Chemical reduction of nitrate, which is a major water contaminant regulated in most countries, by nano-scale zero valent iron (NZVI) particles has attracted great attention recently due to its high potential for nitrate control. However, a proper kinetic model for nitrate reduction by NZVI has not been proposed especially in iron-limiting condition and furthermore, the practical applicability for water treatment, e.g., the effect of other components in natural water bodies has hardly been concerned. In this study, kinetic models for nitrate removal and product generation were proposed and evaluated for different NZVI dose, pH and ionic strength. And the effects of humic acid (HA) on nitrate reduction by NZVI were investigated. HA is adapted as the representative inhibitor and/or promoter because it is the most widely distributed natural organic matter which significantly affects the performance of water treatment processes. Property of HA was also investigated for better understanding of the effects on the performance of NZVI. NZVI for this study was prepared via chemical reduction by borohydride and it showed average particle size of 16.7 nm and BET surface area of $17.623 \plusmm 0.042 $m^2$/g$.
Nitrate removal was described successfully by adsorption kinetic equations and unreacted nitrate at NZVI surface was partially recovered in this study. Parameters of adsorption kinetics showed good correlation with reaction conditions. Pseudo first order reaction kinetic equation, which has been commonly adopted, provided poor fit but the rate constant showed good correlation with reaction conditions. Adsorption models considering catalyst deactivation provided better prediction of nitrate profiles, while global deactivation model predicted equilibrium and concentration dependent deactivation model predicted residual activity. Ammonia generation was described separately because the retardation of ammonia generation has been reported and also observed in this study. It was successfully described by Langmuir-Hinshelwood kinetic models with good correlations between kinetic parameters and reaction conditions indicating nitrate reduction to ammonia is a heterogeneous catalytic reaction.
Nitrate reduction was enhanced at high NZVI dose, lower pH and lower ionic strength. Inhibition of higher initial pH ($\gt9$), which is favorable for iron (hydr)oxide at NZVI surface, was more significant than the promotion of nitrate reduction. The kinetic parameters were highly dependent on ionic strength, indicating electrostatic attraction or hydrogen bonding is predominant for nitrate adsorption on NZVI. It is more supported by the profiles of nitrate and ammonia at high initial pH of 11, which implies the competition between nitrate anions and hydroxyl anion.
Both nitrate reaction rate and removal efficiency increased at low HA concentration (6.25-12.5 mg/L) and they decreased with HA higher than 50 mg/L. Kinetic parameters of pseudo first and second order adsorption equations resulted in good prediction and good correlations between the parameters and HA concentration separately for lower (6.25-25 mg /L) and higher (25-200 mg/L) HA. Nitrate and ammonia profiles with HA of 50-200 mg/L showed two distinct regions corresponding before and after equilibrium of DOC, which implies the competitive adsorption of HA and also the insignificant effects of bulk HA on nitrate removal. Based on the results, it seems clear that HA acts as an electron mediator at low surface mass but it inhibits nitrate reduction by development of condensed structure of natural polymer on iron surface at high surface mass, which was further enhanced by higher ionic strength.
Organic fraction of the samples were analyzed for organic carbon content, UV/VIS spectroscopy, fluorescence spectroscopy and FT-IR spectroscopy, to investigated the behavior of HA during reaction with NZVI. The maximum adsorption amount of HA on NZVI was $34.39713\plusmm0.708$ mg-C/g-NZVI and equilibrium reached less than 1 hr. Indicators of aromaticity and molecular size in UV/VIS spectroscopy ($UV_{254}$, $E_4/E_6$, $\epsilon_{280}$, $\epsilon_{600}$, $SUVA_{254}$, $SUVA_{365}$ and $SCOA_{436}$) increased during reaction and $MW_w$, $MW_n$ estimated from SEC chromatographs also increased, which indicate the aggregation of dissolved HA into higher molecular weight. The separate HA aggregates and those on NZVI particles were observed by TEM images. EEM and SF spectra of aqueous fraction from fluorescence spectroscopy showed “blue-shift” by significant decrease of peaks representing more condensed aromatic structures and much less decrease of them representing smaller molecules when all amount of HA was adsorbed on NZVI, while SF spectra showed “red-shift” when initial HA was higher than maximum adsorption amount. FT-IR spectra of reacted solid fraction also evidences preferential adsorption of aromatic moiety to aliphatic structures and the formation of more conjugated aromatic structure on NZVI surface. It is more supported by the relative increase of peak intensity of aromatic components to aliphatic components in FT-IR spectra. The results indicate that the inhibition of nitrate reduction by HA is contributed by interruption of mass transfer by preferential adsorption of aromatic moiety and aggregation and condensation of them on surface active sites of NZVI.
