토양세척 및 식물복원공법을 이용한 중금속 오염토양의 정화에 관한 연구

Abstract

다양한 경로를 통해 토양으로 유입되는 오염물질 중 일부 중금속들은 토양 중에서 자연적으로 생분해되어 소멸되는 것이 아니라 생태계에 지속적으로 잔류하는 특성들을 가지고 있어 심각한 사회경제적 문제를 일으킬 수 있다. 중금속 오염토양의 복원을 위해 다양한 공법들이 적용되고 있는데, 이 중에서도 단기간에 오염토양의 양을 현저하게 줄일 수 있는 토양세척법이 매우 유용한 방법으로 알려져 있으며, 최근에는 처리시간은 좀더 오래 걸리지만 저비용의 효율적인 처리가 가능한 식물정화공법에 대한 관심이 높아지고 있다. 오염된 부지의 성공적인 복원을 위해서는 정화공법에 따른 효율을 극대화하는 방안과 현장 적용가능성 등에 대한 연구가 필수적이다. 본 연구에서는 과거 30여 년간 지속적으로 토양오염이 이루어진 클레이사격장(Clay shooting range) 납 단일오염토양과 폐탄처리장(Open burning and open detonation site; OBOD stie) 카드뮴․구리․아연 복합오염토양을 정화하기 위해 토양세척법(Soil washing)과 식물복원공법(Phytoremediation)의 적용가능성을 평가하고 정화효율 영향인자들에 대한 최적 조건들을 도출하고자 실험실규모의 실내실험을 실시하였다. 토양세척실험에서는 토양의 오염기간에 따른 토양내 중금속 존재형태별 분포비율을 확인하고 그에 따른 세척효율을 평가하기 위해 인공오염토양을 제조하여 실제현장오염토양과 토양세척실험을 병행하였다. 오염토양의 특성에 적합한 최적 세척제 선정을 위해 무기산, 유기산, 킬레이트제 및 중성염들을 이용한 세척효율을 평가하였으며, 추출시간, 고액비 및 토양입자크기에 따른 세척효율을 평가하였다. 또한 토양세척 전과 후 토양에 대하여 연속추출법을 이용한 단계별 중금속 존재형태에 따른 세척효율을 평가하였다. 실제현장오염토양과 인공오염토양의 중금속 존재형태를 살펴본 결과 납 단일오염토양에서는 치환태 분포비율을 각각 18.04%와 42.28%, 카드뮴․구리․아연 복합오염토양에서는 치환태 분포비율이 각각 카드뮴 24.04%, 44.26%, 구리 0.47%, 53.26%, 아연 8.60%, 53.76%, 철/망간 산화물 결합태 분포비율이 각각 카드뮴 25.81%, 0.73%, 구리 28.67%, 9.32%, 아연 48.23%, 5.73%로 큰 차이를 보였다. 이러한 이유로 대부분의 세척제에서 인공오염토양이 실제현장오염토양보다 추출효율이 우수하게 나타났다. 클레이사격장 납 단일오염토양의 경우 0.5 M HCl(고액비 1:5, 추출시간 2 hr)에 의해 초기오염토양(2,538 mgPb/kg)의 92.9%의 납이 추출되어 잔존토양의 농도가 181 mgPb/kg로 토양오염우려기준(1지역, 200 mgPb/kg) 미만으로 확인되었다. 폐탄처리장 카드뮴․구리․아연 복합오염토양의 경우 0.5M HCl(고액비 1:5, 추출시간 12 hr)에 의해 초기오염토양(6 mgCd/kg, 589 mgCu/kg, 638 mgZn/kg)에서 카드뮴, 구리, 아연이 각각 95.1%, 90.5%, 83.2%가 추출되어 잔존토양의 농도가 각각 0.3 mgCd/kg, 56 mgCu/kg, 107 mgZn/kg)로 토양오염우려기준(1지역, 4 mgCd/kg, 150 mgCu/kg, 300 mgZn/kg) 미만으로 확인되었다. 그러나 세척 후 잔존토양 내 일부 중금속의 존재형태중 이동성이 용이한 치환태와 탄산염결합태의 분포비율이 증가하는 경우가 있어 잔존토양에 대한 적절한 관리가 필요할 것으로 사료된다. 식물정화실험에서는 해바라기와 옥수수 종자를 파종하여 3주후 발아율과 생체량을 측정하였으며, 유묘를 납 단일오염토양과 카드뮴․구리․아연 복합오염토양에 옮겨 심고 3개월 생육 후 각 식물체 기관별 생체량과 중금속 축적능력을 평가하였다. 실제현장오염토양에서 식물체별 발아율과 생체량 측정결과 옥수수가 해바라기보다 더 우수한 것으로 나타났다. 그러나 오염토양에서 3개월 동안 생육시킨 해바라기와 옥수수의 생체량 측정 결과 납 단일오염토양과 카드뮴․구리․아연 복합오염토양 모두에서 해바라기가 옥수수보다 각각 3.2배, 2.7배 많았다. 또한 납 단일오염토양에서 식물체 납 축적량은 해바라기가 옥수수보다 1.1배 높게 나타났다. 그러나 카드뮴․구리․아연 복합오염토양에서 식물체 중금속 축적량은 해바라기보다 옥수수에서 Cd은 5.8배, Cu는 9.5배, Zn은 1.8배로 높게 나타났다. 클레이사격장 납 단일오염토양 부지와 폐탄처리장 카드뮴․구리․아연 복합오염토양 부지의 효율적인 정화를 위해 토양세척실험과 식물정화실험을 실시한 결과 부지특성(오염특성, 정화장소, 정화기간, 정화목표, 정화시간 등)을 고려하여 두 공법을 병행하여 처리하는 것이 적절할 것으로 판단된다. 클레이사격장의 경우 토양세척공법 적용부지에 대해서는 0.5 M HCl(고액비 1:5, 세척시간 2 hr), 식물정화공법 적용부지에 대해서는 해바라기를 이용한 오염토양 정화가 적합할 것으로 판단된다. 폐탄처리장의 경우 토양세척공법 적용부지에 대해서는 0.5 M HCl(고액비 1:5, 세척시간 12 hr), 식물정화공법 적용부지에 대해서는 옥수수를 이용한 오염토양 정화가 적합할 것으로 판단되었다.