Fe-ettringite 및 Al-monosulfate를 이용한 비소처리 신공정 개발

Abstract

비소는 자연적/인위적으로 광범위하게 발생되고 있는 독성 물질로서, 이를 효과적으로 처리하기 위한 다양한 기술들이 연구∙ 개발되었거나 진행 중에 있다. 그 중에서도 수중 비소를 철염(ferric salts)과 함께 응집/공침시키는 처리법은 약품이 싸고 공정이 간단하여 가장 널리 적용되고 있는 비소제거 기술이다. 그러나 이 기술은 높은 제거효율을 얻기 위하여 적정 범위 내로의 pH 조정이 필수적이므로 과량의 약품이 소모될 뿐만 아니라, 비소오염폐수 내 일반적으로 공존하는 중금속을 제거하기 위하여 별도의 추가공정을 필요로 하므로, 효율성 및 경제성이 저하되는 등의 공정상 문제점이 상존한다. 따라서 기존 기술의 단점을 보완 또는 대체할 수 있는 새로운 고효율/저비용 비소제거기술의 개발이 시급한 실정이며, 본 연구에서는 그 대안으로서 시멘트 수화물을 이용한 비소제거기술을 제안하고자 하였다. 시멘트의 수화반응에 의해 생성되는 물질 중 calcium sulfoaluminate계 수화물은 기존 비소오염토양 처리를 위한 고형화/안정화 연구를 통해 강한 염기성 환경 하에서 높은 음이온 고정능을 나타내는 것으로 알려져 있다. 그러므로 이를 비소오염폐수 처리에 적용시 수중의 전기화학적 이온이동의 활성화에 의해 보다 높은 효율의 비소제거가 가능함과 동시에 수화물 내 calcium 성분에 의해 조성된 높은 pH 환경에서 기타 용존중금속의 동시처리가 가능할 것으로 예상되었다. 본 연구에서는 calcium sulfoaluminate계 수화물 중에서도 연구전례가 없고 공정 적용성이 우수할 것으로 예상되는 Fe-ettringite 및 Al-monosulfate 물질들을 선별하여 집중적으로 연구하였으며, 효과적 비소 처리를 위한 특성을 파악하고 제거효율을 평가하는 한편, 이 물질들을 이용한 새로운 비소폐수 처리법을 제안하였다. Fe-ettringite는 비소에 대한 친화력이 높은 3가철(ferric)의 사용과 양이온 column 구조 내 음이온성 오염물질을 고정한 채 안정적으로 형성되는 ettringite의 특성을 결합한 시너지 효과에 의해 높은 비소 제거능을 나타낼 것으로 예상되었으며, 비소제거 동력학 및 평형실험 등을 통해 이를 확인하였다. 먼저 시멘트 내 존재하는 Fe-ettringite의 화학구조식을 기초로 수중 비산염(arsenate)의 몰농도 대비 다양한 주입비의 CaO 및 FeCl3를 실험하였으며, 그 결과 As(V):Ca:Fe의 몰비가 1:6:3이 되도록 8 mmol/L의 CaO 및 4 mmol/L의 FeCl3가 주입되었을 때 반응시작 5분 만에 초기농도 1.33 mmol/L의 As(V) 농도가 약 95% 이상 저감되었다. 특히 동량의 CaO 또는 FeCl3가 단독적으로 control 실험에서는 주목할 만한 비소제거효과를 관찰할 수 없었으며, 이를 통해 비산염 함유 Fe-ettringite의 형성에 의한 비소제거 시너지 효과를 확인할 수 있었다. Fe-ettringite 형성시 pH의 영향을 살펴보기 위하여 pH 2~13의 다양한 범위에서 비소제거 실험을 실시하였으며, pH 2.5 및 pH 11~12 범위에서 높은 비소 제거효율이 관찰되었다. 다량의 염산 주입으로 얻어진 pH 2.5에서는 비소폐수 내 함유된 중금속이 다량 용출될 수 있으므로 실제 공정에 부적합할 것이나, 별도의 약품 조정 없이 적정량의 CaO 및 FeCl3 주입만으로 얻어진 pH 11~12의 범위에서는 Fe-ettringite 형성에 의한 높은 비소제거효율 뿐만 아니라 용존중금속의 수산화물 침전에 의한 동시제거가 가능할 것으로 예상되므로 실제 공정 적용성 측면에서 매우 긍정적인 특성이라 평가되었다. FE-SEM 분석에 의해 pH 7 및 12 범위에서 ettringite phase의 전형적 결정형태인 침상구조가 관찰됨으로써 비산염 함유 Fe-ettirngite의 형성이 확인되었으며, 특히 pH 12 조건에서 더욱 명확한 형태의 결정 형성이 관찰되었다. Al-monosulfate는 칼슘 및 알루미늄으로 이루어진 main layer와 황산염 및 결합수로 구성된 inter layer를 지닌 이중층상수산화물 (LDH)의 구조를 지니고 있다. LDH 물질은 inter layer의 음이온이 외부 음이온성 오염물질과 이온교환되는 능력이 탁월하다 알려져 있으므로, Al-monosulfate 역시 내부의 황산염이 외부의 비산염과 이온교환 하는 기작으로 비소를 처리할 수 있을 것으로 예상되었다. 먼저 Al-monosulfate의 비소제거능을 확인하기 전에 tricalcium aluminate와 gypsum을 1:1의 몰비로 섞은 후 80~90 ˚C에서 36시간 동안 수화 반응시켜 순수한 Al-monosulfate를 합성하였다. 이를 XRD, XRF, TGA 및 SEM/EDS 등을 통해 분석한 결과, 소량의 Al-ettringite 불순물의 존재가 확인되었음에도 불구하고 물리∙ 화학적으로 매우 순도 높은 Al-monosulfate가 합성된 것으로 평가되었다. 이를 이용하여 초기농도 0.67 mmol/L 비소오염 폐수 처리 동력학 실험을 실시하였으며, 그 결과 1.33 mmol/L의 황산염이 존재하도록 Al-monosulfate를 주입한 경우 12시간 내 약 73%의 비소제거 효율을 얻을 수 있었다. 특히 비소농도가 저감됨에 따라 용출되는 황산염의 농도가 증가하는 경향을 나타내어 이온교환에 의한 비소 제거가 이루어졌음을 알 수 있었으며, XRD 및 FT-IR 분석 결과 Al-monosulfate의 결정형태가 유지된 채 황산염과의 이온교환에 의하여 inter layer내 비산염이 존재함을 확인할 수 있었다. 이러한 동력학 실험 결과를 바탕으로 다양한 초기 비소농도에 대한 평형실험이 실시되었으며, 초기 비소농도가 낮은 경우 이온교환에 의한 Langmuir 흡착 등온선과 일치하는 결과가 나타났다. 그러나 고농도 비소 존재시 Al-monosulfate가 ettringite phase로 변형되는 현상이 확인되었으며, SBA 수지와 유사한 ettringite의 특성에 의해 황산염이 공존하는 경우 Al-monosulfate의 비소 제거능이 저하되는 것으로 확인되었다. 상기 연구결과들을 바탕으로, Fe-ettringite 및 Al-monosulfate를 비소처리에 적용시 기존 ferric 공정과 비교하여 보다 간단하고 경제적인 공정의 설계가 가능할 것으로 예상되었다. Fe-ettringite 및 Al-monosulfate는 pH 11~12 범위에서 비소의 고효율 처리(각각 g당 83.5 mg, 84.6 mg 비소제거)가 가능하였으며, 특히 이 같은 강염기성 pH 환경은 별도의 약품주입 등의 추가공정 없이 조성 가능하므로, 기존 공정 대비 단계별 반응조 개수의 획기적 경감이 가능할 것으로 예상되었다. 