Earticle

본 기술보고문은 비소 제거를 위한 전기응집(Electrocoagulation, EC) 기술의 최근 동향과 국내와베트남 현장 실증 사례를 정리하였다. Fe 전극 기반 EC는 단순한 장치 구조와 낮은 비용으로 WHO 음용수 기준(10 µg/L 이하)을 안정적으로 달성할 수 있는 기술로 주목받고 있으며, 공기 음극이나초음파 산화 결합과 같은 개선된 공정의 도입으로 처리 속도가 더욱 향상되고 있다. 국내에서는 금정광산과 충남 지역 지하수에서 Fe 전극 기반 EC 실증이 수행되어 광산배수 및 자연기원 오염수조건에서도 단일 공정으로 WHO 기준을 충족하는 성과를 거두었다. 베트남의 경우, 가정용 소형장치와 마을 상수도 시스템에서 EC가 적용되어 수십~수백 명 규모 주민에게 안전한 식수를 안정적으로 공급할 수 있음을 보여주었다. 특히 태양광 전원과의 결합은 전력 인프라가 부족한 지역에서도 안정적인 운전 가능성을 확인하였으며, 현장 적용성, 경제성, 확장성 측면에서 EC 기술의 유망성을 입증하였다.

This technical report reviews recent advances in electrocoagulation (EC) for use in arsenic removal and highlights field demonstrations in South Korea and Vietnam. Fe-based EC has emerged as a simple, cost-effective method that reliably realizes the World Health Organization (WHO) drinking water guideline of 10 μg/L. Improved processes, such as air-cathode EC and ultrasound oxidation, considerably accelerate the treatment rate. Field applications in South Korea, including the Geumjeong mine, Bonghwa, and naturally contaminated groundwater in Chungnam, confirmed that Fe-based EC consistently satisfied the WHO standard in mining effluent and naturally contaminated groundwater. In Vietnam, EC was applied at household and community scales, demonstrating stable operation and providing safe drinking water for tens to hundreds of residents. Coupling with solar power enabled reliable performances in areas with limited infrastructure. These demonstrations highlight the practicality and scalability of EC as a key technology for use in decentralized water treatment and safe water supply.

본 연구에서는 SAPS조에 흔히 사용하는 버섯퇴비(MC)와 모래 크기 석회석을 혼합한버섯퇴비(MCL)를 두 개의 컬럼에투입하고 9주간 광산배수를 유입시켜 수질을 비교했다. 버섯퇴비층은 중화능력이 없어서 Al, Fe2+ 및 Mn 성분을 통과시켰다. Al은자갈 석회석층에서 침전하여 석회석의 용해를 억제했다. 모래 석회석 혼합 버섯퇴비층은 pH를 5.90으로 증가시켜 Al을 버섯퇴비층에 침전시켰다. 9주 동안 자갈 석회석 층의 빈공간이 유지되었고 pH는 6.58 그리고 알칼리도는 131.09 mg/L CaCO3으로 측정되었다. 버섯퇴비층내 모래 석회석은 pH를 증가시켜서 Al을 제거해서 자갈 크기 석회석 층의 중화기능을 길게 유지하였다.

In this study, mushroom compost (MC) and mushroom compost mixed with sand-sized limestone (MCL), commonly used in successive alkalinity producing systems (SAPS), were loaded into two columns and introduced into acid mine drainage (AMD) for nine weeks to compare water quality. The mushroom compost had no neutralizing ability and, therefore, did not remove the components of Al, Fe2+, and Mn. Aluminum was continuously removed in the gravel-sized limestone layer, thereby reducing the neutralization function of limestone. The mushroom compost with sand limestone increased the pH 3.52 to 5.90 and had Al deposited in the compost layer. For nine weeks, the space of the limestone layer was maintained, pH was measured as 6.58, and alkalinity was measured as 131.09 mg/L CaCO3. Sand limestone in the mushroom compost layer thus increased pH and served as a filter for Al, thus maintaining the neutralization function of the gravel-sized limestone layer for a long time.

채굴이 중단된 폐광산으로부터 배출되는 광산배수는 많은 오염을 일으키는 원인이 되고 있다. 과거에서부터 지금까지 광산배수를 처리하기 위한 석회의 사용으로 인해 침전물인 다량의 슬러지 폐기물을 발생시키고 있다. 따라서 본 연구는 광산배수로부터 발생하는 많은 양의 슬러지 폐기물을 감소시키고, 각 금속들을 회수하여 처리하는 것을 목표로 선정하였다. 산성광산배수 내의 용존 금속 중 Fe, Al을 대상으로 고순도 침전물로 침전시키기 위해 buffer 공정을 추가한 SSP(selective sequential precipitation)시스템을 설치하였다. 침전물의 회수율과 순도는 현장에서 측정 및 화학분석을 통해 평가하였다. SSP 공정을 이용하여 Fe, Al의 회수율과 순도를 평가하였다. Fe 회수율은 99.68~99.70 이며, Al 회수율은 75.00~81.82%로 나타났다. Fe 순도는 96.05~97.61%이며, Al 순도는 81.01~87.48%로 나타났다. Fe와 Al의 고순도 침전물로 회수가 가능할 수 있었다.

