Remediation of Malathion Contaminated Soil Using Zero Valent Iron Nano-Particles ()
1. Introduction
Organophosphates are esters of phosphoric acid, thiophosphoric acid and other phosphoric acids, which are widely used as insecticides and acaricides. [1,2] Malathion {Diethyl 2-[(dimethoxyphosphorothioyl)sulfanyl] butanedioate}, commonly referred to as organophosphorous pesticides (OPPs) is used as insecticides for control of insects on fruits and vegetables. Organophosphorus pesticides (OPPs) are a class of chemicals that generally act as cholinesterase inhibitors and have been widely used in agriculture as the replacement for persistent organochlorinated pesticides [3]. However, organic phosphate pesticides are of great environmental concern primarily because they are toxic to mammals and birds [4]. Malathion is a non systemic wide-spectrum organophosphate insecticide and is one of the earliest organophosphate insecticides developed in 1950. It is used for the control of sucking and chewing insects on fruits and vegetables, and also used to control mosquitoes, flies, household insects, animal parasites (ectoparasites). (Malathion may also be found in formulations with many other pesticides).
Pesticides, like other organic pollutants, are retained by the soil according to their physicochemical properties, nature and composition [2]. Malathion is of low persistence in soil with reported retention half-lives of 1 to 25 days. Its degradation in soil is well known and rapidity of its degradation is related to the degree of soil binding. Breakdown occurs by a combination of biological degradation and nonbiological reaction with water. If released to the air atmosphere, malathion will break down rapidly in presence of sunlight, with a reported half-life of 1.5 days [3-5]. The fate of malathion in soil has been studied extensively. It has been reported that the rate of its degradation increases with increasing moisture/alkaline conditions [6]. It is moderately bound to soil and is soluble in water. Therefore, it may contaminate groundwater or surface water in situations which are less conductive to their breakdown [5].
Due to the environmental concerns associated with the accumulation of the organophosphorus pesticides in food products and water supplies, efforts are focussed to develop safe, convenient and economically feasible methods for pesticides detoxification during the last two decades [7,8]. Malathion, despite of its high toxicity is still extensively used throughout the world [7]. Various environmental friendly materials like soil micro flora and rhizophus oryzae biomass, which are efficient adsorbents for malathion, have been suggested for the decontamination of organophosphorous pesticides (OPPs) from soil [9,10]. Shan et al. [11] studied the removal efficiency of malathion from wastewater of organophosphate pesticide mill, using a bacterium, Acinetobacterjohnsonii MA19 [2,11].
In this study, zero valent iron nanoparticles have been used to degrade malathion in ambient environment. It was reported that the addition of iron compounds (Fe0, Fe2+) enhanced the degradation efficiency of OPPs [12- 14]. The present paper describes the investigation on the remediation of malathion contaminated soil using iron nano particles.
2. Materials and Methods
2.1. Chemicals and Reagents
Malathion used in this study was obtained from Sigma Aldrich and was used as received. Doubly distilled water (Millipore) was used throughout the experiment for the preparation of the reagents. All other reagents were of analytical grade and were used without further purification. Stock solution of malathion, was prepared by dissolving 100 mg of pesticides (technical forms and formulations) in minimum amount of glacial acetic acid [Merck] and then diluting to 100 mL with distilled water. Working standards were prepared by appropriate dilution. An aqueous solution of 0.01% (w/v) N-bromosuccinimide (Merck.), 0.02% (w/v) rhodamine B and 4 mol·L–1 hydrochloric acid were prepared. Freshly prepared NBS solution was used. Glacial acetic acid (Merck) was used.
2.2. Synthesis of Iron Nano Particles
The nano-particles of Fe were prepared by reducing ferric chloride solution (FeCl3·6H2O) with sodium borohydride (NaBH4). 1.6 M sodium borohydride solution was added drop wise to the 0.2 M ferric chloride solution under stirring. The high concentration difference between the two solutions leads to the formation of iron nanoparticles. Optimum stirring speed of the magnetic stirrer and concentration of FeCl3·6H2O were arrived at by carrying the experiments by varying these two parameters and arriving at desired size nano particles [15,16].
2.3. Measurement of Hydrodynamic Diameter of Fe Nanoparticles
Malvern, Zeta-sizer nano zs with MPT-2 Auto-titrator was used for the measurement of hydrodynamic diameter of the Fe nano particles. 1.5 mL of the aliquot was taken in the transparent cell for recording the particle size distribution using Dynamic Light Scattering [NIBS,{Non Invasive Back Scattered}]. By this technique the hydrodynamic diameter in the range of 0.6 nm - 6 mm could be measured. Details of the same are discussed elsewhere. [17] The measurements of zeta potential at different pH values showed that the iso-electric-point of iron nanoparticles occurred around 8.1 - 8.2.
2.4. Simulation Experiments for Contamination of the Soil with Malathion
Soil was contaminated with malathion under ambient atomspheric conditions, where 1000 g of the soil was mixed with a stock solution of malathion. This process was carried out by forming slurry followed by slow addition of malathion under continuous stirring. The soil was then dried, hand crushed and passed through the 2 mm sieve. Unused malathion was quantified as discussed below.