|Heavy metals that are infiltrated into the soil through a variety of channels are not completely biodegraded by natural processes, that continue to exist in the ecology that can lead to serious socioeconomic consequences. A variety of processing methods are applied for remediation of the soil contaminated with heavy metal. Soil washing is considered very useful because it can significantly reduce the amount of contaminants in the soil in a short period of time. However, there is a growing interest in phytoremediation that allows effective and economic soil treatment although processing takes a little longer time. It is now imperative to study on the maximization of the efficiency of phytoremediation and its applicability for the successful remediation of contaminated soil. In this study, an experiment in the laboratory scale was performed to evaluate the applicability of phytoremediation and soil washing for efficient remediation of lead contamination soil in the clay shooting range and cadmium, copper and zinc contamination soil in the open burning and open detonation (OBOD) site that has been contaminated with heavy metals continuously for the past 30 years and to determine the optimal condition for the remediation efficiency parameters. In Soil Washing Experiment, the distribution rate of each heavy metal bonding form in the soil that has been existing for the duration of contamination was investigated and soil washing tests were performed to evaluate the washing efficiency, using the artificial contamination soil and the actual contamination site soil. In order to identify the most extractant for effective remediation of the contaminated soil with given characteristics of the site, washing efficiencies were evaluated for different extraction times, solid/liquid ratios and soil particle sizes, using mineral acid, organic acids, chelating agents and neutral salts. Using the sequential extraction method, washing efficiencies were also performed for each bonding form of heavy metal in the soil for each stage before and after the soil washing. Investigation of the heavy metal bonding forms in the actual contamination site soil and the artificial contamination soil revealed that, in the single Pb contamination soil, the ratio of the exchangeable form was 18.04% and 42.28% respectively while, in the combined Cd, Cu, Zn contamination soil, the ratio of the exchangeable form was 24.04% and 44.26% for cadmium, 0.47% and 53.26% for copper and 8.60% 53.76% for zinc respectively and the ratio of Fe/Mn oxide bonding form was 25.81%, 0.73% for cadmium, 28.67% and 9.32% for copper, and 48.23% and 5.73% for zinc, showing considerable difference between the two forms. For this reason, most extractants showed higher extraction efficiency when they were used on the artificial contamination soil than when they were used on the actual contamination site soil. When 0.5M HCl (solid/liquid ratio; 1:5, extraction time; 2 hr) was used on the single Pb contamination soil from the clay shooting range, 92.9% of lead could be extracted from the initial contamination soil (2,538 mgPb/kg) leaving the residual concentration of 181 mgPb/kg, which is confirmed lower than the Worrisome Level of Soil Contamination (200 mgPb/kg in 1 area). When 0.5M HCl (solid/liquid ratio; 1:5, extraction time; 12 hr) was used on the combined