Surface stoichiometry of zinc sulfide and its effect on the adsorption behaviors of xanthate
© Wang et al 2011
Received: 6 July 2011
Accepted: 24 November 2011
Published: 24 November 2011
In this paper, the surface stoichiometry, acid-base properties as well as the adsorption of xanthate at ZnS surfaces were studied by means of potentiometric titration, adsorption and solution speciation modeling. The surface proton binding site was determined by using Gran plot to evaluate the potentiometric titration data. Testing results implied that for stoichiometric surfaces of zinc sulfide, the proton and hydroxide determine the surface charge. For the nonstoichiometric surfaces, the surface charge is controlled by proton, hydroxide, zinc and sulfide ions depending on specific conditions. The xanthate adsorption decreases with increasing solution pH, which indicates an ion exchange reaction at the surfaces. Based on experimental results, the surface protonation, deprotonation, stoichiometry and xanthate adsorption mechanism were discussed.
Zinc sulfide is one of the most important industrial minerals of zinc metal, which is usually recovered by flotation technology, in which the mineral surfaces are treated hydrophobic. Synthetic zinc sulfide is also well known as electroluminescence material. In fact, due to its wide applications and potential use as a spintronic material in high technology field, it was one of the most studied luminescence [1–10], photo catalyst , special morphology [11–16] and mesoporous materials [17–21] in inorganic synthesis. It was reported that transition metal doped nanocrystals of semiconductor like zinc sulfide can yield both high luminescence efficiencies and life time shortening . The doped semiconductor nanocrystals seem to represent a new class of luminescent material, with wide range application in e.g. display, lighting and lasers [2–10]. In past years the dramatic surface effect of nano sized ZnS was reported , which states that when the surface of a ZnS nanoparticle gets wet, its entire crystal structure rearranges to become more ordered, closer to the structure of a bulk piece of solid ZnS. It was also reported  that zinc sulfide nanoparticles, a mere 10 atoms across have a disordered crystal structure that puts them under constant strain, increasing the stiffness of the particles. Obviously surface chemical property of zinc sulfide plays a critical role in these phenomena. When zinc sulfide are put into aqueous solution, the surface hydration takes place at once, the adsorption of proton and hydroxide ions make the surface either positively or negatively charged in the absence of foreign ions. Due to the sensitivity of sulfide ions to oxygen, redox reactions at the surfaces may take place as well. The surface properties are influenced by overall equilibrium of surface and solution chemical reactions including solution acid base and redox reactions, surface complexations, precipitation and dissolution. All theses reactions are very fundamental to understanding of aforementioned mineral flotation, surface modification and inorganic material synthesis processes. Therefore, it is meaningful to study the acid-base and reduction properties of zinc sulfide as well as their correlation with these processes. Furthermore, the surface stoichiometry of zinc sulfide will not only influence the surface charge but also have some impact on the preparation of luminance material, inorganic synthesis of nanoparticles, which deserve a deep going study.
The surface complexation of zinc sulfide in relation to mineral flotation was studied [24–27] and some surface reaction constants were reported. Xanthate having the structure ROC(= S)S-M+ (R = alkyl; M+ = Na+, K+) is a type of collector widely used in sulfide mineral floatation. The uses of xanthate as capping agents for stabilizing colloids [28, 29] and as precursors for metal sulfide nano-particles [30–33] have also been studied and the adsorption mechanism plays an important role in theses process and deserves further studies. Although the surface stoichiometry of zinc sulfide may profoundly affect its adsorption properties, the systematic knowledge on this aspect is lacking.
In this paper, surface stoichoimetry, acid-base and redox properties as well as the adsorption of xanthate at ZnS surfaces were studied by means of potentiometric titration, adsorption and solution speciation modeling. Testing results indicate that the state of surface stoichiometry of ZnS plays an important role in its adsorption process. The adsorption of xanthate and the surface charge at the ZnS surfaces is in the order of zinc rich ZnS > stoichiometric ZnS > sulfur rich ZnS.
