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AZERBAIJAN CHEMICAL JOURNAL № 2 2019
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UDC 541.64:547.458.81 EFFECT OF ORGANIC MATTER ON HEXAVALENT CHROMIUM REDUCTION BY
NANO ZERO VALENT IRON IN SOIL
G.R.Allahverdiyeva
Baku State University gunel.allahverdiyeva.89@gmail.com
Received 06.11.2018
The experimental data are presented on lowering concentration of chrome(VI) ions in soil (with a purpose of rendering it harmless) by nanoparticles of iron in the presence of high concentration of an organic compound (saccharase) as a reducing agent. The reiterated repetition experiments has proved an effectiveness of synthesized before iron nanoparticles. The basic parameters of an effective reduction of Cr(VI) by nanoparticles of iron have been found out. They are as follows: the time of reduction (2 h) and concentration of organic substances (35.71 g/kg of soil). It has been established that a rise in reduction duration above 2 h lowers its effectiveness. The cause of this, revealed by us, consists in that a high concentration of the organic compound provides for inhibitating of Cr(VI). It occurs on account of 1) adsorption of substance on a surface of iron nanoparticles and as a result the decrease in their active surface, 2) interaction of the organic compound with the products of the reduction reaction, 3) dissolution of saccharase in liquid phase. As result of investigation the necessity of determining the significance of high concentration of the organic substance for an effective reduction of Cr(VI) in soil is exposed. At present the work in this direction is being continued.
Keywords: Fe0 nanoparticles, organic matter, Cr(VI) reduction.
https://doi.org/10.32737/0005-2531-2019-2-11-14
Introduction
Hexavalent chromium is a widely detected in soil and water. The all-round spreading of such contaminant is by the reason of its exploitation in a broad range of industries including tanning of leather, production of stainless steel, wood treatment, refractory industry, chromium mining electroplating, and pigment manufacture [1]. A multitude of waste sites, ground-water and soils are polluted by anthropogenic chromium. Cr(VI) is toxic for plants, microorganisms, animals and humans and is linked with elaboration of numerous chronic health frustrations such as respiratory impairment, organ damage and dermatitis. Reduction of Cr(VI) to Cr(III) is desirable as the latter species is an essential nutrient for maintaining normal physiological function in human organisms, has a low mobility and bioavailabity [2]. Given strong dependence of Cr mobility and toxicity on its re-dox, remediation technologies that reduce Cr(VI) such as reduction by Fe0 nanoparticles are a matter of practical interest [3]. Several studies demonstrated that Fe0 nanoparticles are an efficient and inexpensive reductants for Cr(VI) [4, 5].
A special attention has been directed to evaluating the impact of naturally organic compounds [6] and/or coordinating reagents [7] on the reduction rate of Cr(VI) mediated with Fe0 nanoparticles [8].
Soil organic matter influence the effectiveness of soil remediation processes. Simple aliphatic organics acids with one to three car-boxylic acid groups such as acetic, citric, formic, fumaric, lactic, oxalic, malonic, valeric, succinic, tartaric and p-hydroxybenzoic acid occur frequently in soil and subsurface environments. Such compounds are released by organisms or are produced during the decomposition of larger biopolymers and can alter chemical processes in soils through complexation with metal ions in solution and ligand exchange reactions at soil surfaces [9]. They play a crucial role in chromate reduction by Fe0 nanoparticles via adsorption and complexation with surface sites of Fe0 and/or with the iron corrosion products. Thus, sucrose as non-reducing sugar and having aromatic nucleus with phenolic like humic substances was chosen for the study of organic compound on the reduction of Cr(VI) by Fe0 nanoparticles.
In the present paper, the effect on Cr(VI) reduction rate in the presence of high concentration of organic matter was investigated.
Materials
The soil sample (S) was collected at an agricultural site close to the chromium contaminated site, and it was characterized by a high TOC (Total Organic Carbon) value. Samples were artificially contaminated by applying 50 ml solution of 142.5 g/l of C12H22O11 (sugar) on 50 g of soil. All (soil/solution) was agitated daily for one week, put to mature at room temperature (20±10C). The initial soil characterization was carried out through commonly used protocols [10]. The concentration of Cr(VI) was determined by the colorimetric method of diphe-nylcarbazide [11] after alkaline digestion according to the EPA 3060A method (US EPA, 1996) using a UV-visible spectrophotometer (T80+, PG Instruments, Ltd). The organic compound content is given by Shimadzu Total Organic Carbon Analyzer instrument (TOC-L Model).
Ferrous sulphate heptahydrate (FeSO4-7H2O) and sodium borohydride (NaBH4) were purchased from Tianjin Fuchen Agent Manufactory, China. K2CrO4 was purchased from Fisher Scientific (Pittsburg, PA).