또한 강염기성 조건에서 폐수 내 비소와 공존하는 중금속은 수산화물 형태로 쉽게 침전∙ 제거되므로, 최소한의 공정으로 비소 및 중금속의 동시제거가 가능하기 때문에 설치 및 운영에 소요되는 비용과 부지면적을 최소화할 수 있을 것으로 기대되었다. 기존 비소처리제와의 경제성 비교 결과 Fe-ettringite 및 Al-monosulfate는 각각 1 g 비소 처리시 $0.48 및 $0.05의 비용만을 소모하는 것으로 나타나 매우 높은 경제성을 지닌 것으로 평가되었다. 결론적으로, Fe-ettringite 및 Al-monosulfate를 이용하여 개발된 수중 비소처리 신공법은 고효율/저비용의 처리기술로서 차후 실제 현장에 성공적으로 적용될 수 있을 것으로 사료된다.|Arsenic removal technologies have been variously developed, the coagulation/coprecipitation method using ferric salts is the most common conventional technology among them. But it has some inevitable problems caused by the requirements of excessive reagent consumption for obtaining removal efficiency and desired pH range. Heavy metals generally coexist with arsenic in contaminated water originated from soil washing wastewater and acid mine drainage (AMD), the additional treatments accompanied with pH adjustment are required to remove arsenic and other heavy metals simultaneously. Therefore, the development of new technology for treating arsenic contaminated water replacing conventional method with low-cost and highly effective alternative is desirable. The arsenic removal technology using cement hydrates was considered as an effective alternative process. Ettringite and monosulfate phases, which belong to a group of calcium sulfoaluminate hydrates, were expected that their anion-binding capabilities under highly basic condition can be beneficial in the treatment of arsenic contaminated water because the dissolved heavy metals can be removed simultaneously. Based on the consideration of the originality and applicability of the research concept, Fe-ettringite and Al-monosulfate were selected as effective compounds for As(V) removal from soil washing wastewater and AMD. Fe-ettringite was expected to remove arsenic efficiently because the synergetic effect from combination of both injected ferric iron initially and the formation of ettringite structure in later stage can improve the removal capacity. The As(V) removal capacity by formation of Fe-ettringite was estimated through the kinetic and equilibrium batch experiments. Fe-ettringite was made by injection of calcium oxide and ferric chloride into artificial arsenate contaminated water. The principle of formation of As(V)-containing Fe-ettringite was used as a mechanism of As(V) uptake. The dose of calcium and ferric iron with respect to arsenate was examined in various molar ratios, thus the optimum molar ratio of As(V):Ca:Fe was found to be 1:6:3 (1.33, 8, and 4 mmol/L, respectively) at which approximate 95% efficiency of As(V) uptake was achieved. The control experiment using CaO (8 mmol/L) and FeCl3 (4 mmol/L) were performed in individual sets, they could not achieve the As(V) removal capacity of Fe-ettringite which implies the synergetic effect of the combination system with Fe-ettringite. The equilibrium experiments were performed in various pH ranges to determine the effect of pH on the formation of arsenate-containing Fe-ettringite. Remarkable As(V) removal was achieved in range of pH 10.9~11.8, which corresponds to the favorable pH for formation of ettringite phase. At above pH 12, Fe-ettringite formation was interfered by the consumption of calcium by portlandite (Ca(OH)2) precipitation, which was confirmed by FE-SEM analysis. The well-crystalized Fe-ettringite was observed in the