The widespread usage and storage of petroleum fuels have made petroleum hydrocarbons the most prevalent soil and groundwater contaminants. The treatment of sites contaminated with long chain alkanes and polycyclic aromatic hydrocarbons involved in petroleum fuels has limitations because of their properties such as low volatility, low mobility, low solubility, and low degradability
Bioremediation has been applied to various contaminated sites for several decades, because it has many advantages such as permanent elimination of waste, cheaper biological system, positive public acceptance, minimum site disruption, risk elimination with long-term liability, and combination with other treatment techniques. In a heterogeneous and/or low permeability soil, however, bacteria cannot sufficiently metabolize contaminants due to the transport limitation of bacteria or nutrients, and so an additional management is required.
When a direct current field is introduced across soil deposits through inert electrodes, the electrokinetic (EK) phenomena such as electroosmosis, electrophoresis, and electromigration cause the transport of various compounds even in a low permeability soil. Therefore, application of EK phenomena to bioremediation, namely EK bioremediation, can uniformly and rapidly supplies nutrients, electron donors/acceptors, and bacteria to soil. It is generally known that electrophoresis is an important mechanism for bacterial transport, which depends upon the surface charge density of individual or aggregative bacteria. However, few researches on EK bioremediation have been reported.
In this study, in situ EK bioremediation of a laboratory-prepared petroleum hydrocarbons-contaminated kaolinite was carried out. Appropriate electrolyte circulation method was employedto maintain microbial activity and contaminant removal. Forced electrolyte circulation methods (S3 and S4) supplied continuously nutrients and microorganisms to soil and maintained neutral pH in all compartments. Besides additional supplement and post-treatment of electrolyte solution were unnecessary. Electroosmosis as well as electrophoresis affected microbial transport in soil.
Phenanthrene degradation by Sphingomonas sp. 3Y depended upon oxygen concentration. In EK bioremediation oxygen was electrically generated at cathode, and so removal efficiency increased with current density. For bacterial consortium having various metabolic pathways optimum current density existed presumably because of appropriate environmental conditions for the bacterial growth. Pentadecane removal increased with its initial concentration because of the increased amount of weakly-bound pentadecane onto soil surface, and also energy expenditure increased due to the inhibition of ionic transport.
Requirements for field application were deduced through varying operational parameters. When the forced electrolyte circulation method was used, electric power was consumed proportionally to the square root of current density increase. Both electrolyte type and electrochemical reaction were important to maintain microbial activity. For example, phosphate prevented an abrupt pH change by increasing soil buffering capacity. Magnesium ion of $MgSO_4$ precipitated to form magnesium hydroxide by combining with hydroxyl ions generated at cathode, and so pH in all compartments decreased largely. Therefore, alternative compounds (ex. $Na_2SO_4$ and $(NH_4)_2SO_4$) as sulfate source should be used. When high current density was introduced, the low electrical conductivity of soil at initial operation increased largely soil temperature so that microbial activity and transport decreased severely. Therefore, the application of high current density should be increased gradually. At certain initial water content, pH polarization between bioreactor and soil was observed. Until 35% of initial water content, removal efficiency was enhanced because of the increased mobility of ions and microorganisms.
EK bioremediation removed rapidly petroleum hydrocarbons in a low permeability soil not producing second pollution. For 200 mg/kg phenanthrene-contaminated soil, 137 mg/kg could be degraded within only 1 week. For 5000 mg/kg pentadecane-contaminated soil, 840 mg/kg was removed within 3 weeks. Consequently, the present methods of EK bioremediation demonstrated superiority over the conventional bioremediation, which had inherent demerits of slow degradation and low removal efficiency.
본 연구에서는 석유계 유류오염토양을 대상으로 현장실용성이 있는 동전기 생물학적복원 기술을 개발하고 실험실 규모의 반응기를 제작하여 인공오염토양에 대한 처리를 실행하였다. 처리기간, 전력소비, 제거효율로 나타나는 공정효율을 향상시키기 위해 전류밀도, 전해질 순환, 함수율, 전자수용체 등의 조건을 다르게 하여 동전기 생물학적복원에 대한 결과를 이해하고 현장적용을 고려한 필요사항을 도출하였다.
1. 미생물활성과 오염물제거를 지속적으로 유지할 수 있는 적절한 전해질 순환법을 도입하였다.
① 전해질 강제순환법을 도입한 공정계 (S3와 S4) 에서는 토양내 지속적인 영양원 및 미생물을 공급하고 중성영역의 pH를 유지시킬 수 있었다.
② 전해액 순환을 통해 미생물과 전해액의 추가 공급과 전해액의 2차처리가 불필요하였다.
③ 동전기 생물학적복원에서 전기영동 뿐만 아니라 전기삼투 또한 미생물의 토양내 주요이동수단이 되었다.
2. 동전기 생물학적복원은 저투수토양에서 추출제를 사용하지 않으며 오염지대의 확대없이 유류오염물의 빠른 제거가 가능하였다.
① 200 mg/kg phenanthrene-오염토양에 대해서 1주만에 최대 137 mg/kg을 제거하였다. ② 5000 mg/kg pentadecane-오염토양에 대해서 3주만에 840 mg/kg가 제거하였다.
③ 생물반응기에서는 공정기간내내 미생물활성을 유지시킬 수 있었으며 오염물이 검출되지 않았다.
④ 동전기에 의해 매우 가속화된 제거속도는 기존 생물학적복원의 느린제거속도와 장기간처리와 같은 내재적 문제를 해결하는 대안이 되었다.
3. 대상토양의 오염특성에 따라 제거결과가 다르게 나타났다.
① Phenanthrene-오염토양 정화에 사용한 Sphingomonas sp. 3Y는 산소농도가 충분치 않으면 제거율이 매우 낮아진다. 동전기 생물학적복원에서 전류밀도의 증가에 따라 산소발생이 많아지고 제거율이 상승되었다.
② Pentadecane-오염토양의 정화에 사용한 복합미생물(bacterial consortium)은 다양한 대사특성을 가짐으로써 최적 전류밀도가 존재하는 것으로 판단된다.
③ 오염물농도가 높아질수록 토양과 약한 결합을 하는 비율이 높아지므로 제거량이 상승되나 이온의 이동을 방해하여 전력소비가 증가하였다.
4. 공정변수의 조절을 통해 현장적용을 고려한 필요사항을 도출하였다.
① 전해질 강제순환법은 전류밀도의 상승에 따라 전력이 제곱배로 증가하므로 복원목표와 소요비용 사이에 의사결정과정이 필요하였다.
② 전해질 종류와 그에 따른 전기화학반응은 미생물활성의 유지에 중요하였다. 인산염의 경우 토양의 완충능을 증가시켜 pH의 급격한 변화를 방지하였다.
③ Mg과 같이 수산화이온과 결합하여 침전물 ($Mg(OH)_2$) 을 형성하는 이온은 공정계내 pH를 매우 감소시켜 미생물활성을 소멸시키므로 대체성분 (예, $Na_2SO_4$) 을 이용해야한다.
④ 높은 전류의 도입시 토양의 낮은 전기전도도가 토양온도를 과다 상승시켜 미생물활성이 저해되고 함수율이 낮아지므로 전류를 단계적으로 증가시켜야 한다.
⑤ 토양과 생물반응기 사이에 pH 분극화를 일으키는 함수율이 존재하였다.
⑥ 높은 함수율(35%까지)에서는 이온 및 미생물의 이동성이 향상되어 제거효율이 증가되었다.
