The thiol group binds Cd(II) and Pb(II) to surface complexes because of its high affinity to metal ions. The metals were accumulated in electrode due to reduction reaction, and then dissolved in solution through oxidation reaction. The electrochemical reaction occurred as follows:
Complex step Accumulation step Stripping step Electrochemical process of Pb(II) and Cd(II) on electrodes modified by MPTMS-diatomite was illustrated by Figure 4.
Figure 4. Proposed representation of preconcentration and stripping mechanism of Cd(II) and Pb(II) on MPTMS-diatomite/GCE.
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Figure 5 shows the DP-ASV of Cd(II) and Pb(II) with different concentrations. An obvious anodic peak at around -0.82 and at -0.61 V. These observations can be attributed to the oxidation species of Cd(II) and Pb(II), respectively that have been deposited into the surface of MPTMS-diatomite/GCE during reduction processes. The intensity of anodic peak increased with an increase in Cd(II) and Pb(II) concentration from 20 to 300 ppb. A calibration plot of the anodic current response versus Cd(II) concentration and Pb(II) are presented in the insets of Figure 5. The results show that the current peak response was linear to the Cd(II) concentration with a R2 of 0.999 in the range of 20-300 ppb and the Pb(II) concentration with R2 of 0.994 in the range of 20-150 ppb. The limit of detection (LOD) was calculated on the basis of (3.3 Sa/b) criteria (Sa represents the standard deviation of the intercept while b represents the slope of the calibration curve defined for the LOD concentration range (20-300 ppb)). The LOD for Cd(II) and Pb(II) calculated was 15.9 ppb and 6.9 ppb, respectively.
A comparison of MPTMS-diatomite-GCE developed with other GCEs modified with other nanoparticles for simultaneous determination of Pb(II) and Cd(II) shows that the GCE modified with MPTMS-diatomite developed in the present work shows good LOD compared with other similar materials/GCE reported previously in the literature. This indicates that MPTMS-diatomite is a potential material for electrode modifiers.
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Figure 5. The DP-ASV voltammograms of Cd(II) and Pb(II):
Conditions: the concentrations in the range of 20 to 300 ppb of the Cd(II) and Pb(II); other conditions: 0.1 M ABS (pH 4.5); Eacc = –1.2 V; tacc = 60 s; pulse amplitude (E) = 50 mV; pulse time = 40 ms;
potential step = 6 mV; v = 20 mV s–1; = 2000 rpm. (inset (left): a plot of stripping peak current with concentration of Cd(II); inset (right): a plot of stripping peak current with concentration of Pb(II)).
CONCLUSION
1.The Vietnamese diatomite, which is composed of amorphous silica, has high porosity and surface area. It was studied as an adsorbent for the removal of AB dye from aqueous solution. Solution pH has a significant influence in the adsorption of AB, where the capacity of the adsorbents increases with increasing pH from 4.0 to 11.0. Experimental isothermal data were fitted well to both Langmuir and Freundlich model in the large range of 400-1400 mg L-1. The
U(V)
-0.4 -0.5 -0.6 -0.7 -0.8 -0.9 -1
j(uA)
3.6 3.4 3.2 3 2.8 2.6 2.4 2.2 2
0 100 200 300 400
0.0 0.4 0.8 1.2
IP,Cd (A)
CCd(II), ppb
0 40 80 120 160
0.0 0.3 0.6 0.9 1.2
IP, Pb, (A)
CPb(II), (ppb)
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maximum adsorption capacity, qm = 518.0 ± 13.2 mg g-1, calculated from Freundlich equation and qmom = 528 ± 62 mg.g-1 calculated from Langmuir equation are statistically similar. However, parameters of these equations were effected remarkably on the initial AB concentrations. Both value of qmom and qm increases with the increasing initial AB concentration. The free energy of AB adsorption on diatomite is more negative at higher temperature, which demonstrates that the spontaneity increases with the rise of temperature. Piecewise linear regression as a statistical method for the analysis of experimental adsorption data by the Webber’s intraparticle-diffusion models provide the time periods for each diffusion and results show that the AB adsorption onto diatomite was film diffusion controlled. The rate-limiting step is effected on the initial AB concentration. The adsorption processes obey the pseudo- second-order process in the range of 150-400 mg L-1 and the pseudo- first order one in range of 400-900 mg L-1.
2. Mn-Fe binary oxides have been homogeneously incorporated into diatomite by means of redox reaction of KMnO4 and FeSO4. The redox products consist of multiple oxidation state oxides (Mn3+, Mn4+, Fe3+ and Fe2+. The binary oxides dispersed highly into the diatomite surface, forming a Mn-Fe oxides thin layer covering the diatomite surface. Oxide thin layer possesses approximate 1:10 molar ratio of Mn/Fe in composition regardless of the samples prepared in different pH media. The Fe-Mn binary oxides were tested as catalysts for CWPO of phenol. The catalyst demonstrated well in the aqueous medium with a wide range of pHs 4.7-7. The phenol (1000 mg L-1) and intermediates of dihydroxyl benzene were degraded completely after 50 minutes. The high catalytic activity for
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phenol oxidation is due to catalytically synergized properties of iron and manganese oxide highly dispersed on pores of diatomite. Fe-Mn binary oxides modified diatomite materials are promising catalysts for complete oxidation of phenol with hydrogen peroxide in aqueous solutions.