Results and Discussions
Chemical reactions in ZnS-H2O system
Chemical reaction in Zn2 +-HS- -e--H2O system
(1) Acid-base reactions
H+ + HS- <=> H2S
H+ + HS- <=> H2S(g)
HS- <=> H+ + S2-
H2O <=> H+ + OH-
Zn2+ + 2 HS- <=> Zn(HS)2
Zn2+ + 3 HS- <=> Zn(HS)3-
Zn2+ + 4 HS- <=> Zn(HS)42-
Zn2+ + 2 H2O <=> 2 H+ + Zn(OH)2
Zn2+ + 3 H2O <=> 3 H+ + Zn(OH)3-
Zn2+ + 4 H2O <=> 4 H+ + Zn(OH)4-
2 Zn2+ + 6 H2O <=> 6 H+ + Zn2(OH)62-
2 Zn2+ + H2O <=> H+ + Zn2OH3+
4 Zn2+ + 4 H2O <=> 4 H+ + Zn4(OH)44+
Zn2+ + H2O <=> H+ + ZnOH+
Zn2+ + 2 HS- <=> H+ + ZnS(HS)-
Zn2+ + 3 HS- <=> H+ + ZnS(HS)22-
2 H+ + 2e- <=> H2
2 H+ + 2e- <=> H2(g)
2 H2O <=> 2 H+ + 2e- + H2O2
4 HS- <=> 2 H+ + 6e- + H2S4
5 HS- <=> 3 H+ + 8e- + H2S5
4 HS- <=> 3 H+ + 6e- + HS4-
5 HS- <=> 4 H+ + 8e- + HS5-
2 H2O <=> 4 H+ + 4e- + O2
2 H2O <=> 4 H+ + 4e- + O2(g)
3 H2O <=> 6 H+ + 6e- + O3
3 H2O <=> 6 H+ + 6e- + O3(g)
2 HS- <=> 2 H+ + 3e- + S2-
2 HS- <=> 2 H+ + 2e- + S22-
2 HS- + 3 H2O <=> 8 H+ + 8e- + S2O32-
3 HS- <=> 3 H+ + 4e- + S32-
4 HS- <=> 4 H+ + 6e- + S42-
5 HS- <=> 5 H+ + 8e- + S52-
6 HS- <=> 6 H+ + 10e- + S62-
HS- + 3 H2O <=> 7 H+ + 6e- + SO32-
HS- + 4 H2O <=> 9 H+ + 8e- + SO42-
2 HS- + 3 H2O <=> 6 H+ + 8e- + H2S2O3
HS- + 4 H2O <=> 7 H+ + 8e- + H2SO4
2 H2O <=> 3 H+ + 2e- + HO2-
2 HS- + 3 H2O <=> 7 H+ + 8e- + HS2O3-
HS- + 3 H2O <=> 6 H+ + 6e- + HSO3-
HS- + 4 H2O <=> 8 H+ + 8e- + HSO4-
2 HS- + 6 H2O <=> 14 H+ + 14e- + S2O62-
2 HS- + 8 H2O <=> 18 H+ + 18e- + S2O82
4 HS- + 6 H2O <=> 16 H+ + 18e- + S4O62-
HS- + 2 H2O <=> 5 H+ + 6e- + SO2(aq)
HS- + 2 H2O <=> 5 H+ + 6e- + SO2(g)
Zn2+ + 8 H2O + 2 HS- <=> 18 H+ + 16e- + Zn(SO4)22-
Zn2+ + 16 H2O + 4 HS- <=> 36 H+ + 32 e- + Zn(SO4)46-
Zn2+ + 3 H2O + 2 HS- <=> 8 H+ + 8e- + ZnS2O3
Zn2+ + 4 H2O + HS- <=> 9 H+ + 8e- + ZnSO4
3) Precipitation and dissolution reactions
Zn2+ + 2 H2O <=> 2 H+ + Zn(OH)2(s)
HS- <=> H+ + 2e- + S(s)
2e- + Zn2+ = Zn(c)
Zn2+ + HS- <=> H+ + ZnS(s)
2 Zn2+ + 6 H2O + HS- <=> 11 H+ + 8e- + Zn2(OH)2SO4(s)
3 Zn2+ + 9 H2O +2 HS- <=> 20 H+ + 16e- + Zn3O(SO4)2(s)
4 Zn2+ +10 H2O + HS- <=> 15 H+ + 8e- + Zn4(OH)6SO4(s)
Zn2+ + 4 H2O + HS- <=> 9 H+ + 8e- + ZnSO4(s)
Zn2+ + HS- <=> 9 H+ + 8e- + ZnSO4·6 H2O(s)
Zn2+ + HS- <=> 9 H+ + 8e- + ZnSO4·7 H2O(s)
(4) Surface complexation reactions*
≡SZn + H+ <=> ≡ZnSH+
≡SZn + H2O <=> ≡SZnOH- + H+
≡SZn + 2H+ <=> ≡SH2 + Zn2+
Here, Ha, Va, Hb, Vb, and V0 denote the acid concentration of burette, added acid volume, base concentration of burette, added base volume and original volume respectively.
