Break

The use of a new Fe Substituted Niobium Silicate
(NS91) for Hg(II) ion exchange from aqueous solutions

Z. Lin¹, E.
Fabre^1,2, S.P. Cardoso¹, E. Pereira^2,3,
C. M. Silva¹

¹ CICECO, Department of Chemistry, University of Aveiro, 3810-193
Portugal

² CESAM, Department of Chemistry, University of Aveiro, 3810-193
Portugal

³ LAQV-REQUIMTE, Department of Chemistry, University of Aveiro,
3810-193 Portugal

*simaocardoso@ua.pt

1. Introduction

Many industrial activities are responsible for the release of
mercury into the aquatic bodies, such as battery and lamps production, mining
and metallurgical processes, and chlor-alkali, petrochemical and paint
industries [1]. The persistent character of
mercury allows its accumulation in the living organisms and magnification along
the food chain causing several impacts on environment and human health [2]. The European Union lists mercury
as a priority hazardous substance and encourages its cessation or phasing out of
discharges and emissions by 2021 [3]. Moreover, the Agenda for
Sustainable Development of United Nations of 2030 in one of its goals, promotes
the development of new efficient and low-cost technologies to improve the
quality and reuse of water. In line with that, ion exchange has been widely
applied for water treatment due to its ease of operation and efficacy to treat
large volumes of dilute solutions [4–6]. However, the cost and recover of
ion exchanger is a limiting factor, which drives the search for new materials with
higher capacities and economic viability.

Microporous materials have high surface areas and may be very selectivity
for target metals. In this work, a Fe Substituted Niobium silicate (NS91) has
been synthesized, characterized and applied for Hg(II) removal by ion exchange.
The effect of the pH of solution and increase in the mass of solid have been
investigated. The framework of NS91 is negatively charged electrostatically balanced
with exchangeable cations what makes this material of great interest for
cationic ion exchange.

2.
Experimental

The NS91 was synthesized as follow. Sodium silicate solution,
sodium hydroxide, sodium chloride, potassium chloride, potassium fluoride and
water were mixed. Then a mixture of niobium pentachloride and iron trichloride was
added to above solution with thoroughly stirring. The resulting precursor was
agitated or under ultrasonic bath alternately in a covered container for at
least 2.5 hours and then treated in Teflon in-lined autoclave at 230 ºC for 7
days without agitation. Finally, the solid product was collected by centrifugation,
and washed thoroughly with water, and dried at 80 ºC. The obtained samples were
characterized by powder XRD, SEM, EDS and TG.

The ion exchange studies were performed under batch conditions, at
22 ± 1 °C in 1 dm³ volumetric flasks magnetically stirred at 650
rpm. The spiked solutions were prepared in high purity water (18.2 MΩ cm^-1)
by dilution of the stock solution of Hg(II) in order to obtain the initial
concentration of 50 µg dm^-3. The effect of pH was evaluated in the
range of 3 - 9, adjusted with HNO₃ and NaOH and fixed masses of 7 mg
dm^-3 of NS91. The mass effect was evaluated with (7 and 10 mg dm^-3),
at pH 6.0. Liquid samples were collected, filtered through a Millipore
membrane, and analyzed afterwards by cold vapour atomic fluorescence
spectroscopy (CV-AFS) to quantify Hg(II) concentration in solution. The
experiments were conducted until reaching equilibrium (ca. 120 hours)
and a control solution (solution without solid) was always run in parallel to
check experimental losses.

3. Results and Discussion

From a series of syntheses with different conditions, we have
obtained very similar powder XRD patterns. The SEM micrograph in Figure 1 shows
that the run product consists of aggregate of small bar crystallites with ca.
50 nm. TGA in Figure 1 gives ca. 14 % mass loss between 20 and 800 ºC. With the atomic ration obtained from EDS, the possible
composition of the run product may be (Na,K)₂(Nb,Fe)₂Si₂O₁₀.4H₂O.

After searching in XRD database, we found that the PXRD of our
iron niobium silicate is very similar to that of a rubidium niobium silicate. Using
the structure data of the rubidium niobium silicate, preliminary simulation
gives good agreement to experimental data (see Figure 3). The proposed
structure is shown in Figure 4.

