다황화칼슘을 이용한 지하수 내 카드뮴과 아연 저감 : 침전 기작, 산화 저항성에 관한 연구

학위논문(석사) -- 서울대학교대학원 : 공과대학 건설환경공학부, 2023. 2. 남경필.Heavy metals in sludge or leachate discharged from abandoned mines and smelters flow into the surrounding water system, causing serious water contamination. In particular, heavy metal contamination and acidification in groundwater are serious due to cadmium (Cd2+), zinc (Zn2+), and sulfate (SO42-) generated during the smelting process. In this regard, there are some cases of removing Cd2+ and Zn2+ from groundwater using calcium polysulfide (CPS), one of the reducing agents for remediation of groundwater, however, the mechanism of the heavy metal removal of CPS is not clearly known. In addition, heavy metals precipitated in the form of sulfides could be oxidized and dissolved in aqueous phase by dissolved oxygen (DO). Therefore, this study aimed to quantify the polysulfide (Sx2-), derive the precipitation mechanism of Cd2+ and Zn2+ by CPS, evaluate the effect of DO on precipitate formed from CPS injection, and finally apply the batch results to the contaminated groundwater. As a result of this study, the concentration of Sx2- in CPS 1% (w/v) was 82.2 mM, and 76.8 mM of Cd and 77.6 mM of Zn were removed per 1% of CPS injection. In addition, referring to the X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) analysis, elemental sulfur (S8 or S0), gypsum (CaSO4·2H2O), CdS, and ZnS were present in the precipitate, and hydroxides such as Cd(OH)2 and Zn(OH)2 were not found. Moreover, there was little change in the pH in the aqueous solution before the heavy metal precipitation was completed, but after the heavy metal precipitation was completed, the pH showed a tendency to increase. Taken together, it was concluded that when CPS is injected into a heavy metal contamination source, CPS does not cause a change in pH if overdose is not injected and one S2- molecule is released from Sx2- and precipitates with heavy metals in the form of sulfide, and the rest is precipitated as S8. In addition, by exposing the precipitate to the aerobic conditions, it was revealed that Sx2- and bisulfide (HS-) react with DO suppressing the oxidation of heavy metals. Based on these results, CPS injection into the field groundwater confirmed that Cd2+ and Zn2+ in the groundwater could be successfully removed and the precipitate exists stably without dissolution for up to 21 days.폐광산이나 폐제련소에서 유출된 유출수나 슬러지 내부에 존재하는 중금속들은 주변 수계로 유입되어 심각한 수질 오염을 야기할 수 있다. 특히, 중금속으로 인해 지하수가 오염되거나 산성화되는 문제가 심각한데 이는 제련 과정에서 발생하는 카드뮴 (Cd2+), 아연 (Zn2+), 황산염 (SO42-)으로 인해 야기된다. 이러한 문제를 해결하기 위해, 지하수를 정화하기 위한 환원제 중 하나로 쓰이는 다황화칼슘 (Calcium polysulfide; CPS)를 이용하여 지하수 내 Cd2+와 Zn2+를 제거한 몇몇 사례가 있지만 CPS에 의해 중금속이 제거되는 구체적인 기작에 대해선 명확하게 밝혀진 바가 없다. 또한 중금속이 황화물 형태로 침전할 경우, 용존 산소 (Dissolved oxygen; DO)에 의해 산화되어 수용액 상태로 용출될 수 있는 가능성이 있다. 따라서, 본 연구에서는 다황화물 (polysulfide; Sx2-)의 농도를 정량하고 CPS에 의한 Cd2+와 Zn2+의 침전 기작을 도출한 다음 CPS 주입으로 생성된 침전물의 산화와 관련된 DO의 영향에 관해서 평가하여, 실제 중금속으로 오염된 지하수에 회분식 실험 결과들을 적용해보고자 한다. 본 연구 결과, CPS 1%에는 82.2 mM의 Sx2-가 존재하고 이를 중금속 오염수에 주입할 경우 76.8 mM의 Cd2+, 77.6 mM의 Zn2+가 제거됨을 확인하였다. 또한 X선 회절 분석법 (X-ray diffraction; XRD)과 주사전자현미경 (Scanning Electron Microscope; SEM)을 이용하여 CPS에 의해 elemental sulfur (S8 또는 S0), gypsum (CaSO4·2H2O), CdS, ZnS가 침전되고 수산화물 형태인 Cd(OH)2나 Zn(OH)2는 발견되지 않음을 밝혔다. 이에 더하여, CPS를 주입하더라도 중금속의 침전이 완료되기 전에는 pH에 큰 변화가 없다가 중금속의 침전이 완료된 후에는 pH가 증가하는 경향성을 보인다는 것을 실험을 통해 알 수 있었다. 이러한 결과들을 종합하면, Sx2-에서 하나의 2가 황 (sulfide)만 방출되어 중금속 침전에 관여하고 나머지는 S8 또는 S0로 존재하고 CPS가 중금속으로 오염된 지하수 오염원에 주입될 경우 CPS가 과량 주입되지 않는 이상 지하수 내에는 pH 변화가 크지 않을 것이라고 결론내릴 수 있었다. 또한, 침전물들을 지속적으로 대기중에 노출시켜 산화 환경을 조성한 실험을 통해, Sx2-와 bisulfide (HS-)가 DO와 반응하여 중금속의 산화를 억제한다고 추론할 수 있었다. 이러한 결과들을 토대로, CPS를 실제 중금속으로 오염된 지하수에 주입해보았고 지하수 내 Cd2+와 Zn2+를 성공적으로 제거할 수 있었다. 또한 이렇게 제거된 침전물들은 산소가 지속적으로 공급되는 상황임에도 3주까지 용출되지 않고 안정적으로 존재하였다.1. Introduction 1 1.1 Background 1 1.2 Literature review 3 1.3 Research objectives 5 2. Materials and method 6 2.1 Quantification of polysulfide concentration in CPS 6 2.2 Production of artificially contaminated solution & CPS injection batch 9 2.3 Effect of iron(II) in heavy metal precipitation by CPS 11 2.4 Evaluation of effect of dissolved oxygen 12 2.4.1 Oxidation experiment of precipitates formed by CPS with DO 12 2.4.2 Cadmium sulfide oxidation by DO 12 2.5 Field applicability batch test 13 3. Results 15 3.1 Concentration of polysulfide in CPS 15 3.2 Deriving heavy metal precipitation mechanism by CPS 17 3.2.1 Single heavy metal contaminated solution (1) Cd2+ 17 3.2.1 Single heavy metal contaminated solution (2) Zn2+ 24 3.2.2 Complex heavy metal contaminated solution 31 3.3 Effect of Fe2+ in heavy metal precipitation by CPS 37 3.4 Evaluation of effect of dissolved oxygen 39 3.4.1 Evaluation of oxidation of precipitates formed by CPS with DO 39 3.4.2 Evaluation of oxidation of CdS without elemental sulfur by DO 43 3.5 Field applicability test for CPS 47 4. Conclusions 49 4.1 Precipitation mechanism of Cd2+ and Zn2+ by CPS 49 4.2 Oxidation resistance of heavy metal precipitate from CPS 51 4.3 Field applicability test for CPS 52 5. Discussions 53 Reference 54 국문초록 58석

