Concentrate treatment via membrane distillation/crystallization method

Abstract

Boron is an important semimetal element that is found in water, soil and rocks. Boron is widely used in a variety of industries such as glass, semi-conductors, detergents, cosmetics, drugs and fertilizer. Boron is also an essential nutrient for animals, plants and humans that needs to be found in dietary intake. Although it is beneficial the excessive amount of boron can lead to some health problems for humans and growth problems or death for plants. The boron content in wastewater is increasing due to the increasing industrial activities. Boron can also be found naturally from rocks, salt deposits or rainfalls and approximately 75% of the boron reserves of the world are found in Turkey. In the second chapter of this thesis study a literature review was prepared comprehensively. Boron removal and recovery techniques were reviewed. The increasing demand for clean water resources makes boron removal from water resources much more important. However, the removal is not enough alone in the aspect of sustainability. The removal and recovery techniques both conventional methods like precipitation, adsorption, coagulation, ion exchange and membrane processes are examined to juxtapose the states of the science in these two related—and increasingly important—processes. In the third chapter, the optimization studies of the membrane distillation process used in the thesis study were conducted. 2 different MD configurations, 3 different vacuum pressures, 6 different membranes and 3 different feed water were used to determine the optimum conditions. VAGMD had the higher fluxes since vacuum assistance enhanced fluxes while decreasing specific energy consumption. The boron content in permeate waters was lower than the 0.5 mg/L. In the fourth chapter hydrophobic and superhydrophobic membrane production methods were investigated. Different fabrication techniques as phase inversion and electrospinning were utilized while coating was applied to modify commercial and fabricated membranes. Different coagulation bath mediums for phase inversion method, nanoparticle additives and different coating solutions were compared. It was possible to obtain a superhydrophobic membrane that has a contact angle of 153. A novel hydrophobic blend membrane was fabricated to use in VAGMD within the scope of the fifth chapter. NaCl, synthetic boron solution and real RO concentrate supplied from the boron mining area were tested. A fluoro-containing benzoxazine monomer (Bz) was blended with our base polymer PVDF. The cross-linking of Bz is provided by thermal curing. According to the results, the BisF-Bz membrane showed higher hydrophobicity, more durability and physical stability compared to the pristine membrane. Boron rejection was highest for the BisF-Bz blended and two times thermally cured nanofiber membrane. In chapter six VAGMDC system was utilized to remove and recover boron from the concentrate stream. The concentrate problem is the main drawback of membrane processes so needs additional treatment and management. To address this problem VAGMDC system was tested. Synthetic boron solution was used to optimize process conditions such as concentration, pH and membrane type. Real RO concentrate was tested in the following experiments. Flux values, boron concentration and boron rejections, SEM-EDS analysis and XRF analysis for obtained crystals were conducted. The results showed that VGMDC could be successfully used for the removal and recovery of boron. The pilot scale VAGMDC system was tested in the scope of chapter seven. Different recovery ratios and operating modes were tested. Highly boron-contented raw river water and RO concentrate of that water were fed to the pilot scale MD system. Conductivity, flow rate and vacuum pressure were observed during the experiments. Boron, arsenic, hardness and conductivity removals were analyzed and determined. After VAGMD operation crystallization was applied to the obtained concentrate. 43.9% B2O3 contented crystals could be obtained. In conclusion, the relatively new VAGMDC process is an effective method for boron removal and recovery.