Mine drainage discharged from abandoned mines are causing a lot of pollution. Conventional lime dosing process is generates a lot of waste due to the generation of the sludge in the process. Therefore, this study aims to assess the feasibility of both treating the mine drainage while recovering individual metals and decreasing the amount of waste by decreasing the amount of sludge. In order to selectively recovery Fe and Al in the coal mine drainage as high-purity precipitation, we installed a SSP(selective sequential precipitation) system with a buffer tank. The recovery rate and purity of the precipitates were evaluated from field measurement and chemical analysis. We also evaluated the recovery rate and the purity of Fe and Al. The recovery rate of Fe and Al are 99.68~99.70 and 75.00~81.82%, respectively. The purity of Fe and Al was determined to be 96.05~97.61% and 81.01~97.48%, respectively.

국내 산성광산배수 처리장에서 회수된 수산화알루미늄 슬러지를 효율적으로 활용하기 위해서 폴리염화비닐 (PVC)과 혼합해 PVC-Al(OH)3 비드를 제조하고 비소 이온 제거를 위해 사용하고자 하였다. pH를 2에서12로 변화시키며 비소이온의 흡착거동을 조사한 결과 모든 구간에서 비소 5가 이온의 제거율이 비소 3가 이온보다 높았으며, pH가 중성영역에서 제거율은 500 분 내에 100%를 나타냈다. 비소 제거율은 pH가 2와 12에서감소하였는데, 이는 수산화알루미늄이 이 영역에서 용해되는 것이 원인으로 분석되었다. 비소 5가 이온의 경우초기 농도 200 mmol/L일 때 흡착량은 200 mmol/g까지 증가하였다.

PVC-Al(OH)3 beads were prepared to remove arsenic (As) ions using aluminium hydroxide sludge for an effective re-use. The sludge was obtained from domestic treatment facilities of acid mine drainages. The absorption behavior of arsenic ion was investigated in the pH range within 2 and 12. As a result, the removal efficiencies of As(V) ion were higher than that of As(III), over the entire pH range of this study. The removal efficiencies of As(V) ion increased to 100% at neutral pHs within 500 min of reaction duration. The efficiencies were low at pH 2 and pH 12, because of the high dissolution of aluminum hydroxide at those pHs. The absorbed amount of As(V) ions increased to 200 mmol/g at 200 mmol/L initial As concentration.

광산배수 내 용존 철 제거속도를 향상시키기 위한 방법으로 폭기를 활용하고자 서로 다른 특성을 띄는 광산배수를 대상으로 폭기실험을 수행하였다. 대기와의 평형반응을 통해 광산배수 내 DO 농도는 증가하고 DCO2 농도는 감소하게 되는데 탈기된 DCO2는 광산배수의 pH를 상승시킨다. 상승된 DO 농도와 pH 값은 Fe2+가 산화되는데 유리한 환경을 제공하여 Fe2+ 산화속도를 크게 향상시킬 수 있었다. 이 과정에서 폭기는 광산배수-대기 간 접촉면적을 증가시킴으로써 기체교환속도를 큰 폭으로 향상시키는 역할을 하였다. Fe2+의 산화반응은 용존 철 제거과정에서 속도결정단계였으므로 폭기는 광산배수 내 용존 철 제거속도를 향상시키는 효과적인 방법이 될 수 있다. 다만 순 산성을 띄는 광산배수의 경우에는 DCO2 탈기에 의한 pH 증가를 기대할 수 없어 폭기를 통해 광산배수 처리효율을 향상시킬 수 없었다.

Aeration was applied on three different mine drainages to verify the removal rate enhancement of dissolved iron. Equilibrium with the atmosphere accelerated the increase in DO and decrease in DCO2 where the latter affected the eventual increase in pH level. The combined effect of the increase in both DO and pH provided a favorable environment for Fe2+ oxidation and this led to the boost in oxidation rate. Aeration, in this process, played an important role in escalating gas exchange rates by increasing the contact area between mine drainages and the atmosphere. Since Fe2+ oxidation was a rate determining step in removing dissolved iron, aeration could be an effective method to expedite the iron removal in mine drainages. In case of net acidic mine drainages, however, aeration could not boost the treatment efficiency due to lack of decarbonation effect which increases the pH of the mine drainages.

