유체역학 캐비테이션 기반의 세척 이젝터를 이용한 오염 토양으로부터 유해 중금속 분리 및 제거

Abstract

The increase in population worldwide has resulted in rapid industrialization and urbanization. As a result, environmental pollution problems such as air, water quality, and soil are increasing. Heavy metal-contaminated soil adversely affects humans and surrounding ecosystems through various pathways. Heavy metals in the soil exist in various forms, such as ion exchange, adsorption, precipitation, and complex formation. Physical, chemical, and biological treatment technologies are typically being developed to purify soil contaminated with heavy metals. Recently, interest in research on eco-friendly processing technology has continued to increase worldwide. Hydrodynamic cavitation is widely applied to various industrial fields in terms of the scalability and efficiency of technology. Cavitation-based physical treatment technology is capable of treating heavy metals in the form of aggregation or particulate particles in the soil. Depending on soil characteristics, it can be used in combination with chemical treatment techniques such as individual processes or soil washing. Cavitation-based soil pollution treatment technology can increase the efficiency of chemical extraction through a simple physical separation and has advantages such as treatment time and reduced washing agent usage. This study aims to improve the remediation efficiency of heavy metal-contaminated soil by using a hydrodynamic cavitation-based washing ejector. In this study, 1) the cavitation washing ejector was first designed and manufactured, and the ejector's flow characteristics and key factor characteristics by shape were investigated through computational fluid dynamics modeling. Based on this, an optimal condition experiment was conducted in which 2) cavitation-based physical treatment and 3) physical and chemical treatment processes were combined to separate contaminated soil. In addition remediation efficiency was confirmed by examining physical and chemical characteristics such as the properties/structure of components in the soil treated in each process and the form of heavy metal presence. First, as a result of examining the flow characteristics and key factor characteristics of the washing ejector by shape, it was confirmed that the diameter of the venturi nozzle, inlet pressure, and the ratio of the length/diameter of the mixing chamber affect the fluid flow. As the diameter of the venturi nozzle increased, the difference in the speed of the fluid and the pressure of the mixing chamber increased. In addition, the smaller the diameter of the mixing chamber and the longer the length, the higher the flow rate of the fluid passing through, and the pressure difference between the inlet and outlet of the mixing chamber also increased. The cavitation number is affected by the inlet pressure of the venturi nozzle, the inlet pressure is below 3 MPa, and the cavitation number is below 1. The size of the cavitation bubbles ranged from 0.49 mm to 0.87 mm. As the venturi nozzle's diameter and inlet pressure increased, the cavitation bubble's size also increased. As the bubble size increased, the bubble surface area flux increased from 131.45 cm2/s/cm2 to 309.01 cm2/s/cm2. The fluid in the washing ejector showed turbulent flow, and the Reynolds number increased as the diameter and inlet pressure of the venturi nozzle increased. The critical factor affecting the removal of heavy metals according to the washing ejector's operating conditions and soil characteristics was identified as the inlet pressure of the venturi nozzle. The inlet pressure affects the cavitation number, bubble size, bubble surface area flux, and Reynolds number, and the higher the clay content in the soil, the higher the removal efficiency of heavy metals. Second, a cavitation-based washing ejector was used to examine the physical and chemical properties of contaminated and treated soil due to physical separation, and the washing ejector was effective in removing fine particles bound to silicate minerals and dispersing aggregated soil. Cavitation-based physical treatment can improve the separation efficiency of heavy metals in particulates by collisions and friction between cavitation bubbles generated in the ejector and soil particles or soil particles. As a result of the removal experiment of heavy metals (As, Zn, and Pb), the removal efficiency was about 40-60%, and the volume of particulates in the purified soil decreased by about 28-47% compared to the contaminated soil. On the other hand, in physical separation, it is judged that the content of fine particles present as oxides, organic materials, and clay minerals in the soil greatly influences removing heavy metals. Third, the remediation efficiency of heavy metal contaminated soil was confirmed using a physical/chemical treatment process that combines washing ejectors and soil washing, and it was more effective than a single chemical treatment. As, Cu, and Pb were removed at 89.17%, 78.22%, and 90.47%, respectively. The heavy metal particles comprised a metal phase captured in the mineral and a metal phase weakly bonded to the mineral surface. The washing ejector primarily removes fine particles bound to the surface of the coarse particle and disperses the aggregated soil. The chemical treatment method using phosphoric acid is believed to effectively remove arsenic adsorbed on amorphous iron oxides. Therefore, it was confirmed that the physical/chemical treatment method was more effective in purifying contaminated soil. As a result, the main factors of the washing ejector that affect the remediation of heavy metal contaminated soil were identified as the venturi nozzle inlet pressure, the mixing chamber shape, cavitation number, the surface area flux, and Reynolds. When cavitation-based washing ejector technology is applied to the remediation of heavy metal contaminated soil in the future, the separation/removal system established in this study can be used for single processes, combination process, and industrial processes along with preliminary investigations by soil characteristics.