Experimental part
Nanoscale zero-valent iron particles can be prepared in aqueous solutions via the reduction of ferric iron Fe(III) or ferrous iron(II) with sodium borohydride. Zero-valent iron particles can also be prepared from hydrogen reduction of iron oxides. In this work, the method of synthesis of nanoscale iron particles with the sodium borohydride was used. Fe0 nanoparticles were prepared from 1 g/L Fe2+ aqueous solution, by the reduction of sulphate heptahydrate with potassium borohydride at room temperature and in a free oxygen atmosphere. Carboxy Methyl Cellulose (CMC) was used as stabilizer. A key advantage of this method is its simplicity. It can be safely done in most chemistry laboratories with simple chemical reagents. Specifically, the synthesis of iron nanoparticles was conducted in a flask reactor with three open
necks. The central neck was housed with a magnetic stirrer at 350 rpm. Typically, 2:1 volume ratio of NaBH4 and FeSO47H2O were vigorously mixed in the flask reactor for additional 30 min after the titration. The reduction of Fe2+ to Fe0 occurred according to the following reaction [12]:
Fe+2+BH4-+6H2O^Fe0+2B(OH)3+7H2|. (1)
The generated iron particles were harvested with vacuum filtration and stabilized with a large volume of deionized water to wash, and at the end, with diluted ethanol.
A 0.86 g/L solution of Fe0 nanoparticles was obtained and used for the reduction tests. Batch experiments were carried by mixing 5 g of soil in orbital shaker at 120 rpm with 50 ml of the reducing solution. The stoichiometric amount of Fe0 nanoparticles was calculated according to the following equation:
2CrO4-3Fe°+10H+~2Cr(OH)3+3Fe2+2H2O. (2)
As regards the organic compound effect in soil, the experiments were performed at room temperature (20±10C), and at the end of each test, the soil sample was filtered through a 0.45 p,m Whatman membrane filter, and the reaction was stopped by washing the soil with distilled water. The residue amount of Cr(VI) and organic compound in soil were determined after treatment.
Results and discussion
Effect of soil organic matter on Cr(VI) reduction. The study reveals that treating polluted soil with a Fe0 nanoparticles aqueous solution, the chromium removal was time dependent and increasing with Fe0 nanoparticles. Experiments show that a decrease in the rate of Cr(VI) reduction in the soils during two hours treatment made with reducing solution of Fe0 nanoparticles (x25) from 91% to 12% in the presence of organic matter (sugar). The inhibition might be due to the fact that the organic compound adsorbed on the surface Fe0 nanoparti-cles was quite a lot, which made the active reaction sites of the iron Fe0 nanoparticles occupied. It seemed that the availability of Fe0 nanoparti-
G.R.ALLAHVERDIYEVA et al.
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cles active sites for adsorption and reductive reactions provoked the reaction.
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30 -20 -10 -0
0 5 10 15 20 25 30 Contact time, h
Cr(VI) reduction in soil sample.
Figure shows that only a slight decrease (about 12%) in the amount of Cr(VI) in the soils occurred after 2 hours of treatment by the Fe0 nanoparticles (x25) reducing solution. These results suggest that high organic matter concentration had an inhibitory effect on the Cr(VI) removal by Fe0 nanoparticles. After 1 min of reaction, the reaction rate was obviously lower. And prolonging the time up to 16 h, the rate of Cr(VI) reduction was stable at 36%, indicating the end of the reaction. Even prolonging the reaction time up to 16 h, only a constant value of 36% reduction of Cr(VI) was calculated, thus indicating that steady conditions were attained. Regarding the reaction mechanism, it has been suggested that Fe0 nanoparticles first reduce Cr(VI) to Cr(III), then Cr(III) also precipitates as Fe(III)-Cr(III) hydroxides on the Fe0 nanoparticles surfaces. Steady conditions were therefore due to the passivation of the nanoparticles surface, with the consequent loss of reactivity [13]. In fact, the nature of organic matter strongly influences the reaction mechanisms. Acidic or chelating compounds, such as humic and fulvic substances in soil, generally increase the reduction effectiveness, by forming complexes with reaction products as Fe(III) and Cr(III) thus preventing their precipitation and the passivation of iron nanoparticles. On the contrary, in the presence of non humic organic matter, such as organic wastes, the reduction inhibition was attributed to the solubilization of the organic matter of soil and its adsorption of organic compounds onto the surface of iron na-
noparticles, thus reducing the availability of free active reaction sites.
Moreover, Table reports the mean parameters in the natural and treated soil. A significant reduction of the organic carbon in soil occurred during treatment; due to organics dissolution in the extract was also observed. At the same the TOC of the liquid phase in the reactor correspondingly increased.
