REDUCTION OF NITRATE IN AQUEOUS WASTE

Transcription

1 Environmental Geochemistry, Vo/.8, No.I &2, pp , 2005 REDUCTION OF NITRATE IN AQUEOUS WASTE D. Mandai Chemical Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai Abstract: Removal of nitrate form aqueous waste as well as from drinking water is a serious global problem. Denitrification processes are broadly classified into biological, thermal and chemical processes. C_hemical reduction is one process to remove nitrate from aqueous waste, groundwater and surface water body, as all the nitrate slats are highly soluble in water. This paper describes the use of metallic iron powder as a reductant for the reduction of nitrite in water with high nitrate concentration in the range of I g/1. The effect of ph on the rate of denitration and products of nitrite reduction was studied with a fixed dosage of metallic iron powder. The ph of the solution was maintained at the desired level using dilute sulphuric acid. The reduction of nitrite by metallic iron was a pseudo-zero-order reaction under the experimental conditions and the reduction rate of nitrite was increased with decreasing the ph of reaction solution. Keywords: Nitrate, wastewater treatment INTRODUCTION Most of the manufacturing industries generate and discharge wastewater containing oxides of nitrogen, such as nitrate and nitrite. Nitrate and nitrite contamination in drinking water causes methemoglobinema, a disease in which oxygenbearing capacity of blood is reduced (Mirvish, 1985; Murphy, 1991). Nitrate ion in the stomach may form N-nitrosoamine, which causes stomach cancer (Mirvish: 1985). Nitrite may be more toxic to aquatic living things and human health than nitrate (Lin and Wu, 1996). Recently, new strict standards of nitrate and nitrite for environmental water were issued in Japan (Nagasaka, 1999), European countries (Murphy, 1991) and also in India (CPCB, 1998). Biological denitrification process is the most widely used technique for the removal of nitrogen oxides from water (Hiscock et al., 1991). However, the rate of denitrification in biological process is relatively low, and the addition of organic compounds is required for the treatment of inorganic wastewaters (Thalasso et.al, 1997). As an alternative process, the nitrate present in aqueous solution can be converted into solid residue by thermal evaporation and the residue nitrate can be thermally decomposed to nitrogen and nitrogen oxides (Cox et.al, 1994). In recent years, chemical reduction has received a lot of attention as an alternative technique for removing nitrate from water (Young 1964; 2004; Heeke 1990; Cox 1994; Horold 1993; Kalu 1996; Lin and Wu, 1996; Cheng 1997; Huang 1998; Hu 2000; 2001). Chemical reduction of nitrogen oxides in water could achieve higher removal rate than biological denitrification process (Lin and Wu, 1996; Cheng 1997). Jesus Blanco (2003) studied the performance of copper/nickel oxide catalyst supported on ceramic monoliths of y alumina!silicate, for the selective catalytic reduction of nitrogen oxides in the flue gas. One of the most used reductants for the chemical reduction of nitrogen oxides is gaseous hydrogen (Hz) [Harold 1993). However, there are some practical problems when using Hz as a repuctant such as the selection of optimal catalyst, generation and /or storage of Hz, and the addition of Hz to water because of its relatively low aqueous solubility. Recently, many researchers have reported the possibility of metallic iron as a reductant of nitrate (Cheng 1997; Huang 1998; Zawaideh 1998; Hu 2001, Chih-Hsiang Liao, 2003, Hong Ying Hu 2001; Paul Westerhoff & Jennifer James, 2003; Seunghee Choe 2004; Yong H. Huang & Tian, 2004). Metallic iron (Fe 0 ) powder is thermodynamically unstable in water and causes iron to react with water producing hydrogen gas and hydroxide (Chih-Hsiang 2003). Mechanisms for electrochemical reduction are still unresolved in many cases, and may involve direct electron exchange between nitrate and Fe 0 that is reduced to ferrous or ferric ion or production of hydrogen gas which subsequent exchanges electrons with a contaminant as given below. (3) 2H-7Hz

