Application of Fe 2 O 3 nanoparticles in Heavy Metal Removal

Transcription

1 Application of Fe 2 O 3 nanoparticles in Heavy Metal Removal

2 5.1 Introduction Different contaminants are released to water bodies due to the rapid industrialization of human society, including heavy metal ions, organics, bacteria, viruses, and so on, which are serious harmful to human health (Wang et al. 2012). Among the wide diversity of chemical contaminants affecting water resources, heavy metals receive particular concern considering their strong toxicity even at low concentrations (Marcovecchio et al. 2007). Toxic heavy metal can cause serious health effects with varied symptoms depending on the nature and quantity of the metal ingested (Adepoju-Bello and Alabi, 2005).Human beings are generally exposed to the metals such as Aluminum, Arsenic Cadmium, Lead, and Mercury. They exist in water in colloidal, particulate and dissolved phases (Adepoju-Bello et al. 2009) Heavy metals released into the surface and groundwater have been a major preoccupation for many years because of their increased discharge, acute toxicity, non-biodegradable nature and tendency for bioaccumulation ( Xu et al. 2010). Low concentration (below 5 mg/l) of heavy metals is difficult to treat economically using chemical precipitation methodologies. Ion exchange and reverse osmosis have high operation and maintenance costs. Conventional treatment methods do not remove pollutants such as toxic heavy metals, pesticides etc at appreciable level. New approaches are continually being examined to supplement traditional water treatment methods. There is a need to develop more cost effective and efficient techniques. Nanotechnology has great potential for providing efficient, cost effective, and environmentally acceptable solutions for improving water quality. Several bimetallic nanoparticles have also been shown to be useful in decontaminating water containing toxic heavy metals. Nanoscale iron particles represent a new generation of environmental remediation technologies that could provide cost-effective solutions to some of the most challenging environmental cleanup 65 July 2013

3 problems. Several nano-sized iron containing minerals have been studied for adsorption processes in wastewater treatment Various Forms of Iron oxide Nanoparticles The two most commonly studied iron oxides have been magnetite (Fe 3 O 4 ) and maghemite (γ- Fe 2 O 3 ) (Gupta and Gupta 2005).The small size of those nanoparticles, typically 2-3 orders of magnitude smaller than bacteria, provides a much larger surface area than ferric oxide typically usedin water treatment. The magnetic iron oxide nminerals are collectively known as ferrites. Various ferrites have been employed to enhance removal of cobalt and iron from simulated groundwater (Tiwari et al. 2008) Mode of Action of Iron Nanoparticles Due to its low cost-effective, high efficiency, and simple to operate for removing trace levels of heavy metal ions, adsorption technology is regarded as the most promising one to remove heavy metal ions from effluents( Zamboulis et al. 2011) Adsorption mechanism, or surface complexation, is the reaction between adsorbate, an ion or molecule, and the functional group of an adsorbent surface. The classification of an adsorption mechanism depends on the surface complex formed at the adsorption. Surface complexes are divided into outer- and inner-sphere surface complexes, which previously were called physical and chemical adsorption, respectively. With outer-sphere surface complexation, the surface charge is crucial in complex formation, because electrostatic interactions and van der Waals forces (both are weak forces) are involved. The outer-sphere surface complex is separated from the inner-sphere surface complex by the existence of the water layer between the ion or molecule (adsorbate) and the surface functional group of adsorbent.with inner-sphere surface complexation, a covalent or ionic bond (strong forces) is formed, and it can occur regardless of the surface charge. Formed complexes are named as monodentate and bidentate, according to the 66 July 2013

4 number of oxygen bonds to the adsorbed metal (Sparks 2003) 5.2 Materials and Method 5.2.1Synthesis of Fe 2 O 3 Nanoparticles The nanoparticles were synthesized by co precipitation method as discussed by Massart(1981). All chemicals used were analytical grade purchased from MERCK (India). In the synthesis of iron oxide nanoparticles 0.01 M ferric nitrate [Fe(NO 3 ) 3 ] was added to 0.5M sodium hydroxide(naoh) with vigorous stirring using a magnetic stirrer till ph=10.7.the precipitate formed was washed with distilled water and the ph was adjusted to 8.7.Then 1 ml HCl was added to dissolve the precipitate and 0.1M NaH 2 PO 4 was added for re-precipitation. Heated this solution to 100 o C, washed and dried the precipitate Immobilization and Characterization of Nanoparticles We used one step encapsulation method for immobilization of nanoparticles in semi permeable alginate beads. A solution containing Fe 2 O 3 nanoparticles (2.0 wt %) and sodium alginate (2.0 wt %) was prepared with distilled water, and stirred for 30 min at 85ºC. Afterwards, the solution was extruded as small drops by means of syringe into a stirred solution of calcium chloride (10.0 wt %), where spherical gel beads were formed with a size of 2-3 mm. The gel beads were retained in the CaCl 2 solution for 12 h for hardening and then washed with distilled water. Characteristics of entrapped beads were carried out using Scanning Electron Microscope equipped with EDX (Horiba SU-6600) Preparation of Various Metal Solutions Stock solution of lead, arsenic and chromium were prepared. The working solutions (influent) with different concentration were prepared by appropriate dilution of the stock solution. Stock solutions of arsenite (100 mg/l) were prepared by dissolving appropriate quantity of arsenic trioxide, (As 2 O 3 ) 67 July 2013