SEM micrographs of samples of pH 7 and 12, the needle-like crystals of pH 12 were more abundant and clear. Al-monosulfate was also examined as the novel material for As(V) removal since its structure of layered double hydroxide is expected to exhibit high removal efficiency by ion-exchange. Pure phase of Al-monosulfate was synthesized by the hydration at 80~90 ˚C for 36 hours using the stoichiometric mixture of tricalcium aluminate (calcined at 1,300 ˚C) and gypsum. The characterization of chemical compound of synthesized Al-monosulfate was conducted using XRD, XRF, and SEM/EDS, the results indicated that the highly-pure Al-monosulfate was successfully synthesized with small impurity of Al-ettringite. The kinetic batch experiment was carried out to estimate the removal capacity of Al-monosulfate for As(V). The reaction reached equilibrium in 12 hours, the results revealed the close relation between As(V) uptake and SO42- release. The intercalation of arsenate was elucidated in XRD and FT-IR analyses, therefore, ion-exchange by sulfate replacement was considered as one of the most plausible mechanism despite of the partial precipitation of calcium-arsenate solid by the dissolution of Al-monosulfate. The equilibrium batch experiment was performed to investigate the sorption behavior of As(V) to Al-monosulfate, different sorption behaviors were presented with the increase of initial As(V) concentration. In the relatively low concentration of As(V), Langmuir-type ion exchange reaction. But in the As(V)-rich condition, the transformation of Al-monosulfate into ettringite phase is more evident than ion-exchange. This formed ettringite phase seldom removed As(V) because it is thought to prefer dissolved SO42- like a SBA resin. The formation and transformation of ettringite phase were clearly observed by FE-SEM. A novel process for arsenic removal using Fe-ettringite and Al-monosulfate can be designed in more simple and cost-effective way compared with the conventional process using ferric salts. The arsenic removal reactions by Fe-ettringite and Al-monosulfate are promoted in pH range of 11~12, this basic condition can be obtained just by controlling the proper dose of CaO/FeCl3 (Fe-ettringite) or Al-monosulfate. Therefore, any additional process for adjusting the pH to enhance arsenic removal is not required, which leads to remarkable reduction of the consumption as well as numbers of the reactor required. Besides, in certain pH condition, coexisting heavy metals can also be effectively eliminated by forming hydroxide precipitates, simultaneously. This indicates that the additional processes for heavy metal removal in conventional treatment are no longer required in the new process. The new proposed process is more economical because it requires less capital and operating cost and area due to the simpler design. High arsenic removal efficiency of Fe-ettringite (83.5 mg As(V)/g) and Al-monosulfate (84.6 mg As(V)/g) will also help to reduce reagent consumption and generated sludge to disposal, together with the simultaneous removal of arsenic and heavy metals without pH adjustment. The cost of Fe-ettringite/Al-monosulfate compared with the other reagents, they appeared to be most cost-effective as $0.48 and $0.05/g-As(V), respectively. Consequently, the new proposed process using Fe-ettringite/Al-monosulfate is