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수질에 영향을 미치는 유역인자에 대한 통계분석법과 GIS를 이용한 분석 : 한강유역을 대상으로

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Feasibility Evaluation for Remediation of Groundwater Contaminated with Heavy Metal using Calcium Polysulfide in Homogeneous media

In this study, column tests using relatively uniform Jumunjin sand media were conducted to evaluate the feasibility ofcalcium polysulfide (CaSx, CPS) in removing high concentration of Zn2+ in groundwater. The injected CPS solutionreacted rapidly with Zn2+ in artificial groundwater and effectively reduced Zn2+ by more than 99% through metal sulfideprecipitation. Since the density (d = 1.27 g/cm3) of CPS solution was greater than that of water, CPS solution settled downrapidly while capturing Zn2+ and formed stable CPS layer similar to dense nonaqueous phase liquid. Mass balance analysison Zn2+ in CPS solution suggested that CPS solution effectively reacted with Zn2+ to form metal sulfide precipitates exceptfor high groundwater seepage velocity of 400 cm/d. With greater groundwater seepage velocity, injected CPS did notcompletely dissolve at the CPS-water interface, but a partially-misible CPS layer continuously moved and reacted withZn2+ in the direction of groundwater flow. Since hydraulic conductivity (Kh) decreased slightly due to the generated metalprecipitates in the inter-pores of media, injection of CPS solution should be optimized to prevent clogging. As evidencedby both XRF and SEM/EDS results, ZnS precipitates were clearly observed through the reaction between the CPSsolution and Zn2+. Further study is warranted to evaluate the feasibility of CPS to remove high-concentration heavy metalcontaminatedgroundwater in complex and heterogeneous media.N