2 D.Mandal Both mechanisms rely upon the oxidation of Fe 0 and production of ferrous ion. Ferrous may convert to ferric ion in presence of air or any other oxidizing agent. The nascent hydrogen (H) produced in reaction (2) reduces N0 3. to N0 2. as shown in reaction ( 4) and subsequently nitrogen gas (N 2 ) as shown in reaction (5). Previous work: Hong-Ying Hu (200 1) used iron powder for the removal of nitrate of ground and surface water with maximum nitrate concentration of 180 mg/1. They studied the effect of ph on the rate and products of nitrate reduction with the fixed dosage of metallic iron powder (12 mol Fe/mol-N, size of the powder used 80 mesh). The reduction of nitrate by metallic iron was a pseudo-zero-order reaction under the experimental conditions. The reduction rate of nitrate was increased with decreasing ph of reaction solution and the pseudozero-order reaction rate constants were 180, 130, 60, 15, 10 and 1 mm/h at ph=2,3,4,5, 6 and 7, respectively. The reaction products of nitrate were nitrogen gas and ammonium. They claimed that nitrite in water can be completely reduced by metallic iron powder to nitrogen gas and ammonium under weak acidic or neutral conditions. From the experimental results it was observed that by optimizing the reaction conditions nitrite in aqueous solution can be completely reduced to nitrogen gas with no or less formation of ammonium. Paul Westerhoff and Jennifer James (2003) have studied the reduction of nitrate in a continuousflow zero-valent iron packed bed columns with different process parameters viz., ph, dissolved oxygen and nitrate concentration. They observed that nitrate removal was accompanied by a ph increase, dissolved oxygen decrease and soluble iron increases. Chih-Hsiang et. a!, (2003) used iron powder along with UV light of W intensity and reduction of nitrate carried out in presence of propanol and H They observed that controlled the ph at 4 and they observed that removal of nitrate increases with increasing Fe 0 dosage; but the removal makes no difference as the Fe 0 dosage is greater than 2 gil. UV irradiation retards the dissolution of ferrous and removal of nitrate. The propanol plays a role of organic inhibitor for Fe 0 corrosion. The presence of H appears to inactivate all reactions for the Fe 0 of size 10 lim and for final H remain unchanged but for Fe 0 particle of size 150 /Im, the H decomposed rapidly. Recently, Yong and Tian (2004) studied the effect of low ph (2-4.5) on nitrate reduction with iron powder. The observed that nitrate reduced to ammonia at ph A black eating, consisted of both Fe (II) and Fe (III), was formed on the surface of iron grains as an iron corrosion product, which was unstable evolved with time into other oxide under certain conditions. They proposed a kinetic model incorporating the effects of ph on nitrate reduction and Langmuir adsorption. The nitrate concentration of the feed solution was 248mg/l. Seunghee Choe et. a!, (2004) studied the reduction of nitrate at ph 2, 3, and 4 with initial feed concentration 442 mg/1. They reported that NH 3 1NH/ were the only N products. The nitrate ion concentration was reduced to mg/1 (78 mgn/lit). In all the above mentioned previous works of nitrate reduction with metallic iron, the maximum nitrate concentration in the feed solution was 1.0 g/lit and the nitrate reduction were carried out with excess quantity metallic iron powder, under different ph ranges and the major products are ammonium and nitrogen gas. In this present study, we used the nitrate concentration of the feed solution in the range of g/lit. Reduction of nitrate in the aqueous solution has been carried out at different ph with slow addition of iron powder of minimum quantity. Process was repeated with the same solution after complete removal of iron form the treated solution to achieve the higher efficiency. Chemicals: MATERIALS AND METHODS Iron Powder: Metallic iron powders (purity: >99.8%, size: 100 mesh) No pretreatment of the iron powder was done. Sodium nitrite (L.R Grade) Potassium nitrite (L.R Grade) Manganese' oxide: Technical Grade Sodium Carbonate: Technical Nitrite reduction experiments: Glass beakers of 2000 ml capacity, glass funnel, glass rods, portable agitator and ph meter were used for the experiments. The total volume of the feed solution taken was 1000 ml. The ph of

