Drainage Water Sodium Removal by Biochar

Wastewater desalination and its 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 and costs are much higher. The irrigation water salinity is a very serious problem in different parts of the world, especially in arid and semi-arid regions. The increased fresh-water demand due to the population growth will cause the pressure on water resources to increase in future causing the water supply through saline and unconventional water to become a serious issue, especially in areas facing water scarcity.

As reuse of the agricultural wastewater reduces the pressure on water resources and improves the environmental conditions, and some field wastewater is rich in sodium, this research has studied the sodium removability by sugarcane bagasse sorbents

Materials and Methods

To prepare biochar, this research used the sugarcane bagasse as a 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 5 ° 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 it from oxidation. Biomass was kept at 600 ° C for 2 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 5 g biochar was produced after the carbonization process; the biochar production efficiency under these conditions was about 25%.

Nano biochar (N-BC) was produced by a planetary ball mill with ceramic cup and bullets where the bullet-to-biochar weight ratio was 15-to-1 and the rotation speed was 300 rpm. The good mill-activity time was 2, 4 and 6 hrs and it worked for 3 min and rested for 1 min to prevent the temperature to rise and cohesive masses to form in the samples; as size and uniformity of particles were important, use was made of a gradation device.

To prepare the biochar, this research used the sugarcane bagasse as a 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.

Saline samples were taken from sugarcane drains where the sodium content was about 4 g/l and the amount of sodium in samples was set to be 2, 4 and 8 g/l using sodium nitrate and deionized water.



Results and Discussion

In all treatments, increasing the initial sodium concentration increased the sodium removal, which was, on average, 74.4% more by activated nano biochar than activated non-nano biochar. Magnetizing nano-absorbents reduced the sodium removal by 18.8%, on average. The highest and lowest sodium removal reductions due to the adsorbent magnetization were 31.6 and 10.9%, respectively. The highest sodium removal in all three activated non-nano, activated nano and magnetic activated nano adsorbents (200 and 400 W treatments) was measured for an activator-to-biochar ratio of 3. According to the results, sodium adsorption by magnetic 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 sodium concentration from 3 to 25 g/l, increased the sodium removal by the magnetic activated nano absorbent by 3, 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 sodium-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. Therefore, the Langmuir isothermal model conformed better to the measured data than the Freundlich model.