Removal of Direct Blue 71 and chromium from aqueous solutions by metal coating organic adsorbents, metal coating biochar and biochar-metal composite

Introduction
Widespread entry of effluents, which are toxic and non-biodegradable, from factories and various industries into the environment, and accordingly, pollution of water and soil resources lead to many dangers for humans and other organisms. Therefore, modification of these resources is important. Currently, multiple technologies from the physical, chemical, and biological perspectives have been established for the remediation of contaminated water. However, most of them involve energy consumption, high-cost instruments, low efficiency, complicated implementation, or secondary pollution. Therefore, it is critical to develop more convenient, economic, and environmentally harmonious strategies for the decontamination of polluted water. Biochar is an emerging material that is manufactured by the decomposition of carbon-rich biomass under oxygen-limited pyrolysis. Remarkable progress has been made in the understanding of biochar as an environmentally friendly and low-cost material for carbon sequestration, energy recovery, contamination relief, and nutrient supplementation. In recent decades, biochar has gained significant attention in the remediation of contamination in terrestrial and aquatic environments. However, biochar only has limited adsorption ability to anionic contaminants in water. This is because biochar often has a negative surface charge, hindering it to absorb negatively charged compounds such as Cr(VI) and Direct Blue71. Various modification methods thus have been developed to improve its affiliation to anionic contaminants by introducing metal oxides onto the carbon surface within its pore networks. Studies showed that the application of biochar and metal-coated biochar, effectively leads to the removal of significant amounts of contaminants from water and soil, but so far the impact of metal-carbon composite on the removal of contaminants, especially anionic contaminants has not been reviewed. Therefore, the purpose of this research is to produce different types of biochar-metal composites, to investigate the effectiveness of different types of composites in removing Direct Blue 71 and chromium from aqueous solution, and also to compare the composites with: plant biomass, metal-coated biomass, biochar, and metal coated biochar.
 
Materials and Methods
In this study, the effect of plant biomass, metal-coated biomass, biochar, metal-coated biochar, and metal-biochar composite at two temperatures (300 and 600  ̊C) on the removal of Direct Blue 71 and Chromium contaminants from the water was investigated. The biochar used in this experiment was produced from the thermal decomposition of rice straw, which is abundant in the region. At first, the sample was passed through a 2 mm sieve, then they pyrolyzed at 300 and 600  ̊C for three hours. Metal-coated biochars and metal-biochar composites were prepared from the combination of metals (manganese, zinc, copper, iron, and aluminum) with a concentration of 10,000 mg L^-1 with agricultural residues (rice straw) as a raw material or biochar. The samples were mixed with metals with a ratio of 1:50 (1 gram of sample, 50 mL of metal solution) and shaken for 24 hours. Then the samples were filtered and oven dried at 70  ̊C. After the preparation of adsorbents, a specific amount of adsorbents and pollutants with a concentration of 20 mg L^-1 were combined and shaken for three hours until they reached equilibrium. All the samples were centrifuged for 5 minutes at 6000 rpm. After filtration, the final concentration of pollutants was determined and the removal percentage of Direct Blue 71 and chromium was calculated.
 
Results and Discussion
Based on our results, the application of different adsorbents has a significant effect on the removal percentage of Direct Blue 71 and chromium from the water. Our data showed that high-temperature adsorbents were more efficient in removing Direct Blue 71 and chromium. For example, by increasing the biochar pyrolysis temperature from 300 to 600 °C, the Direct Blue 71 removal percentage has increased from 10.733 to 63.695 %. According to the results of the current study, it can be observed that covering the agricultural residues and biochars with metals has been able to increase the efficiency of the adsorbents in pollutant removal due to the creation of a cationic bridge. In general, the results of this study showed that the application of aluminum and iron composite and aluminum and iron coated biochar produced at 600°C was beneficial in the remediation of contaminated water and these adsorbents could remove 98.303, 88.847, 98.302 and 96.777 % of Direct Blue 71 and 97.983, 78.733, 96.75 and 92.167 % of chromium pollutant from the aqueous solution, respectively. Therefore, the application of these adsorbents can be useful to modify water polluted with these contaminants.
 
Conclusion
According to the results of this research, it can be observed that the addition of metal-coated biochar and biochar-metal composite to water has reduced the amount of direct blue 71 and chromium pollutants, so their use in water contaminated with these pollutants can be beneficial. Organic materials and biochar are among the adsorbents that are widely used to reduce pollutants from water and soil, but anionic pollutants are not well absorbed due to the dominant negative surface charge of these adsorbents. Therefore, it seems that for more effective use of organic adsorbents, it is necessary to combine these materials with metals or other materials so that their absorption capacity increases and they can be effective in removing anionic pollutants. The composite has a new physical and chemical nature compared to Biomass which is pyrolyzed alone (biochar). Even composite can be significantly different from metal-coated biochar.