3 Reduction of Nitrate in aqueous waste reaction solution was maintained at the desired level by adding using sulfuric acid. The denitration reaction by adding iron power was carried out in batches for duration of 4 hours. The reactor was kept open to the atmosphere throughout the experimental period of each batch and the operations were carried out at room temperature. The reactant mixture was stirred by a portable agitator at rpm. The experiments were carried out with different concentration of nitrates, ranging from 20 g/1 to 50 gil and the solution volume used in each experiments were 1000 mi. Iron powder was added (iron dosage: 1.2 gm Fe-powder I gm N0 3 ) to the reactor. Nitrate and ammonium in the reaction solution were measured at proper intervals during the experiments. In the second sets of experiments dissolved iron was completely removed by adding manganese oxide powder, heating the solution up to 60 C for 30 minutes on continuous stirred condition and then slowly adding sodium carbonate powder up to ph 7.0. Iron precipitates as ferric carbonate which was removed by filtration and washing. The filtrate volume was made up to 1000 ml by adding distilled water. Denitration of this solution was carried out as per the process stated above. Analytical methods: A small amount of samples (2-3 ml) was drawn from the reaction mixtures at minutes time intervals. The nitriteconcentration was measured by taking UV absorption spectra of the solution at 220 nm, at room temperature and with adequate dilution against distilled waster as blank in a spectrophotometer (Make: Jasco, Model: V-550). The method of nitrate concentration by UVabsorption is suitable for nitrate concentration in' the range of 0-11 mg/1. Necessary dilution of the samples was done as per requirement. The method is applicable only for nitrate ion, not nitrite ion. Therefore, the analytical results obtained here are the concentrations of N0 3 - in the samples. Iron was measured by atomic absorption spectrophotometer (Make: GBC. Model: Avanta-PM). RESULTS AND DISCUSSION The experimental data are shown in Table 1 for the first four sets of experiments at an average ph level of 3. Fig 1 shows the pattern of decrease of nitrate level versus time. The nitrite concentration dropped down rapidly in first 80 minutes from the start of the reaction then drops slowly for next 100 minutes and then become almost steady. Similar results were observed (Fig 2 and Table 2) in the second set of experiments i.e., after removal of iron and repeat of the process of denitration. The removal of iron is necessary when the nitrate concentration of the feed solution is higher than 10 g/1. As the nitrate concentration is more iron should be removed more times as noted above. Within the first 80 minutes, the relation between the reaction time and the nitrite concentrations at different ph conditions was almost linear. This indicates that the reduction of nitrite by metallic iron under the experimental conditions in this study was a pseudo-zero-order reaction. There are two major steps that may control the reduction rate of nitrite under our experimental conditions: diffusion of nitrite to the surface of iron particles and the reduction of nitrite at the surface of iron particles. The fact that nitrite reduction can be considered as a pseudo-zero-order reaction suggested that the reduction of nitrite at the surface of iron particles was a limiting-step under the experimental conditions. Under our experimental conditions, the reduction of nitrite by ferrous ion may not occur. The increases in the generation rate of hydrogen will enhance the indirect reduction of nitrite. On the other hand, the increase in dissolution rate of iron may keep the surface of the iron powder fresh during the reaction, and thereby may enhance the direct reduction of n_itrite by metallic iron. Due to the slow addition of iron powder no ammonia formation was observed. This was confirmed by smell of the vapor and placing wet HCl-paper on the reaction vessel. Similar results were obtained for different ph levels. As the ph increases the rate of nitrate removal was increased. But, at a higher ph value the formation of Fe (OH) 2 and/or Fe (OH) 3 precipitates on the surface of iron powder which slows down the reduction rate of nitrite. It was observed that 4 is the optimum ph level. 246

4 D.Mandal Sl. No l II Table I : Nitrate ion concentration with respect to time, first batch. Time SET-I SET-II SET-III SET-IV (minute) Noj- ph NO.l- ph Noj- ph Noj- Con. Con. Con. Con. (g/1) (g/1) (g/1) (g/1) ph S.N I II Table 2 : Nitrate ion concentration with respect to time, second batch processes repeated after complete removal of iron. Time SET-I SET-II SET-III SET-IV (minute) Noj- ph No-- ph Noj ph Noj- Con. Con. Con. Con. (g/1) (g/1) (g/1) (g/1) ! ISO ph