백운석 광산에서 발생되는 분진세척수에 대하여 응집침전에 의한 탁도제거 특성과 분진을 활용한 산성광산배수 중화처리 가능성을 평가하였다. 실험을 위해 S 백운석 광산의 분진을 이용해서 인공 분진 세척수를 제조하였다. 초기 탁도는 1,558 NTU 였으며, pH는 8.51이었다. 분진의 중수위 입도(D50)는 16.4 μm 였으며, 대부분이 10～30 μm의 입자크기를 갖고 있었다. 응집제와 관계없이 pH 3과 5에서는 90%이상의 탁도가 제거되었다. 무기 응집제인 PAC의 경우 pH 7에서, 유기응집제인 Nalco 9601의 경우 pH 11에서 탁도제거 효율이 좋았다. 산성광산배수에 대한 중화능력 실험 결과 백운석 분진은 10분경과 후 pH 2.89인 산성광산배수의 pH를 6.7 이상으로 상승시켰고, 용존되어 있던 Fe와 Al을 100% 제거하였다.

제강 슬래그를 이용해 수용액상의 비소를 제거할 때 발생하는 주 기작을 평가하기 위해 초기 pH를 변화시킨 비소용액과 제강 슬래그의 흡착실험을 수행하고 이에 대한 지구화학적 평형을 Visual MINTEQ을 이용해 모델링하였다. 제강 슬래그와 비소의 화학반응 과정에서 다량의 칼슘이 용출되었고 이는 제강 슬래그로 인해 상승된 용액의 pH에 의해 다시 방해석 및 amorphous calcium carbonate로 침전되면서 공침, 흡착을 통해 비소를 제거하는 것으로 확인된다. 흡착반응 완료 후 용액의 pH가 산성인 경우 칼슘 침전물은 형성되지 못하는 것으로 모사되었으나 ferric oxide, ferrous hydroxide, aluminum oxide, aluminum oxide hydroxide 등이 HAsO4 2-, H2AsO4-형태의 비소를 흡착하는 것으로 판단된다. Visual MINTEQ을 통한 제강 슬래그-비소 반응의 지구화학적 평형 모델링 결과는 선행연구들과 유사한 결과를 보이며 높은 적용 가능성을 보였다.

It was performed to evaluate dominant mechanisms occurring when steel making slag removes arsenic in aqueous solution that batch experiments with initial pH and its geochemical equilibrium modeling using Visual MINTEQ. A large amount of calcium was leached from the slag in process of the chemical reaction with the arsenic solution and were precipitated as calcite or amorphous calcium carbonate due to increased solution pH by the slag removing arsenic by co-precipitation or adsorption. When the solution pH was acidic after the batch experiments, calcium precipitates could not be formed. However, it is estimated that ferric oxide, ferrous hydroxide, aluminum oxide, aluminum oxide hydroxide would adsorb arsenic in form of HAsO4 2-, H2AsO4 - instead of calcium precipitates in acidic condition. Results of geochemical equilibrium modeling using Visual MINTEQ proved its good applicability being similar with those of previous literatures.

Various methods are used to remediate soil contaminated with heavy metals or petroleum. In recent years, harsh physical and chemical remediation methods are being used to increase remediation efficiency, however, such processes could affect soil properties and degrade the ecological functions of the soil. Effects of soil washing, thermal desorption, and land farming, which are the most frequently used remediation methods, on the physicochemical properties of remediated soil were investigated in this study. For soils smaller than 2 mm, the soil texture were changed from sandy clay loam to sandy loam because of the decrease in the clay content after soil washing, and from loamy sand to sandy loam because of the decrease in the sand content and increase in silt content during thermal desorption, however, the soil texture remained unchanged after land farming process. The water-holding capacity, organic matter content, and total nitrogen concentration of the tested soil decreased after soil washing. A change in soil color and an increase in the available phosphate concentration were observed after thermal desorption. Exchangeable cations, total nitrogen, and available phosphate concentration were found to decrease after land farming; these components were probably used by microorganisms during as well as after the land farming process because microbial processes remain active even after land farming. A study of these changes can provide information useful for the reuse of remediated soil. However, it is insufficient to assess only soil physicochemical properties from the viewpoint of the reuse of remediated soil. Potential risks and ecological functions of remediated soil should also be considered to realize sustainable soil use.

