Adsorption of nitrate from aqueous solution by biochar and Fe–coated biochar

Introduction
Nitrate pollution in groundwater and drinking water reservoirs has increased alarmingly in different parts of the world. The high concentration of nitrate in surface and groundwaters is due to the excessive use of chemical fertilizers and improper disposal of wastes caused by human activities and animal manure. Due to its high mobility, nitrate anion is easily washed from the soil and enters the surface and groundwaters. If the concentration of nitrate exceeds the limit (50 mg l^-1), it causes the disease of children with methemoglobinemia, as well as the formation of carcinogenic nitrosamines. Various methods have been proposed to remove nitrate. These methods besides having side effects on water, are not economically viable. In recent years, the development of effective technologies for keeping nitrates in the soil has received much attention. Adding biochar to the soil is one of the effective ways to reduce nitrate leaching. Biochar is a carbon-rich and porous substance, that is produced by heating biomass such as organic waste, animal manure, plant residues, sewage sludge, wood, etc. in limited or oxygen-free conditions. Due to its high specific surface area, high porosity, and diverse functional groups, biochar increases the water retention capacity, cation exchange capacity, and surface absorption capacity after adding it to the soil. Therefore, this research aims to investigate the effect of biochar and biochar coated with trivalent iron on the amount of nitrate absorption from aqueous solution.
 
Materials and Methods
Biochar can be produced from materials with low economic value and is a suitable and inexpensive adsorbent for nitrate removal from water sources. According to the studies conducted for biochar production, the temperature and duration of storage in the furnace are the most important factors controlling the quality and strength of biochar in nitrate removal. In this research, four types of rice straw, rice husk, sugarcane bagasse, and dicer wood chips were used to produce biochars. First, the samples were passed through a 2 mm sieve and dried in an oven at 70°C for 24 h. Then they were converted to biochar for 3 h at 300 and 600°C in an electric furnace under oxygen-free conditions. To determine the best adsorbent with maximum nitrate absorption, 0.5 gr of each adsorbent was weighed and poured into a 50 ml centrifuge tube. Then, it was contacted at a constant time (60 min) at an initial concentration of 50 mg l^-1 of nitrate solution. After determining the best adsorbent, kinetic experiments were done to determine the equilibrium time, optimum pH, and adsorbent dosage. The adsorption isotherms were conducted for soil, rice husk 300˚C (RSB 300), and Fe-coated RSB 300.
 
Results and Discussion
The results showed that among the eight types of biochar produced at two temperature conditions of 300 and 600 ˚C, RSB 300, with the initial concentration of nitrate solution of 50 mg l^-1 and contact time of 60 min, had the most amount of nitrate absorption. The kinetic experiments were continued on this type of biochar. The kinetic experiment results showed adsorption nitrate with an initial concentration of 50 mg l^-1 an equilibrium time of 90 min, pH 7, and an adsorbent dosage of 1.25 g l^-1 was 23580 mg kg^-1. The result of the adsorption isotherms study showed that the adsorption of nitrate on RSB and Fe-coated RSB were fitted to the Langmuir isotherm model. This result indicates the uniform or single-layer distribution of active sites on the absorbent surface. The maximum adsorption capacity of nitrate by RSB and Fe-coated RSB were 38.16 and 43.66 mg g^-1, respectively.
 
Conclusion
The use of cheap absorbents can be a suitable solution for removing environmental pollution. In general, biochar can absorb pollutants and nutrients by its potential physicochemical properties, including high specific surface area, high porosity, high cation and anion exchange capacity, high surface charge density, low volume mass, and the presence of functional groups. The results showed that among the eight types of biochar tested, RSB with the initial concentration of nitrate solution of 50 mg l^-1 and contact time of 60 min, had the highest absorption rate. The optimal conditions for nitrate absorption are estimated at 90 min of contact time, pH 7, and adsorbent dosage of 1.25 g l^-1. The results showed that wastewater treatment by surface absorption process using biochar produced from vegetable waste is a very useful and effective method. Besides, the results of isotherm adsorption on the nitrate adsorption test data by biochar produced from RSB and Biochar Fe-coated RSB showed that nitrate adsorption on these adsorbents according to its correlation coefficient (R²=0.994) is consistent with the Langmuir isotherm model. The maximum absorption capacity RSB is 38.16 mg l^-1, which is more absorbable than other studies. Now, when the above biochar was coated with Fe, the maximum nitrate absorption capacity increased by 43.66 mg g^-1, which is a very high absorption. It can be concluded that RSB, especially when it has a Fe-coating, is a suitable adsorbent for removing nitrate from water. Therefore, it is suggested to investigate the effect of biochar covered with different cations on the mobility of other pollutants that are in anionic form.