5 Reduction of Nitrate in aqueous waste -+- feednirale:10.254g~ feed N~rale gn..,... feed N~rale gn --;;- feednftrale:80.154g,1 Acknowledgements: The author would like to thank Shri S. K. Ghosh, Head Chemical Engineering Division, BARC for his valuable guidance and Shri M.Y.S Natarajan, Shri R. S. Tanksali and Shri M. P. Bellary for their contributions to the experiments. REFERENCES 30 " 21'. ~: ~ " Time (minu\es) Figure-1. Nitrale concen lraijon vs.lime (ph:-3) -t- Set-1: 2nd batch: Feed Nitrate=0.141g/l - ~---Set nd batch, Feed nitrate: 0.546g/l ~- se t- I ll. 2nd ba tch; Feed nitrat e=10.563g/l ~ ~ X Set-IV.2nd batch, Feed nitrate=27.985g/l B 10 ~. ~ 14 g 13 >; l;~~ 3. ~ 2 ~ ~ - ~~~~~ Tine (minutes I F~ure-2. Seco nd balch: Nitrate c on cen t ra l~n vs. lime (ph-1 CONCLUSION The study clearly showed that higher level of nitrite in water (more than 1 g/1) can be reduced by metallic iron. For higher feed nitrate concentration the process may be repeated after removal of added iron to increase the nitrate removal efficiency. When the iron powder is added slowly the reaction product is only o nitrogen gas under weak acidic conditions (ph: 3-4). Decreasing the ph in reaction solution accelerated the reduction rate of nitrite, but lowered the yield of nitrogen gas. It is possible for us to reduce nitrite directly to nitrogen gas with no or less formation of ammonium by optimizing the reaction conditions. Cheng L F., Muftikian R., Fernando Q. and Korte N. 1997, Reduction of nitrate to ammonia by zerovalent iron. Chemosphere 35 (II), Cox J. L., Hallen R. T. and Lilga M. A. 1994, Thermochemical nitrate destmction. Environ. Sci. Techno!. 28 (3), CPCB-Pollution Control Acts, Rules & Notification issued there under, Report No. PCU2/1992 & Amendment thereafter ( 1998). Chih-Hsiang Liao, Shyh-Fang Kang, Yu-Wei Hsu, Zero-valent iron reduction of nitrate in the presence of ultraviolet li ght, organic matter and hydrogen peroxide; Water Research, Vol-37, Heeke K. V., Cleemmput 0. V. and Baert L Chemodenitrification of nitrate-polluted water. Environ. Pollution 63, Harold S., Tacke T. and Vorlop K.-D. 1993, Catalytical removal of nitrate and nitrite from drinking water: Screening for hydrogenation catalysts and influence of reaction conditions on activity and selecti vity. Environ. Techno!. 14, Hiscock K. M, Lloyd J. W, Lerner D. N, 1991, Review of natural and artificial denitrification of ground water, Water Research, 25 (9), I Hu H.-Y., Fujie K. and Goto N., 2000, Electrochemical reductive removal of nitrate from water and wastewater. In Proceedings of Ninth Japan-Korea Symposium on Water Environment, Hu H.-Y., Goto N. and Fujie K., 2001, Reductive treatment characteristics of nitrate by metallic iron in aquatic solution. J. Chern. Eng. Japan, submitted for publication. Fig. 7. Effect of ph on the composition of reduction products of nitrite by metallic iron. Hong-Ying Hu, Naohiro Goto and Koichi Fujie, Effect of ph on the reduction of nitrite in water by metallic iron ; Water Res. Yo. 35, No II, pp Huang C. P., Wang H. -W. and Chiu P. C Nitrate reduction by metallic iron. Water Res. 32 (8), Jesus Blanco, Pedro Avila, Silvia Suarez, Malcolm Yates et. al, 2003 ; CuO/NiO monolithic catalysts for NOx removal from nitric acid flue gas; Chemical Engineering Journal; 97, 1-9. Kalu E. E., WhiteR. E. and Hobbs D. T. 1996, Use of a hydrogen anode for nitrate waste reduction. J. Electrochem. Soc. 143(10), Lin S. H. and Wu C. L., 1996, Electrochemical removal of nitrite and ammonia for aquaculture. Water Res. 30(2), :248

6 D.Mandal Lin S. H. and Wu C. L., 1996, Remove of nitrogenous compounds from aqueous solution by ozonation and ion exchange. Water Res. 30(8), Mandai D, Denitration of liquid waste, Indian Chemical Engineer, 45 (3) Mirvish S.S., 1985, Gastric cancer and salivary nitrate and nitrite, Nature, 315, Murphy, A.P, 1991; Chemical removal of nitrate from water, Nature; 350, Nagasaka Y., New water environmental standards for human health. Environ. Management 35(9), Paul Westerhoff and Jennifer James, 2003, Nitrate removal in zero-valent iron packed columns, Water Research Vol-37, Seunghee Choe. Howard M. Liljestrand, Jeehyeong Khim, 2004, Nitrate reduction by zero-valent iron under different ph regimes; Applied Geochemistry, 19, Thalasso F., Vallecillo A., Garcia-Encina P. and Fdz Polanco F., 1997: The use of methane as a sole carbon source for wastewater denitrification. Water Res. 31 (I), Warna J., Turunen I., Salmi T. and Maunula T., 1994 Kinetics of nitrate reduction in monolith reactor. Chem.Eng. Sci. 49(24B), Young G. K., Bungay H. R., Brown L. M. and Parsons W. A., 1964, Chemical reduction of nitrate in water. J. WPCF 36(3), Yong H. Huang and Tian C. Zhang, Effects of low ph on nitrate reduction by iron powder; Water Research, Vol-38, 2004, p Zawaideh LL, & Zhang T.C, 1998; The effects of ph and addition of an organic buffer (HEPES) on nitrate transformation in Fe 0 -water systems; Water Sci. Techno!. 38,