5 in distilled water. Similarly lead and chromium solutions of 1000ppm were sprepared by dissolving lead nitrate and Cr (NO 3 ) 3 in 1000ml of distilled water Kinetic Studies Column studies were carried out in a column made of pyrex glass of 1.8cm internal diameter and 30cm length. The column was fitted with adsorbent by tapping so that maximum amount of adsorbent was packed without gaps. The influent solution containing known concentration was transferred to the column. All experiments were carried out at room temperature. The effluent solution was collected at regular intervals of time and concentration of metals after adsorption was determined using photometric and voltametric method. The effect of various parameters such as contact time (10, 20, 30, 40, 50, 60 and 90 min), ph (2, 4, 6,8,10 and 12), adsorbent dosage (10, 15, 20, 25, 30 and 40 g) and initial metal concentration (0.50, 1.0, 2.0 and 3.0 mg/l) were studied in term of their effect on reaction processes. The study of adsorption isotherms in water treatment is significant as it provides valuable insights into the application of design. An isotherm describes the relationship between the quantity adsorbed and that remaining in the solution at a fixed temperature at equilibrium. 5.3 Results and Discussion 5.3.1Toxic Heavy Metals Removal from Water Using Fe 2 O 3 Nanoparticles Fe 2 O 3 nanoparticles synthesized by chemical precipitation method were characterized using SEM (Fig.5.1) and EDS (Fig.5.2) spectra. Fe 2 O 3 nanoparticles synthesized by chemical precipitation method shows very good promise for practical applicability of Arsenic(III),Lead(II) and Chromium (VI ) removal from aqueous solution. 90% removal is possible for these metals at ph 12 (Fig.5.3) with in 30 min. Batch study was conducted to find out the optimum adsorbent dose and the contact time for maximum possible removal of arsenic and chromium 68 July 2013

6 Thesis Fig.5.1 SEM image of Iron nanoparticles Fig5.2 EDS analysis of Fe nanoparticles Fig. 5.3 Percentage of adsorption of As (III), Pb(II) and Cr (VI) as a function of time. Initial concentration of metals: 2 ppm, ph Effect of ph The ph is one of the important factors in heavy metal removal using nanoparticle from drinking water. The effect of ph on chromium and arsenic removal was investigated in the initial ph range of Variation of the adsorption capacity of chromium and arsenic with ph was shown in Fig: 5.4. From Fig.5.4, it is evident that about 95% of As (III) was adsorbed on the alginate surface in a ph range of at an initial As (III) concentration of 2.0 mg/l. The percentage 69 July 2013

7 removal decreases rapidly with further increase in ph. As (III) adsorption on iron oxide loaded alginate bead surface was almost ph independent in the range of 4-10, with slight higher adsorption in the acidic ph range. In the case of chromium it is observed that the maximum removal occurred at ph 2.5. % of Adsorption ph As Cr % 0f Adsorption As Cr Time, min Fig: 5.4 Percentage adsorption as a function of ph Fig: 5.5 Percentage adsorption as a function time Effect of Contact Time Kinetic analysis for the adsorption process was studied on adsorption of 1.5mg/L of metals at ph=4.5 with adsorbent dosage of 25g.The contact time was varied from 30 to 90 min. It is evident from Fig. 5.5 that adsorption increased at initial stages and after 75 min the maximum removal percentage of 99.33% was obtained. No further change was observed when contact time was increased to 90 min in the case of chromium Effect of Adsorbent Dosage The effect of adsorbent on adsorption is depicted in Fig: 5.6. The removal is more for higher adsorbent dose. This may be due to the availability of more adsorption sites. Arsenic removal is about 75% for 10 g alginate bead containing concentration of 1.5 g of iron and it increases up to 70 July 2013

8 97.5% for adsorbent dose of 25 g at 2 mg/l arsenic, Increase in adsorbent dosage resulted in an increase in removal of Cr(VI) also. At 10g of adsorbent dosage, the removal of Cr(VI) was 90.66%.Removal efficiency of 100% was achieved at the adsorbent dosage of 25g in the experiment. Removal efficiency was found to increase proportionally with the amount of the adsorbent until a certain value was reached; afterwards, the removal efficiency is maintained constant even if adsorbent is added. The percent removal increases rapidly with increase in the dose of the adsorbent due to the greater availability of the adsorption sites on surface area. Adsorption efficency,% Cr As Weight of Adsorbent g Fig: 5.6 Percentage adsorption as a function of adsorbent dose Effect of Initial Metal Concentration Studies on the effect of initial concentration were conducted by varying it from 0.5 mg/l to 3.0 mg/l keeping adsorbent dosage of 25 g at neutral ph (7) and contact time of 60 minutes. From Fig.6, it is evident that the adsorbent can remove metals completely (100%) when the concentration of arsenic was 0.5 mg/l.in the case of arsenic when the initial concentration increased to 1 mg/l the efficiency of removal was decreased to 98.7%, which remains unaltered for 2 mg/l also. Efficiency of removal was decreased to 83 % at 3 mg/l. It is observed that there was a decrease in 71 July 2013