Cd, Cu, Zn contamination soil from the OBOD site, 95.1% of cadmium, 90.5% of copper and 83.2% of zinc could be extracted from the initial contamination soil (6 mgCd/kg, 589 mgCu/kg, 638 mgZn/kg resp.) leaving the residual concentration of 0.3 mgCd/kg, 56 mgCu/kg, 107 mgZn/kg respectively for cadmium, copper and zinc, which are confirmed lower than the Worrisome Level of Soil Contamination (4 mgCd/kg, 150 mgCu/kg, 300 mgZn/kg resp. in 1 area). However, appropriate management of the residual soil is necessary since some of the heavy metals in post-wash residual soil show increased distribution rate of the exchangeable form and the carbonate bonding form that are easy mobilities. In Phytoremediation Experiment, Helianthus annuus and Zea mays seeds were sown and the germination rate and the biomass of each were measured after 3 weeks from sowing. Seedlings of the two plants were then transplanted to both the single Pb contamination soil and the combined Cd, Cu, Zn contamination soil and the heavy metal accumulation rate and the biomass of the various organs of the two plants were measured after 3 months of growing. The measurements of the germination rate and the biomass of the two plants grown in the actual contamination site soil showed superior performance of Zea mays to Helianthus annuus. However, the measurements of the biomass of the two plants after 3 months of growing in the contaminated soil revealed 3.2 times and 2.7 times higher results of Helianthus annuus than Zea mays respectively from the growth in the single Pb contamination soil and the combined Cd, Cu, Zn contamination soil. Lead accumulation rate in the two plant after the growth in the single Pb contamination soil showed 1.1 times higher result of Helianthus annuus than Zea mays, while heavy metal accumulation rates after the growth in the combined Cd, Cu, Zn contamination soil showed 5.8 times more cadmium, 9.5 times more copper and 1.8 times more zinc in Zea mays than in Helianthus annuus. Based on the experiment results of the Soil Washing and the Phytoremediation methods for the effective remediation of the single Pb contamination soil in the clay shooting range and the combined Cd, Cu, Zn contamination soil in the OBOD site, we believe that combined use of both methods taking consideration of the characteristics of the site (contamination characteristics, location of treatment, duration of treatment, treatment targets, treatment time, etc). For the clay shooting range, use of 0.5M HCl (solid/liquid ratio; 1:5, washing time; 2 hr) in the soil washing method application areas and use of Helianthus annuus for the phytoremedian method application areas seems ideal for the treatment of contaminated soil. For the OBOD site, use of 0.5M HCl (solid/liquid ratio; 1:5, washing time; 12 hr) in the soil washing method application areas and use of Zea mays for the phytoremediation method application areas seems ideal for the treatment of contaminated soil.