a maximum level of 50 mg/L was set as well as a quality indicator value of 25 mg/L [6]. NO\({}_{3}\)\({}^{-}\)-contaminated water consumption is a serious global concern, especially in regions where available water sources are scarce. That is because exposure to high concentrations of nitrate can cause miscarriages, cancers (e.g., stomach, esophagus, bladder, kidney), and various ecological problems, including eutrophication, harmful algal proliferation, hypoxia/anoxia and loss of aquatic biodiversity [7]. Nitrate levels ranging from 17 to 120 mg/L have been reported in the Peddavagu region (South India) [8], 0.85-11.31 mg/L in the Yamuna River (India) [9], 1.17 mg/L in the Siriri River (Brazil) and 27.77 mg/L in the Ravi River (Pakistan) [10], showing a correlation between the number of inhabitants near water sources and the deterioration of water quality [11]. In Colombia, the Ministry of Health and Social Protection (MPS), the Ministry of Environment and Sustainable Development and the Ministry of Housing, City and Territory of Colombia regulate through Decree 1575 of 2007 [12] and Resolution 2115 of 2007 [13] the characteristics, basic instruments and frequencies of the water quality control and monitoring system with recommended levels in drinking water for human consumption. Between 2017 and 2018, values of 0.85 mg/L have been reported in samples taken at the outlet of a treatment plant in Santander [14], 0.62 mg/L at Gambote, 1.02 mg/L at Sincerin [15] and 8.46 mg/L in Matute Stream [16] in water for human consumption. Therefore, there is a need to remove these nutrients in excess from water. A variety of common methods for removing dissolved nutrients from water and wastewater have been investigated, including biological methods, ion exchange, adsorption methods, chemical precipitation, electrocoagulation and membrane processes [17]. Adsorption has been widely explored as a nitrate removal technology and offers satisfactory results, especially with mineral-based or surface-modified adsorbents [18]. Recently, the development of high-efficiency and environmentally friendly adsorbents has become one of the main topics in the field of water treatment. Biochar has been gradually applied as a low-cost adsorbent in water treatment to successfully remove contaminants from wastewater [19; 20]. That is because it is a carbon (C)-rich material formed through the thermal decomposition of biomasses, including agricultural and forestry wastes, municipal sludge, manure and other C-rich materials [21]. The characteristics of biochar that make it feasible for sorption include, among others, large specific surface area, well-developed pore structure, high C-content, large oxygen-containing functional groups, high cation exchange capacity and the presence of metallic elements in its structure [22; 23]. Previous studies have examined nitrate adsorption using biochar made from various agricultural and forestry residues; when evaluating biochar prepared from rice hulls, obtained by microwave-assisted pyrolysis, a 75% nitrate removal efficiency was achieved [24]; biochar prepared with corn cob at 600 \({}^{\circ}\)C showed a nitrate adsorption capacity of 14.46 mg/g [25]; and by using beechwood sawdust as an adsorbent, it was found that by increasing the initial concentration from 1 to 20 mg/L, the nitrate removal capacity increases. These experiments show the effectiveness of biochar-based sorbents for the removal of nitrate in an aqueous medium.