Parameter Natural soil (S) After treatment (S)
OC, g/kg 35.71 8.11
Cr, mg/kg 104.7 104.1
Cr(VI) mg/kg 51.48 45.25
Conclusion
The reduction of hexavalent chromium performed using nanoparticle zero valent iron proved to be fast and efficient, but it depended on the contact time, the concentration of Fe0 nanoparticles and the chemical composition (namely, organic matter content) of the soil. Iron nanoparticles were synthesized with the borohydride method. In this study, the effect of the presence of organic matter on the removal Cr(VI) by the CMC-Fe0 nanoparticle was investigated on artificially contaminated soils. Regarding the effect of presence of organic compounds on the removal of Cr(VI) by Fe0 nano-particles a significant decrease in the rate of Cr(VI) reduction was observed in the presence of high level of organic matter in the soil. Addition, we note a significant reduction of the organic carbon in treated soil that to say 77.5% which is probably due to the degradation of organic matter and the dissolution in the liquid phase. This behavior has been attributed to the dissolution of organic matter into the liquid phase, and its adsorption onto nanoparticles.
References
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TORPAQDA ÜZVÍ MADDONÍN ALTI VALENTLÍ XROMUN DOMÍR Fe0 NANOHÍSSOCÍKLORÍ ÍLO
reduksíyasina TOSÍRÍ
G.R.Allahverdiyeva
Hazirki içda yüksak qatiliqda üzvi madda (çakar) içtirakinda Cr(VI)-nin reduksiya edici agent kimi damir Fe0 nanohissaciklari içtirakinda reduksiyasi tacrübalari taqdim edilmiçdir. Dövri tacrübalar avval sintez edilmiç damir nano hissaciklarin effektivliyini sübut etmiçdir. Damir nanohissaciklari ila Cr(VI) reduksiyasinin effektivliyinin asas parametrlari tapilmiçdir. Onlar açagidakilardir: reduksiyanin müddati (2 saat) va üzvi maddanin qatiligi. Müayyan edilmiçdir ki, reduksiya müddatinin 2 saatdan cox olmasi effektivliyi azaldir. Buna sababi yüksak qatiliqli üzvi madda Cr(VI) reduksiyasina inhibitor tasiri göstarmasidir. Bu 1) üzvi maddanin damir nanohissaciklarinin sathina adsorbsiyasi hesabina baç verir va natica olaraq aktiv sahalarin azalmasina sabab olur, 2) reduksiya mahsullari ila üzvi maddanin reaksiyasi, 3) saxarozanin maye fazada hall olmasi sayasinda hesabina baç verir. Aparilan araçdirmalarin naticasi olar aq torpaqda Cr(VI)-un effektiv reduksiyasinda uzvi maddanin yüksak qatiligimn tayin edilmasinin ahamiyyati müayyan edilmiçdir. Hazirda bu istiqamatda içlar davam etdirilir.
Açar sözlar: Fe" nanohissaciklari, üzvi madda, Cr(VI) reduksiyasi.
ВЛИЯНИЕ ОРГАНИЧЕСКОГО СОЕДИНЕНИЯ НА ВОСТАНОВЛЕНИЕ ШЕСТИВАЛЕНТНОГО ХРОМА В ПОЧВЕ НАНОЧАСТИЦАМИ ЖЕЛЕЗА Fe0
Г.Р.Аллахвердиева
Представлены экспериментальные данные по снижению концентрации ионов в почве (с целью ее обезвреживания) наночастицами железа Fe0 в присутствии высоких концентраций органического соединения (сахароза, Q^On) в качестве восстанавливающего агента. Многократное повторение экспериментов доказало эффективность синтезированных нами ранее наночастиц Fe0. Выявлены основные параметры эффективного восстановления Cr(VI) наночастицами Fe0. Это - время реакции (2 ч) и концентрация органического вещества (35.71 г/кг почвы). Установлено, что увеличение продолжительности восстановления свыше 2 ч снижает его эффективность. Вскрытая нами причина снижения эффективности реакции заключается в том, что высокие концентрации органического соединения способствуют ингибированию восстановления ионов Cr(VI). Это происходит за счет: l) адсорбции органического вещества на поверхности наночастиц Fe0 и в результате - уменьшения их активной поверхности; 2) взаимодействия органического соединения с продуктами реакции восстановления; 3) растворения сахарозы в жидкой фазе. В результате исследования выявлена необходимость определения строгой концентрации органического вещества для эффективного восстановления хрома в почве. В этом направлении работа продолжается.
Ключевые слова: наночастицы Fe0, органическое соединение, восстановление Cr(VI).