Regarding the ion exchange, the effect of pH is one of the most
important in metal removal processes. The pH impacts the degree of ionization
and speciation of mercury, and the competition with coexisting ions in solution.
Table 1 shows the influence of the pH of the solution on the Hg(II)
elimination. It is clearly shown that this variable strongly affects the
process, the mercury removal increases at pH 5 and then decreases at pH 9. As
the pH increases the concentration of H⁺ in solution is lower and
the Hg(II) removal enhances until the optimum pH 5. The reduction on the % of
removal observed in the range of 5 – 9 may be related with the adjustment of
the pH with NaOH which provides more Na⁺ ions in solution. The
exchangeable cations of NS91 are Na⁺ ions and their higher
concentration in solution reduces the gradient concentration leading to weaker
driving forces for mass transport and hence decreases the ion exchange.

Table 1. The influence of pH of the
solution on the Hg(II) removal .

The NS91 showed high affinity to remove Hg(II) from aqueous
solutions, with only 7 mg dm^-3 of solid, 75 % of mercury was
removed. These results are ascribed to the increase in the number of available sorption
sites with the addition of higher mass.

Acknowledgements

This work was
developed within the scope of the project CICECO-Aveiro Institute of Materials,
FCT Ref. UID/CTM/50011/2019, financed by national funds through the FCT/MCTES,
and the Smart Green Homes Project POCI-01-0247-FEDER-007678, a co-promotion
between Bosch Termotecnologia S.A. and the University of Aveiro. These projects
are financed by Portugal 2020 under the Competitiveness and
Internationalization Operational Program and by the European Regional
Development Fund (FEDER). This work has also
been performed with the support of CNPq (Conselho Nacional de
Desenvolvimento Científico e Tecnológico, Brazil).

Simão P. Cardoso also thanks
the post-doc Grant (BPD/CICECO/5247/2017) financed through the Smart Green Homes
Project. 

References

[1]       C. Liu, J.
Peng, L. Zhang, S. Wang, S. Ju, C. Liu, Mercury adsorption from aqueous
solution by regenerated activated carbon produced from depleted mercury-containing
catalyst by microwave-assisted decontamination, J. Clean. Prod. 196 (2018)
109–121. doi:10.1016/J.JCLEPRO.2018.06.027.

[2]       M.E.
Mahmoud, A.A. Yakout, M.M. Osman, Dowex anion exchanger-loaded-baker’s yeast as
bi-functionalized biosorbents for selective extraction of anionic and cationic
mercury(II) species, J. Hazard. Mater. 164 (2009) 1036–1044.
doi:10.1016/j.jhazmat.2008.09.017.

[3]       Directive
2013/39/eu of the European Parliament and of the Council of 12 August 2013
amending Directives 2000/60/EC and 2008/105/EC as regards priority substances
in the field of water policy, Off. J. Eur. Union. (2013).
https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32013L0039...
(accessed June 18, 2019).

[4]       A.
Da̧browski, Z. Hubicki, P. Podkościelny, E. Robens, Selective removal
of the heavy metal ions from waters and industrial wastewaters by ion-exchange
method, Chemosphere. 56 (2004) 91–106. doi:10.1016/j.chemosphere.2004.03.006.

[5]       C.B.
Lopes, M. Otero, Z. Lin, C.M. Silva, J. Rocha, E. Pereira, A.C. Duarte, Removal
of Hg²⁺ ions from aqueous solution by ETS-4 microporous
titanosilicate-Kinetic and equilibrium studies, Chem. Eng. J. 151 (2009)
247–254. doi:10.1016/j.cej.2009.02.035.

[6]       S.P.
Cardoso, I.S. Azenha, Z. Lin, I. Portugal, A.E. Rodrigues, C.M. Silva,
Experimental measurement and modeling of ion exchange equilibrium and kinetics
of cadmium(II) solutions over microporous stannosilicate AV-6, Chem. Eng. J.
295 (2016) 139–151. doi:10.1016/j.cej.2016.03.007.