presumably considered to be a highly effective and low cost technology for arsenic removal from water. It can be successfully applied to treat real arsenic contaminated water in the field.; Arsenic removal technologies have been variously developed, the coagulation/coprecipitation method using ferric salts is the most common conventional technology among them. But it has some inevitable problems caused by the requirements of excessive reagent consumption for obtaining removal efficiency and desired pH range. Heavy metals generally coexist with arsenic in contaminated water originated from soil washing wastewater and acid mine drainage (AMD), the additional treatments accompanied with pH adjustment are required to remove arsenic and other heavy metals simultaneously. Therefore, the development of new technology for treating arsenic contaminated water replacing conventional method with low-cost and highly effective alternative is desirable. The arsenic removal technology using cement hydrates was considered as an effective alternative process. Ettringite and monosulfate phases, which belong to a group of calcium sulfoaluminate hydrates, were expected that their anion-binding capabilities under highly basic condition can be beneficial in the treatment of arsenic contaminated water because the dissolved heavy metals can be removed simultaneously. Based on the consideration of the originality and applicability of the research concept, Fe-ettringite and Al-monosulfate were selected as effective compounds for As(V) removal from soil washing wastewater and AMD. Fe-ettringite was expected to remove arsenic efficiently because the synergetic effect from combination of both injected ferric iron initially and the formation of ettringite structure in later stage can improve the removal capacity. The As(V) removal capacity by formation of Fe-ettringite was estimated through the kinetic and equilibrium batch experiments. Fe-ettringite was made by injection of calcium oxide and ferric chloride into artificial arsenate contaminated water. The principle of formation of As(V)-containing Fe-ettringite was used as a mechanism of As(V) uptake. The dose of calcium and ferric iron with respect to arsenate was examined in various molar ratios, thus the optimum molar ratio of As(V):Ca:Fe was found to be 1:6:3 (1.33, 8, and 4 mmol/L, respectively) at which approximate 95% efficiency of As(V) uptake was achieved. The control experiment using CaO (8 mmol/L) and FeCl3 (4 mmol/L) were performed in individual sets, they could not achieve the As(V) removal capacity of Fe-ettringite which implies the synergetic effect of the combination system with Fe-ettringite. The equilibrium experiments were performed in various pH ranges to determine the effect of pH on the formation of arsenate-containing Fe-ettringite. Remarkable As(V) removal was achieved in range of pH 10.9~11.8, which corresponds to the favorable pH for formation of ettringite phase. At above pH 12, Fe-ettringite formation was interfered by the consumption of calcium by portlandite (Ca(OH)2) precipitation, which was confirmed by FE-SEM analysis. The well-crystalized Fe-ettringite was observed in the SEM micrographs of samples of pH 7 and 12, the needle-like crystals of pH 12 were more abundant and clear. Al-monosulfate was also examined as the novel material for As(V) removal since