3. The effects of functionalization conditions on the loading of MPTMS in diatomite were investigated. Diatomite from Phu Yen consists of mainly amorphous structure. The crystallization to form quartz occurs at more than 300°C. The diatomite with thermal treatment in the range of 100-300°C is favorable for functionalization. The humidity of diatomite also affects the functionalization level significantly. The MPTMS loading around 9.8% peaked as diatomite was hydrated for 3hours. We have demonstrated that MPTMS-diatomite is useful to prepare chemically modified glassy carbon electrodes. The electrode modified by MPTMS-diatomite exhibited potential for the use of the simultaneous determination of cadmium and lead by DP-ASV. The stripping peak currents of the two metal ions had linear relationships with the concentrations in the range of 20 to 300 ppb (i (àA) = ‒ 0.0202 + 0.0036 C (ppb), R2 = 0.9997) for Cd(II) and 20 to 150 ppb (i (àA) = ‒0.1179 + 0.0069 C (ppb), R2 = 0.9943) for Pb(II). The LOD for Cd(II) and Pb(II) calculated was 15.9 ppb and 6.9 ppb.
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List of articles related to dissertation
1. Bui Hai Dang Son, Vo Quang Mai, Dang Xuan Du, Le Cong Nhan, Tran Thai Hoa, Dinh Quang Khieu (2013), “Freundlich and Langmuir adsorption isotherms for removal astrazon black AFDL dye onto Phu Yen diatomite from aqueous solution”, Vietnam Journal of chemistry, Vol. 59(2AB), 296-301.
2. Bui Hai Dang Son, Vo Quang Mai, Dang Xuan Du, Le Cong Nhan, Tran Thai Hoa, Dinh Quang Khieu (2013), “A kinetic study on astrazon black AFDL dye adsorption onto Phu Yen diatomite”, Vietnam Journal of chemistry, Vol. 51(3AB), pp. 1-5.
3. Bui Hai Dang Son, Vo Quang Mai, Dang Xuan Du, Le Cong Nhan, Tran Thai Hoa, Dinh Quang Khieu (2013), “Study on synthesis of mercaptopropyl- functionalized diatomite”, Journal of Catalysis and Adsorption, vol 2, pp. 136-141.
4. Bui Hai Dang Son, Vo Quang Mai, Dang Xuan Du, Le Cong Nhan, Phan Thi Chi, Dinh Quang Khieu (2014), “Mn-Fe binary oxide incorporated into diatomite: an efficient catalyst for phenol oxidation reaction”, Journal of Science and Technology, 52(5B), pp. 490-495.
5. Bui Hai Dang Son, Vo Quang Mai, Dang Xuan Du, Le Cong Nhan, Dinh Quang Khieu (2015), “Study on the cadimium adsorption over 3- mercaptopropymethosilane modified diatomite from aqueous solution”, Vietnam Journal of chemistry, vol 53(3E12), pp. 238-241.
6. Bui Hai Dang Son, Nguyen Thi Ngoc Trinh, Nguyen Dang Ngọc, Dinh Quang Khieu (2015), “A comparison of physicochemical properties of Phu Yen diatomite and Merck diatomite”, Journal of Catalysis and Adsorption, vol 4(4A), pp. 115-119.
7. Bui Hai Dang Son,Vo Quang Mai, Phan Thi Chi, Nguyen Thi Ngoc Trinh, Dang Xuan Du, Le Cong Nhan, Dinh Quang Khieu (2014), “Study on synthesis of Mn-Fe @ diatomite”, Journal of Catalysis and Adsorption, 3, pp. 127-133.
8. Bui Hai Đang Son, Mai Xuan Tinh, Tran Thanh Minh, Nguyen Dang Ngoc (2016),
“Arsenite and arsenate adsorption by Fe-Mn binary oxides modified diatomite”, Hue University Journal of Science, vol 3, pp.117-124.
9. Bui Hai Dang Son, Vo Quang Mai, Dang Xuan Du, Nguyen Hai Phong, Dinh Quang Khieu (2016), “A Study on Astrazon Black AFDL Dye Adsorption onto Vietnamese Diatomite”, Journal of Chemistry, pp. 1-11;
dx.doi.org/10.1155/2016/8685437. (I.F. = 0.996, ISSN = 2090-9071).
10. Bui Hai Dang Son, Vo Quang Mai, Dang Xuan Du, Nguyen Hai Phong, Nguyen Duc Cuong, Dinh Quang Khieu (2016), “Catalytic wet peroxide oxidation of phenol solution over Fe-Mn binary oxides diatomit composite”, Journal of Porous Material,pp.1-11;doi 10.1007/s10934-016-0296-7. (I.F. = 1.4, ISSN: 1380-2224).
11. Bui Hai Dang Son, Dinh Quang Khieu, Vo Thi Thanh Chau, Pham Dinh Du, Nguyen Hai Phong, Nguyen Thi Diem Chau (2017), “3- mercaptopropyltrimethoxysilane modified diatomite: preparation and application for voltammetric determination of lead (II) and cadmium (II)” Journal of Chemistry; pp.1-10, doi.org/10.1155/2017/9560293. (I.F. = 0.996, ISSN = 2090- 9071).