2.5. Determination of Malathion
Malathion was estimated indirectly by spectrophotometry. The N-bromosuccinimide (NBS) was used to gradually oxidise malathion [6,18]. Excess NBS, when reacted with rhodamine B, decrease its absorbance for which maximum absorbance was at 550 nm [18]. The decrease in the absorbance is directly proportional to the amount of NBS. From these values, reacted NBS and in turn amounts of malathion were calculated. Beer’s law was obeyed in the concentration range of 0.1 - 1.0 µg·mL–1 [6,18]. All the measurement of malathion have Instrumental RSD < 0.5% whereas the method RSD is <2%.
2.6. Mixing of Malathion Contaminated Soil with Zerovalent Iron Nanoparticles
0.1 g of iron nanoparticle powder was mixed manually with 1000 g of dry soil. To have a moisture equivalent of about 20%, 200 mL of deionised water was added dropwise to this soil. Then it was homogenized with a Teflon rod attached to a mechanical stirrer for 30 minutes and kept for equilibration for 12 h. A control soil sample, without adding iron nanoparticles, was prepared in a similar manner.
2.7. Leaching of Malathion from the Soil
Malathion was leached from the contaminated and control soil samples using ground water having pH in the range of 7.3 - 8.5. The protocol for the leaching of the malathion from the soil was standardized by using an inhouse soil standard. Entire set of experiments consisting of preparation of malathion contaminated soil, control sample and leaching of malathion were completed within two days to avoid possible degradation of malathion in soil.
3. Results and Discussion
Various physicochemical characteristics of the soil and groundwater used in the present study are given in the Tables 1 and 2 respectively. Four different types of soils have been used for this study. No major variation of various soil parameters were observed among these soils (Table 1). Ground water samples also did not show major variation in the characteristics like pH, conductance and redox potential as well as levels of different elements present.
3.1. Optimization of Leaching of Malathion from the Soil
Leaching of malathion was carried out using groundwater in the pH range of 4.5 to 8.5 to arrive at optimum pH. The concentration values as a function of pH are plotted in Figure 1. From the figure it is clear that leaching was low at pH 4.5 (30%) and gradually increased with pH. It was observed that at pH 8.2 about 80% of the spiked malathion was leached from the soil and thereafter percentage of leaching of malathion from soil remained nearly constant. Recovery of malathion from the soil was verified from the results of freshly prepared in-house soil standard and the trends were similar. The cause of preferential leaching of malathion in groundwater at pH 8.2 could be due to preferential binding of OH– over the malathion with the organic matter of the soil. Once the pesticide enters the soil it is partitioned between soil particles and soil solution. In soil particles, malathion particularly binds with the soil organic matter. If organic matter is associated with soil particles, then it is favorable for binding pesticides. However when pH is increased from 4.5 to 8.2, the adsorbed pesticide molecules on soil organic matter are desorbed [1,19].
3.2. Characterization of Zerovalent Iron Nanoparticles
Distribution of hydrodynamic diameter of the zerovalent iron nanoparticles in aqueous suspension is shown in Figure 2 and it suggest that the hydrodynamic diameter was 33 nm. The size standardization was done by using latex particles dispersed in 0.92% NaCl medium having a mean hydrodynamic diameter of 60 nm. The suspension has been monitored for 16 h by taking the aliquots at 1 hour intervals. The various measurements show similar size distribution indicating the stability of the suspension.
The XRD diffractogram of freshly synthesized zero valent iron nanoparticle is shown in Figure 3. The XRay Diffraction (XRD) pattern recorded using Cu Ka radiation in the 2q range between 10˚ - 90˚. The figure indicates that iron is mainly in its Fe0 state, characterized by the basic reflection appearing at a 2-theta value of 44.9˚ and additional peaks at 65.22˚, and 82.50˚ These peaks represent bcc (body-centered cubic crystal) Fe(0) lattice planes(110), bcc Fe(0) (200), and bcc Fe(0) (211).
Table 1. Range of physicochemical parameters observed in case of four different soil.
Table 2. Physicochemical characteristics of the ground water.
Figure 1. Variation of the percentage leached malathion as a function of pH.
Figure 2. Hydrodynamic particle size distribution of iron nanoparticles.
Figure 3. XRD pattern of zero valent iron nanoparticles.
The particles were further characterize by measuring the BET surface area of the particles which comes out to be 23 - 25 m2·g–1.
3.3. Optimization of Concentration of Zerovalent Iron Nanoparticles
Soil contaminated with 10 µg·g–1 of malathion was treated with zero valent Fe nanoparticles in the range of 10 - 130 mg·kg–1 of the soil. It is observed from the Figure 4 that there is a decrease in the concentration of malathion with increase in the concentration of Fe nanoparticles upto 100 mg·kg–1 of the soil and malathion is completely degraded. Therefore, it can be concluded that the optimum value of zero valent Fe nanoparticles is 0.1 g·kg–1 of the soil. This is mainly due to oxidation potential [Fe(0) to Fe(+2) is 0.44 V)] generated by 100 mg·kg–1 of zero valent iron nanoparticles is sufficient to oxidise 10 mg (10 µg·g–1 × 1000 g) malathion in soil.
3.4. Validation of Optimized Concentration of Fe-Nanoparicles
Figure 5 gives the percentage degradation of malathion keeping the concentration of Fe nanoparticles as 0.3 g·kg–1 of the soil. The concentration of malathion was varied from 10 to 50 mg·kg–1 of the soil. It was observed that 100% degradation of malathion was there up to 30 mg·kg–1 and thereafter degradation was only 80% and 60% respectively in the soils having malathion contamination of 40 and 50 mg·kg–1.