A new type of chemical oxidation technology utilizing micro bubble ozone oxidizer and a pneumatic fracturing equipment was developed to enhance field applicability of a traditional chemical oxidation technology using hydrogen peroxide as an oxidizer for in-situ soil remediation. To find an efficient way to dissolve gaseous ozone into hydrogen peroxide, ozone was injected into water as micro bubble form then dissolved ozone concentration and its duration time were measured compared to those of simple aeration of gaseous ozone. As a result, dissolved ozone concentration in water increased by 31% (1.6 ppm ${\rightarrow}$ 2.1 ppm) and elapsed time for which maximum ozone concentration decreased by half lengthened from 9 min to 33 min. When the developed pneumatic fracturing technology was applied in sandy loam, cracks were developed and grown in soil for 5~30 seconds so that the radius of influence got longer by 71% from 392 cm to 671 cm. The remediation system using the micro bubble ozone oxidizer and the pneumatic fracturing equipment for field application was made and demonstrated its remediation efficiency at petroleum contaminated site. The system showed enhanced remediation capacity than the traditional chemical oxidation technology using hydrogen peroxide with reduced remediation time by about 33%.

Geochemical and ecological properties of remediated soil and gas exhausted from a low-temperature thermal desorption (LTTD) process were analyzed to assess the environmental impact of LTTD treatment. Soil characteristics were examined with regard to the chemical (EC, CEC, and organic matter) and the ecological (dehydrogenase activity, germination rate of Brassica juncea, and growth of Eisenia andrei) properties. The exhaust gases were analyzed based on the Air Quality Act in Korea as well as volatile organic compounds (VOCs) and mixed odor. Level of organic Organic matter of the soil treated by LTTD process was slightly decreased compared to that of the original soil because the heating temperature ($200^{\circ}C$) and retention time (less than 15 minutes) were neither high nor long enough for the oxidation of organic matter. The LTTD process results in reducing TPH of the contaminated soil from $5,133{\pm}508$ mg/kg to $272{\pm}107$ mg/kg while preserving soil properties. Analysis results of the exhaust gases from the LTTD process satisfied discharge standard of Air Quality Law in Korea. Concentration of VOCs including acetaldehyde, propionaldehyde, butyraldehyde and valeraldehyde in circulation gas volatilized from contaminated soil were effectively reduced in the regenerative thermal oxidizer and all satisfied the legal standards. Showing ecologically improved properties of contaminated soil after LTTD process and environmentally tolerable impact of the exhaust gas, LTTD treatment of TPH-contaminated soil is an environmentally acceptable technology.

본 연구에서는 지하수에 존재하는 비소를 산업 폐기물인 제강 슬래그를 이용하여 제거하고자, 제강 슬래그에 대한 비소의 흡착특성을 규명하였다. 이를 위해 등온흡착 실험과 동적흡착 실험을 수행하였고, 흡착반응 후 용액의 화학적 특성을 분석하였다. 실험 결과, 흡착 실험을 수행한 모든 용액은 염기 상태로 존재하였으며(pH 9이상), 칼슘의 농도가 가장 높았다(30~50mg/L). 등온흡착 실험결과는 Langmuir 모델보다Freundlich 모델에 적용하는 젓이 더 합리적이였으며, 제강 슬래그에 As(V)가 As(III)보다 약 87% 더 많이 흡착되는 것으로 확인됐다. 동적흡착 실험결과의 경우, 유사일차모델보다 유사이차모델을 통해 해석하는 것이 더 적합하였다. 비소의 초기농도가 높을수록 평형 흡착량($q_e$)과 완화시간($t_r$) 이 모두 증가하였으며, As(V)는 As(III)보다 평형 흡착량이 많고 완화시간은 짧은 것을 확인할 수 있었다. 또한, 유사이차모델을 통해 예측된 평형 흡착량이 등온흡착 실험에서 구한 평형 흡착량과 유사해 동적흡착 실험결과로 등온흡착 실험결과를 예측하는 것이 가능함을 확인하였다.

Sorption characteristics of arsenic on furnace slag were investigated to remove arsenic from groundwater using furnace slag, which is industrial waste generated from steel company. Adsorption isotherm experiments and kinetic sorption experiments were performed and the chemical characteristics of supernatants from these experiments were analyzed. Results showed that all supernatants were alkaline (above pH 9) and the highest ion concentration in the solution was found with calcium (30~50 mg/L). Results of adsorption isotherms were more adequately described by the Freundlich model than Langmuir model. From adsorption isotherms experiments, it was noted that the adsorption amount of As(V) was 87% higher than that of As(III). Results of kinetic sorption experiments were more properly fitted by pseudo second order (PSO) model than pseudo first order model. Equilibrium adsorption amount ($q_e$) and relaxation time ($t_r$) calculated from PSO model increased with initial concentration of arsenic. Equilibrium adsorption amount of As(V) was higher than that of As(III) and relaxation time of As(V) was shorter than that of As(III). Adsorption isotherm results could be predicted by kinetic adsorption results, since equilibrium adsorption amount calculated through PSO model generally agreed with equilibrium adsorption amount measured from adsorption isotherm.