Chlorine removal from agricultural wastewater using sugarcane bagasse magnetic nano biochar

Introduction
Irrigation water salinity is a very serious problem in different parts of the world, especially in arid and semi-arid regions. Increasing fresh-water demand due to population growth causes the pressure on water resources to increase in the future causing the water supply through saline and unconventional water to become a serious issue, especially in areas facing water scarcity. On the other hand, agriculture is the world's greatest water consumer where saline water reduces the products, destroys the soil structure, and damages the environment. Wastewater desalination and water reuse is a relatively new approach in the water industry that solves saline-water problems through various methods. But it is uneconomical due to high equipment costs and energy consumption, especially in agriculture where water consumption is much higher. To remove pollutants, various studies have used different adsorbents such as biochar, activated carbon, zeolite, and resin among which biochar can effectively remove pollutants from aquatic environments because it is an effective, inexpensive, polar, high-porosity adsorbent. Ion exchange, complex formation, surface adsorption, electron sharing, and biochar (carboxylic and pHenolic) - functional group interaction are among various mechanisms where the presence of negative charge on the biochar surface and positive charge on metal ions improve the adsorption process. As the activated carbon is made from cheap materials as wood, coal, oil, coke, sawdust, and plant waste, it is quite economical and highly capable of removing a wide range of organic and inorganic pollutants from aquatic and gaseous environments.
 
Materials and Methods
To prepare biochar, this research used the sugarcane bagasse as primary biomass by 1) washing it several times with ordinary and distilled water and drying it in the open air to remove its remaining salts, 2) crushing it with an industrial mill and placing it in an oven at 60 °C for 24 hrs to remove its excess moisture, 3) grinding the crushed bagasse with a small mill for further milling, 4) passing it through 60 and 100 mesh sieves in two stages for more uniformity and 5) placing it in closed containers. Biomass was converted to biochar (BC) using a heat-programmable electric furnace where the temperature rise was set at five °C/min for a uniform heat distribution. Bagasse was placed inside a steel reactor into which nitrogen gas was injected at a fixed flow rate and prevented oxidation. Biomass was kept at 600 °C for two hrs thereafter the furnace was turned off, while nitrogen gas was injected, and the temperature was slowly lowered to that of the lab. Considering the sizes of the furnace and reactor, each time 20 g biomass was placed in the reactor and about five g biochar was produced after the carbonization process; the biochar production efficiency under these conditions was about 25%. Nano biochar (N-BC) was made by a planetary ball mill with ceramic cups and bullets where the bullet-to-biochar weight ratio was 15-to-one and the rotation speed was 300 rpm. The good mill-activity time was two, four, and six hrs. It worked for three min and rested for one minute to prevent the temperature from rising and cohesive masses from forming in the samples; as size and uniformity of particles were important, use was made of a gradation device.
 
Results and Discussion
In all treatments, by increasing the initial Chlorine concentration, the Chlorine removal had an increasing trend. on average, this was, using activated nano biochar 74.4% more than activated non-nano biochar. Magnetizing nano-absorbents reduced the Chlorine removal by 18.8%, on average. The highest and lowest Chlorine removal reductions due to the adsorbent magnetization were 31.6 and 10.9%, respectively. The highest Chlorine removal in all three activated non-nano, activated nano, and magnetically activated nano adsorbents (200 and 400 W treatments) was measured for an activator-to-biochar ratio of three. According to the results, Chlorine adsorption by magnetically activated nano absorbent reached equilibrium after 480 min in the treatment with 200 and 700 W microwave power and after 540 min in treatment with 400 W microwave power. Increasing the initial Chlorine concentration from three to 25 g l^-1, increased the Chlorine removal by the magnetically activated nano absorbent by three, 3.5, and 2.6 times in 200, 400, and 700 W microwave power treatments, respectively.
 
Conclusion
The pseudo-first-order kinetic model had a good correlation with the data and the pseudo-second-order kinetic model did not correlate well with the data in times less than 60 min; hence, the dominating adsorption mechanism was not chemical in this interval. Intraparticle diffusion was an effective Chlorine-adsorption factor from the beginning of the adsorption process. Considering the correlation coefficient and sum of squared errors, the pseudo-first-order kinetic model and the intraparticle diffusion model had the highest correlation with the measured data. The average correlation coefficient for Langmuir and Freundlich models was found to be 0.9938 and 0.886, respectively. Therefore, the Langmuir isothermal model conformed better to the measured data than the Freundlich model.