its structure of layered double hydroxide is expected to exhibit high removal efficiency by ion-exchange. Pure phase of Al-monosulfate was synthesized by the hydration at 80~90 ˚C for 36 hours using the stoichiometric mixture of tricalcium aluminate (calcined at 1,300 ˚C) and gypsum. The characterization of chemical compound of synthesized Al-monosulfate was conducted using XRD, XRF, and SEM/EDS, the results indicated that the highly-pure Al-monosulfate was successfully synthesized with small impurity of Al-ettringite. The kinetic batch experiment was carried out to estimate the removal capacity of Al-monosulfate for As(V). The reaction reached equilibrium in 12 hours, the results revealed the close relation between As(V) uptake and SO42- release. The intercalation of arsenate was elucidated in XRD and FT-IR analyses, therefore, ion-exchange by sulfate replacement was considered as one of the most plausible mechanism despite of the partial precipitation of calcium-arsenate solid by the dissolution of Al-monosulfate. The equilibrium batch experiment was performed to investigate the sorption behavior of As(V) to Al-monosulfate, different sorption behaviors were presented with the increase of initial As(V) concentration. In the relatively low concentration of As(V), Langmuir-type ion exchange reaction. But in the As(V)-rich condition, the transformation of Al-monosulfate into ettringite phase is more evident than ion-exchange. This formed ettringite phase seldom removed As(V) because it is thought to prefer dissolved SO42- like a SBA resin. The formation and transformation of ettringite phase were clearly observed by FE-SEM. A novel process for arsenic removal using Fe-ettringite and Al-monosulfate can be designed in more simple and cost-effective way compared with the conventional process using ferric salts. The arsenic removal reactions by Fe-ettringite and Al-monosulfate are promoted in pH range of 11~12, this basic condition can be obtained just by controlling the proper dose of CaO/FeCl3 (Fe-ettringite) or Al-monosulfate. Therefore, any additional process for adjusting the pH to enhance arsenic removal is not required, which leads to remarkable reduction of the consumption as well as numbers of the reactor required. Besides, in certain pH condition, coexisting heavy metals can also be effectively eliminated by forming hydroxide precipitates, simultaneously. This indicates that the additional processes for heavy metal removal in conventional treatment are no longer required in the new process. The new proposed process is more economical because it requires less capital and operating cost and area due to the simpler design. High arsenic removal efficiency of Fe-ettringite (83.5 mg As(V)/g) and Al-monosulfate (84.6 mg As(V)/g) will also help to reduce reagent consumption and generated sludge to disposal, together with the simultaneous removal of arsenic and heavy metals without pH adjustment. The cost of Fe-ettringite/Al-monosulfate compared with the other reagents, they appeared to be most cost-effective as $0.48 and $0.05/g-As(V), respectively. Consequently, the new proposed process using Fe-ettringite/Al-monosulfate is presumably considered to be a highly effective and low cost technology for arsenic removal from water. It can be successfully applied to treat real arsenic contaminated water in the field.