(63ak) Insight into the Mechanism of Toxic Chromium (VI) from Wastewater Using Magnetite Coated PINE Powder

INSIGHT
INTO THE MECHANISM OF TOXIC CHROMIUM (VI) FROM WASTEWATER USING MAGNETITE
COATED PINE POWDER

                
Pholosi A., Ofomaja A., Naidoo E.B.

Biosorption and Wastewater
Treatment Research Laboratory, Department of Chemistry,

Faculty
of Applied and Computer Sciences, Vaal University of Technology, P. Bag X021,
Vanderbiljpark, 1900 South Africa.

INTRODUCTION

A knowledge of the interaction between
pollutant molecules or ions and active sites on an adsorbent surface is an
important phenomenon in many industrial processes such as catalysis, adsorption
and other separation techniques. The use of lignocellulosic materials for
Cr(VI) adsorption has gained interest over the years (Miretzky and Cirelli,
2010). The adsorption properties of lignocellulosic materials has been
attributed to acidic functional groups on their surfaces which can take part in
ion-exchange, complexation, chelation and hydrogen bonding with pollutant species
in solution (Vithanage et al., 2015). Two mechanisms for lignocellulosic
adsorption of Cr(VI) have been proposed; (i) an electrostatic mechanism in
which at low pH, the protonated acidic functional groups on the biomaterial
carrying a positive charge attracts the negatively charged Cr(VI) species (Demirbas,
2005) and (ii) the adsorption-coupled reduction mechanism
in which Cr(VI) is reduced to Cr(III) either in solution (Direct reduction) or
after being adsorbed (Indirect reduction) by
electron-donor groups of the biomaterial that have
lower reduction potential values than that of Cr(VI) (Gao et al., 2008).
The Cr(III) ions formed in solution may then form complexes with
Cr(III)-binding groups on the biomaterial surface or released into bulk
solution by repulsion between Cr(III) and other positively charged groups on
the biomaterial (Miretzky and Cirelli, 2010).

METHODS

Sodium hydroxide treated pine coated with
magnetite (NTP-NC) was prepared by co-precipitation method. The effect of
solution pH was performed in a batch system in which 0.5 g of the adsorbent was
contacted in six 250 cm³ beakers containing 75 cm³ of 75
mg/dm³ Cr(VI) solutions set at pH 1, 2, 4, 6, 8 and 10 at 26 °C
and agitated at 200 rpm for 2 h and the clear solution analysed for Cr(V/I)
left using UV-Vis spectrophotometry at 460 nm

RESULTS

The results of the effect of initial
solution pH on the adsorption of Cr(VI) onto NTP-NC is shown in Fig. 1. To
confirm the operating mechanism, the change in [H⁺] concentration at
each initial pH, the oxidation/reduction potential before and after adsorption
and the amounts of Total Cr, Cr(VI) and Cr(III) present in solution at each pH
was monitored for NTP-NC and the results displayed in Fig1 . The change in [H⁺]
concentration (Δ[H⁺] = [H⁺] _final – [H⁺]
_initial) at the end of the adsorption process for each initial solution
pH was recorded and plotted against the initial solution pH. The results showed
that the change in [H⁺] concentrations at the end of the adsorption
process where higher for lower initial solution pH's (where lower amounts of Cr
was found in solution) than for higher initial solution pH's (where higher
amounts of Cr was found in solution) and the change in [H⁺]
concentration became small and almost constant after initial solution pH 4.
This is consistent with the assumption that the removal of Cr from solution is
associated with [H⁺] ion consumption and that as the concentration
of [H⁺] ions reduces in solution, the Cr removal reduces. Cr(VI)
ion in acidic solution has a positive redox potential value (above 1.3 V at
standard conditions) and in the presence of an electron donor becomes unstable
and can be reduced to Cr(III) (Park et al., 2008). The results show that
the initial ORP values (before adsorption) for the Cr solutions at pH's of 1,
2, 4, 6, 8, and 10  were 339, 280, 165, 55, -53 and -172 mV for NTP-NC
respectively. The positive values of ORP at lower solution pH's indicates
higher possibility for reducing Cr(VI), while the lower and negative ORP values
indicates lower possibility for reducing Cr(VI). On addition of the adsorbents
into the solutions, a change in ORP were observed . The values of ORP at the
end of the adsorption process became 330, 268, 30, 9 -29 and -61 for NTP-NC
respectively. A reduction in ORP at the end of the adsorption stage signifies
that reduction has occurred in the system. Yu et al. (2014) showed the
amount of Cr(VI) removed from aqueous solution in contact with zero-valent iron
(nZVI) could be related to the change in ORP and pH of the solution. The
authors observed a decrease in ORP from 400 to 100 mV and a corresponding 10
unit pH increase as Cr(VI) was removed from solution.