9 the percentage of removal of arsenic corresponding to an increased initial arsenic concentration. Results indicated that the removal percentage of Cr (VI) on iron oxide nanoparticles increased at high concentration. Moreover, the rate of this adsorptive reaction in the optimized period of contact varies directly with the concentration of the adsorbate Kinetic Studies of As (III) and Cr (III) Adsorption Adsorption isotherms, which are the presentation of the amount of solute adsorbed per unit of adsorbent as a function of equilibrium concentration in bulk solution at constant temperature, were also studied. Freundlich and Langmuir equations were used to find the patterns of adsorption by adsorbent iron oxide for arsenic and chromium removal. The freundlich isotherm is an empirical model that is based on adsorption on heterogeneous surface. The Freundlich equation deals with physico-chemical adsorption on heterogenous surfaces (indicates theadsorptive capacity or loading factor). The linearized form of the Freundlich equation is given as: Log(x/m) = log K + 1/n log Ce where x is the amount of the solute adsorbed, m is the mass of adsorbent used, Ce (mg/l) the equilibrium solute concentration in solution, and K, a constant, which is a measure of the adsorption capacity, and n is a measure of the adsorption intensity. The Langmuir isotherm is valid for single-layer adsorption. It is based on the assumption that all the adsorption sites have equal affinity for molecules of the adsorbate and there is no transmigration of the adsorbate in the plane of the surface. The linear form of the Langmuir equation is Ce/ (x/m) =1/a + (1/b) Ce where x is the amount of the solute adsorbed, m is the mass of the adsorbent, Ce (mg/l) is the concentration of arsenic at equilibrium, a is the amount of solute adsorbed per unit mass of adsorbent required for monolayer coverage of the surface, also called monolayer capacity, and b (Lmg _ 1) is the 72 July 2013

10 Langmuir constant related to the affinity between the sorbent and the sorbate. According to Langmuir model, adsorption occurs uniformly on the active sites of the adsorbent and once an adsorbate occupies a site, no further adsorption can takes place from this site. The system was equilibrated with adsorbent concentration of 0.5g for 30 min with different metal concentrations at room temperature. The equilibrium data obtained were fitted to the Freundlich and Langmuir isotherms. The linearity of the plot log Ce versus log (x/m) for shows the applicability of Freundlich adsorption isotherm for the adsorption in the case of arsenic. The values of K and n were obtained from the slope and intercept of the plot between log (x/m) and log Ce for arsenic removal (Fig. 5.8a). The plots of 1/X against Ce for adsorption of As (III) gave a straight line (Fig.5.8b). The estimated goodness of fit is r 2 =0.711 and enables applicability of the Langmuir model to As (III) adsorption on the surface-functionalized nanoparticles. The Langmuir constants a and b for the present study are 0.33 mg/g and respectively. The linearity of the plot (Fig. 5.7a) for log Ce versus log (x/m) for chromium shows the applicability of Freundlich adsorption isotherm for the adsorption of chromium using iron oxides nanoparticles. The value of 1/n<1 represents favourable adsorption of chromium on to iron oxide The value of K is found to be The plot of Ce with Ce/(x/m) yields straight line (Fig: 5.7 b) indicating the applicability of Langmuir equation. The value of Langmuir constant is In the present study, the values of K and n were found to be mg/g and The isotherm fitted very well for the adsorbent with a correlation coefficient Significantly, higher values of the adsorption capacity (K) obtained with nanoparticles encapsulated alginate indicates that it can be effectively used for the removal of these metals from water. 73 July 2013

11 Fig 5.7a Langmuir adsorption isotherm of Chromium Fig.5.7b Freundlich adsorption of isotherm Chromium Fig 5.8a Langmuir adsorption isotherm of Arsenic. Fig.5.8b Freundlich adsorption of isotherm Arsenic 5.4 Summary Fe 2 O 3 nanoparticles synthesized by chemical precipitation method shows very good promise for practical applicability of As (III) removal from aqueous solution. Up to 99.9% removal efficiency for Arsenic (III) and Chromium (VI) was obtained by Fe 2 O 3 nanoparticles from aqueous solution at a low adsorbent dose, and very short time over a wide range of ph. It could be concluded that the removal efficiency were enhanced with increasing of Fe 2 O 3 dosage and reaction time, but decreased with increasing metal concentration. The influences of the initial ph, temperature, contact time and dosage of the adsorbent on adsorption performance have been experimentally verified by column 74 July 2013

12 method. The equilibrium data were fitted to the Langmuir and Freundlich isotherm equations. The obtained results showed that iron oxide impregnated sodium alginate can be used to remove toxic metals from aqueous solutions even at low concentration. 75 July 2013