질산은 대부분의 국가에서 규제되고 있는 주요 수질오염물질로서, 나노 철 입자 (Nano-scale zero valent iron, NZVI)에 의한 질산의 화학적 환원은 그 잠재력이 매우 높아 최근에 크게 주목받고 있다. 그러나, 특히 NZVI 제한 조건에서, NZVI에 의한 질산의 화학적 환원반응에 관한 적합한 반응속도 모델이 제시되지 않았으며, 자연수계에 존재하는 다른 성분의 영향 등, 수처리에 대한 실 적용 가능성은 거의 검토되지 않고 있다. 본 연구에서는 질산 제거와 반응산물 생성에 관한 반응속도 모델을 제시하였으며, 이 모델은 NZVI 주입량, pH 및 이온 강도 등의 반응조건에 대해 평가하였다. 또한, 자연에 널리 존재하는 자연 유기물로 수처리 공정의 성능에 크게 영향을 미치는 휴믹산을 대표적인 저해물질 또는 촉진물질로 선정하여, NZVI에 의한 질산 환원에 미치는 영향을 연구하였으며, 휴믹산이 미치는 영향을 보다 잘 이해하기 위해, 휴믹산의 특성 역시 조사하였다. 본 연구에 이용된 NZVI는 화학적 환원법에 의해 합성하였고 평균 입경은 16.7 nm, 비표면적은 $17.623\plusmm0.042$ $m^2$/g이었다.
NZVI에 의한 질산의 제거는 흡착속도식에 의해 잘 묘사되었으며 NZVI 표면에 흡착되어 있는 미반응 질산이 부분적으로 회수되었다. 흡착속도상수는 반응조건과 높은 상관관계를 가졌다. 가장 많이 적용되어 온 1차 반응속도식은 실험결과에 적합하지 않으나, 반응속도상수와 반응조건은 높은 상관관계를 가졌다. 흡착속도식은 촉매의 활성저하를 고려하는 경우, 실험값을 보다 잘 묘사하였으며, 반응물질 농도와 무관한 활성저하 모델 (global deactivation) 은 완전한 활성 상실을, 반응물질 농도를 고려한 활성저하 모델의 경우에는 활성이 잔류하는 것으로 예측하였다. 반응생성물인 암모니아의 생성은 질산의 제거 속도와 다른 양상을 보이므로, 별도로 해석되었으며, Langmuir-Hinshelwood 반응속도식에 의해 성공적으로 묘사되었고, 이는 NZVI에 의한 질산의 환원이 불균일 촉매 반응임을 나타낸다.
NZVI에 의한 질산의 환원은 높은 NZVI 주입량, 낮은 초기 pH 및 낮은 이온강도에서 향상되었고, 이온강도에 민감하였다. 철 수산화물의 생성이 용이한 높은 초기 pH ($\gt9$) 에서는 반응속도가 현저히 저하되었으며, 수산화 이온과 질산이온의 경쟁흡착 현상이 관찰되었다.
질산의 제거와 암모니아의 생성속도는 휴믹산이 전자전달 매개체로 작용함에 따라 낮은 휴믹산 농도에서 향상되었고 (6.25-12.5 mg/L), 그 이상에서는 NZVI 표면에서 휴믹산의 흡착량이 증가하고, 그 구성요소들이 보다 조밀한 구조를 형성하여 활성점으로의 물질전달이 저해됨에 따라 저하되었다. 50 mg/L 이상의 높은 휴믹산 농도에서 질산과 암모니아 농도는 용존유기물질 농도의 평형 시점 (1 시간 이내) 전과 후로 2개의 구역으로 나누어지며, 흡착 및 반응 속도상수도 상이한 값을 나타내었다.
휴믹산에 기인한 용존 유기물의 특성을 UV/VIS 분광법, fluorescence 분광법 그리고 SEC chromatography에 의해 분석한 결과, 방향족성 증가, 평균분자량 증가 및 방향족 화합물의 지방족 화합물에 대한 우선흡착 현상이 관찰되었고 이는 반응 종류 후 수집된 고형물의 FT-IR 스펙트럼에도 나타났다. 또한 NZVI와 용존 휴믹산의 응집체(aggregates)가 투과전자현미경에 의해 관측되어, 휴믹산이 NZVI와의 반응에 의해 고분자화되어 NZVI의 활성점을 감소시켜 반응물의 물질전달이 저해되어 반응속도와 효율이 감소시키는 원인이 됨을 확인되었다.