It can be seen that between pH 4 to 10 the solution of 0.1 M NaNO3 has no buffer capacity. When the concentration of ZnS sample suspension is 0.5 g/L, the pH buffer range is between pH 7 to 9 and the buffer capacity and range are getting bigger and widen when the solid concentration is increased. With increasing the solid concentration from 0 to 2 g/L, the increase of pH buffer capacity of sample suspension indicates clearly the influence of the concentration of surface binding site and the acid-base properties of these sites. As well known that the solubility of zinc sulfide is very limited, the pH buffer from the soluble zinc ions should be negligible therefore the pH buffer of solution can only originate from the surfaces. If so, the change of solid concentration will correspondingly change the buffer capacity and it is indeed the case. We have qualitatively checked the soluble species in the supernatant of ZnS suspension by titrating with acid solution and found out that there is no pH buffer capacity, which means the soluble ions is negligible. From Figure 3 it can be seen clearly that the pH change behaviors of zinc sulfide suspension with changing total concentration of acid varies as a function of solid concentration. The higher the solid concentration of zinc sulfide is the more the pH buffer capacity of corresponding suspension.
It can be seen from Figure 6 that when 20 mL 0.1 M sulfide solution is titrated with 0.1 M zinc ion solution, the equivalent volume of zinc ion solution should be about 20 mL and is indeed 20 mL. However, when the 20 mL 0.1 M zinc ion solution is titrated with 0.1 M sulfide solution the equivalent volume of sulfide is about 19 mL. This phenomenon may be expressed as that when zinc ion solution is titrated with sulfide ion solution, the equivalent volume corresponds to the surface state of 5 in Figure 5. After the excess zinc ions in solution is consumed by forming bulk zinc sulfide and zinc rich zinc sulfide surfaces, the addition of sulfide ions induces the quick increase of hydroxide in solution and sudden jump of the pH value.
Determination of surface proton binding site
Here Hs, Had and Hex denote surface proton binding site, total added proton and excess proton respectively. The Hex can be determined by Ve of acid-base titration.
Zeta potential measurements
In the process of inorganic synthesis of nanoparticles, employing template such as xanthate [31–33] may be needed in order to obtain ideal products. The interaction of template with solid surface and transformation of precursor into metal sulfide nanoparticles are necessary steps. The surface stoichiometry and sorption properties of zinc sulfide shall directly influence the quality of final nanoparticle products, which dictates the releasing rate of metal ions from metal xanthate precursors. Therefore the studies of the surface fundamental properties will help to understand relevant process regarding inorganic synthesis and to elucidate the mechanisms so as to optimize the synthesis process.
Zinc nitrate, sodium sulfide, sodium hydroxide, potassium hydroxide, hydrochloric acid, sodium nitrate, carbon disulfide, acetone, petroleum ether, ethanol, octanol and ethyl xanthate are all analytical grades and were purchased from Sinopharm Chemical Reagent Co., Ltd. Octyl xanthate were synthesized using carbon disulfide, octanol and potassium hydroxide as source materials and recrystallized three times using acetone and petroleum ether to get enough purity (> 99%). Doubly distilled water was used throughout entire study.
Stoichiometric zinc sulfide sample was prepared by precipitation from a stoichiometric mixture of aqueous zinc nitrate and sodium sulfide and the specific surface area of this sample was determined by Nova 2000e BET instrument from Quantachrome to be 37.6 m2/g. The zinc rich zinc sulfide sample was prepared by precipitation from a nonstoichiometric mixture of 2 parts of aqueous zinc nitrate (0.2 M) and 1 part of sodium sulfide (0.1 M). The sulfide rich zinc sulfide sample was prepared by precipitation from a nonstoichiometric mixture of 2 parts of sodium sulfide (0.2 M) and 1 part of aqueous zinc nitrate (0.1 M). The BET surface for sulfide rich and zinc rich samples are considered to be about the same as that of the stoichiometric sample because both the synthesis process and the mass of solid formed are basically the same. All samples were rinsed with doubly distilled water three times and dried in an oven at 80°C for over 8 hours.
