(533r) A Comprehensive Study to Evaluate the Removal Capacity of Heavy Metal Ions and Dissolved Organics from Produced Water Using Vadose Zone Soils for Soil Aquifer Treatment

Produced water is one of the largest streams of wastewater generated from the oil and gas industries and its production is exponentially increasing in the upcoming years due to the unpredictable increase in the demand for oil and gas over the world. Since produced water is acting as complex contaminated toxic slurry because of the presence of dissolved and dispersed oil components, production chemicals, dissolved formation minerals, produced solids, and dissolved gases (including CO₂ and H₂S), it displayed detrimental effects on the surface, soil, aquatic system, and underground water as this often discharged to surface water without appropriate treatment. However, the nature of contaminants and their percentages constituting the produced water can vary drastically based on the geolocation and the type of petroleum products. The treatment of produced water is now very important due to the treatment will not only decline the drastic environmental effects of the industry, moreover, it takes the edge off the stress on freshwater resources. As a result of the great demand for oil and gas, a large quantity of produced water is generated, hence major problem facing the treatment of produced water should need to handle large volumes with covering the removal of a range of contaminants. The UAE is one of the countries facing the problem of high production of produced water and its treatment as it is very rich in oil and gas industries. Currently, soil aquifer treatment (SAT) is an attractive method in the area of wastewater treatment in developed and developing countries because of its advantages such as removing multiple contaminants, being robust, environment-friendly, and minimum use of energy and chemicals. SAT denotes the artificial recharge or infiltration of wastewater through the unsaturated zone (vadose zone) to boost the underlying aquifers. Since, SAT improves the physical, chemical, and microbial quality of contaminated water by removing particles during soil passage, as well as provides long-term storage without evaporative losses and less external re-contamination, it can thus substitute or support other water treatment methods. But the major limitation of this method is the leaching of aquifer materials such as iron, manganese, calcium, selenium, arsenic, chromium or fluoride, etc in extracted water under reducing conditions. Thus, a detailed study on the behaviour or interaction of soils from the vadose zone with contaminated water, and the investigation on the mechanism of removal should be conducted for the SAT treatment method before it becomes a part of the water treatment process or independent treatment. Since, SAT can handle the treatment of large volume of contaminated water as well as environmentally friendly and cost effective method, it can be applied for the treatment of produced water from oil and gas industries. The present study focussed on the suitability of four soils such as sand, sandstone, siltstone, and conglomerate which were collected from the vadose zone of Abu Dhabi, UAE, for the treatment of produced water for SAT method. This comprehensive study was aimed to monitor the removal capacity on two heavy metals (Ni (II), and Zn (II)), and organic contaminants such as phenol with the same concentration as their in produced water using selected four soil samples. The four soil samples were utilized without further treatment or modification in order to keep a cost-effective approach. The detailed characterizations of soil samples were conducted using different techniques (SEM, EDX, XRF, FTIR, and XRD analysis) to analyze their chemical composition and physicochemical properties. The results found that sandstone and conglomerate were rich in calcium carbonate-based minerals, and sand and siltstone were based on silica-based minerals along with other minerals. To understand the interaction between metal ions/phenol and soil samples, batch experiments were conducted on a laboratory scale at normal pH at room temperature for two weeks, and data were assessed to find the conditions of maximum removal along with relevant isotherms and kinetic equations. The results displayed the soil samples followed the order sandstone > conglomerate > siltstone > sand for the removal of Ni (II), and Zn (II), and phenol from synthetic produced water for 250 mg/L for the soil samples. The sandstone displayed the highest removal of 98-99% while conglomerate and siltstone presented 89-98% for Ni (II) and Zn (II), and 32% for phenol, respectively. The experimental data were fitted using the Langmuir isotherm, Freundlich isotherm, Temkin isotherm model, and D-R isotherm, and the results described the best fit for the Langmuir isotherm for Ni (II) and Zn (II) and Freundlich isotherm for phenol. The maximum adsorption capacity was corresponding to complete single monolayer coverage from Langmuir isotherm of sandstone for Ni (II), and Zn (II) is 2.3 mg/g and 4.4 mg/g, respectively which is consistent with experimental values (2.6 mg/g for Ni (II) and 3.6 mg/g for Zn (II)). The equilibrium kinetic model studies were presented four samples that followed the pseudo-second-order kinetics for the removal of metal ions and phenol than pseudo-first-order kinetics and Elovich kinetics. The 2D correlation analysis combined with Fourier transform infrared spectroscopy in attenuated total reflectance geometry (2D COS FTIR) was applied to investigate the mechanism of removal of Ni (II) and Zn (II), and phenol. The 2D-COS FTIR analysis was verified that removal of metal ions/phenol can be driven by both silica and carbonate groups present in the minerals of four soil samples. The asymmetric Si-O as the prime response in Ni (II), and Zn (II) removal mechanism for sand and siltstone and asymmetric CO₃^2-bands decided for the sandstone and conglomerate. While the interaction of phenol and Si-O bands was predominant in the removal mechanisms for phenol along with electrostatic interaction between phenol and carboxylate in four soil samples. The characterization studies such as SEM, EDX, FTIR, and XRD analysis after the treatment of soil samples with produced water were indicated calcite minerals in sandstone and conglomerate, and silica-based minerals in siltstone and sand are the main responsible factor for the highest removal of Ni (II), Zn (II), and phenol which is consistent with results of 2D-COS FTIR analysis. However, the ICP-MS analysis of treated produced water presented that some components such as magnesium, calcium, and sodium from four soils were discharged into treated water even though high removal performance. The percentages of leached materials in treated water were different from each other because of the difference in the percentage composition of minerals present in each soil samples which was confirmed by XRF analysis. In summary, sandstone, conglomerate, and siltstone provided a high degree of contaminants removal with minimum treatment cost, thus these soils can be effectively used for SAT process for the treatment of produced water. Furthermore, the 2D-COS FTIR analysis offered a unique insight into a clear picture of the mechanism of removal on the surface of the soil by the interaction between surface functional groups containing Si-O bond, and CO₃^2- groups of soil samples and metal ions or phenol.