Finally, analysis of the prepared Cr
solutions at the various solution pH's showed that the concentration of Total
Cr obtained from ICP and that of Cr(VI) using UV-vis measurements were very
similar, suggesting that Total Cr was almost equal to Cr(VI) in solution. After
contacting the various solutions with the adsorbents, the concentration of
Total Cr in the pH 1 solution was observed to have reduced to 21.56 mg/dm³
for NTP-NC, while the Cr(VI) concentrations was 10.66 mg/dm³ for
NTP-NC. With solution pH 2, the concentration Cr(III) left in solution was
lower than that of solution pH 1 for both samples. The reason is that at pH 1
there is an abundance of H⁺ ions in solution which accumulates on
the functional groups on the adsorbent making the surfaces of the adsorbent
positively charged and repelling away the positively charged Cr(III) ions from
the adsorbent surfaces (Suksabye et al., 2009). At solution pH 2, it is
believed that there is less accumulation of H⁺ ions at the adsorbent
surfaces which led to reduced electrostatic repulsion and increased adsorption
of Cr(III) from solution. This increase in Cr(III) adsorption is also confirmed
by the reduction in Total Cr in solution between solution pH 1 and 2. The
concentrations of Cr(VI) left in solution at pH 2 was higher than at pH 1 for
NTP while for NTP-NC the Cr(VI) concentrations were lower at pH 2. For solution
pH's higher than 2 and up to 10, the residual Cr(III) in solution becomes very
low and almost constant. This is consistent with the small change in [H⁺]
in solution which is almost constant at this pH range.

FTIR Evidence for the adsorption of
Cr(III)

When
Cr(III) is formed in solution via adsorption-coupled reduction mechanism it may
undergo hydrolysis and/or complexation depending on the solution pH and the
total Cr(III) concentration. Hydrolysis may lead to the formation of various
Cr(III) species with an increase in solution pH which gives rise to a reduction
in its solubility  and increasing the chances of precipitation of the Cr(III) species.
At solution pH 1 – 4, Cr³⁺ and CrOH²⁺
are the major species which can be adsorbed by negatively charged
ligands on the biomaterial surface. On the other hand, at solution pH range of
4 – 10, Cr(III) species (hydroxo complex), such as CrOH²⁺, and Cr(OH)₃,
are the dominant forms in the aqueous system and at high pH is readily transformed
into soluble complex,  . When NTP-NC
was contacted with Cr(VI) solution for 2 hr, the final solution pH was
increased to pH 2.53. The FTIR spectra for NTP-NC (Fig. 2) shows the presence
of characteristic Fe-O peaks of bulk Fe₃O₄ at 553.26 cm^-1, 564.36 cm^-1, 570.21 cm^-1
and 576 21 cm^-1. Functional groups associated with the pine includes
the broad band at 3250.99 cm^-1 representing –OH of cellulose,
2905.17 cm^-1 representing C-H stretching, while the peaks at 1627.69
and 1602.87 cm^-1 shows the COO^- and C=O bonds
respectively. After 2 hr contact with Cr(VI) solution, the peak at 564.34 cm^-1
was observed to have split into several new peaks (Fig. 3b (ii)). These emerging
peaks are characteristic of Fe-O bonds of maghemite (γ-Fe₃O₄),
for example the characteristics peaks of maghemite was observed at 595.21,
685.65 cm^-1 and 582.70, 634.91 cm^-1
suggesting a change in the iron oxide phase (Fe₃O₄
to γ-Fe₂O₃) due to the conversion of Cr(VI) to
Cr(III) by magnetite nanoparticles. According to Peterson et al. (1997),
Cr(III)(hydr)oxide produced is precipitated on the
magnetite surface. In this study, α-Cr₂O₃ peak were
observed at 604.63 cm^-1 indicating that Cr(III) was formed during
the adsorption process. Finally it was also observed that the carboxylate group
at 1635.55 cm^-1 was shifted to 1637.73 cm^-1 while a new
peak was observed at 1737.72 cm^-1 suggesting that complexation
between Cr(III) and Carboxylate ions also occurred.
   

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Miretzky,
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Environ. Manage. 151 (2015) 443-449.

D. Park, C.K. Ahn, Y.M. Kim, Y.-S. Yun,
J.M. Park, J. Hazard. Mater. 160 (2008) 422–427.

P. Suksabye, A. Nakajima, P. Thiravetyan, Y. Baba, W. Nakbanpote.. J. Hazar. Mater. 161 (2009) 1103–1108.

R.F., Yu, F.H.,
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M.L. Peterson,
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Fig. 1 Effect of solution pH on the
adsorption of Cr(VI)

Fig, 2: FTIR of NTP-NC