Sample surface characterization
Fourier transforms infrared (FT-IR) measurements samples in KBr pellets were carried out by means of a Bruker Vertex 70 FT-IR spectrometer. One centimetre diameter and constant weight KBr pellets were prepared by mixing the sample with KBr at 1:100 ratios. The spectra were measured in the wavenumber range of 400-4000 cm-1 with 100 scans at 4 cm-1 resolution.
Zeta potential of samples was measured using a Zeta meter (JS94H) based on the mobility of electrophoresis. The zeta potential measurements of sample as a function of pH were carried out in the solution with the ionic strength of 1 mM NaNO3. In this procedure, a series of sample in a closed 50 mL polyethylene test tube with solid sample concentration of 0.8 g L-1 was conditioned in a thermostatic shaker at 25°C. The pHs of the suspensions were adjusted using either acid (0.1 M HCl) or base (0.1 M NaOH) solutions. After shaking overnight (24 h), the samples were allowed to stand for 5 min to let larger particles settle. An aliquot taken from the supernatant was used to measure the zeta potentials. The average of 5 measurements was taken to represent each measured potential and the error range of the measurements is also marked in the presented diagram.
The sulfide samples of different solid weight (0.05 g, 0.10 g and 0.20 g) in a titration vessel with 100 mL 0.1 M NaNO3 solution (with a stop and without special protection from an inert gas) were titrated respectively with acid (0.1133 M HCl) or base solution (0.10 M NaOH), and after conditioned magnetically for 5 minutes, the resulting pH was recorded. The glass electrode was calibrated using the standard pH buffer solutions before each titration.
The adsorption studies of ethyl and octyl xanthate by various ZnS samples (0.8 g/L) were conducted by a batch technique at 25°C. For these adsorption studies, a series of 50 mL polyethylene test tube containing 25 mL of xanthate solutions of 0.05 mM concentrations closed with a stop were employed at a varying pH. All pH adjustments were made by adding the requisite amount of dilute acid (HCl) and dilute base (NaOH) and the influence of pH was then determined by examining the adsorption of xanthate at a desired concentration over a pH range of 5.0-12.0. These in a thermostatic shaker were shaking continuously for 60 minutes to attain equilibration and then a part of the suspension was withdrawn and centrifuged at 16000 rpm for 10 minutes. The residual concentration of xanthate in the supernatant was then determined spectrophotometrically by measuring the absorbance at λmax of 301 nm against the standard calibration curve with linearity of R2 = 0.9991 between 0.0 and 0.1 mM. The pH value of the ZnS suspension before centrifugation was recorded using a combined glass electrode, which was calibrated using standard pH buffer solutions before use.
The speciation modeling of solution species was carried out using computer program MEDUSA and the relevant equilibrium constants were collected from the MEDUSA database . In performing the speciation modeling, we need assigning the concentration for each involved component and selecting the diagram type before running MEDUSA program to get calculation results and relevant diagrams. For example, to get the predominant Eh-pH diagram in Zn2+-HS--e--H2O system, we need giving the concentration for Zn2+ and HS-, the Eh and pH range before running MEDUSA. The calculations cover all possible chemical reactions in Zn2+-HS--e--H2O system, including acid-base reactions, redox reactions and precipitation and dissolution reactions presented in Table 1.
In the present work we have evaluated the surface properties of stoichiometric, zinc rich and sulfur rich synthetic zinc sulfide nanoparticles by acid base potentiometric titration in combination with UV-Visible spectroscopic measurements for xanthate adsorption. Following conclusions can be drawn from our studies:
1. Potentiometric titration is a useful tool in the determination of surface proton binding site of zinc sulfide and in the evaluation of surface stoichiometry. The surface proton binding site determined was found to be increased quantitatively with increasing solid concentration of zinc sulfide.
2. The surface stoichiometry strongly affects the surface properties of zinc sulfide; either zinc rich or sulfur rich surface will bring different acid base properties, i.e. different surface acidity constants.
3. Xanthate adsorption at the surface of zinc sulfide decreased with decreasing surface zinc ions and increasing solution pH indicating the importance of surface zinc site and an ion exchange process between the hydroxide ions in mineral suspension and adsorbed xanthate ions at the surfaces.
Financial support from Chinese Natural Science Foundation (No.50874052; No. 20677022) and National Basic Research Program of China (No. 2011CB933700) is gratefully acknowledged.
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