This study characterized a natural adsorbent material, a dacitic tuff, from Aguas Blancas in Zimapan Hgo. by emission spectroscopy of inductive coupling plasma, X-ray diffraction and scanning electronic microscopy. Also, the influences of concentration, contact time, and pH on the adsorption capacity of arsenic (As) on a dacitic tuff doped with MgO by batch technique were assessed. The results show that the treated material can increase adsorption capacity by 95% compared to natural, untreated material. The pH range for better As adsorption is 3–7 at ambient temperature. The data were evaluated using Langmuir and Freundlich models; these were better suited to the Langmuir model with a capacity of adsorption of 1,897.1 μg g−1. Finally, the mechanism proposed between adsorbent and As compounds is an anion exchange, according to the Kaganer–Dubinin–Radushkevich model, which is provided by functional groups MgOH2+ that were fixed on the material.

INTRODUCTION

Owing to the negative effects of arsenic (As) on human health and the low availability of groundwater free of As, different processes have been proposed to decrease it (Vaclavikova 2008). Flocculation–coagulation is a process commonly used in the treatment of waste-water with high levels of this element. This process is highly dependent upon initial concentration, coagulant dosage, pH, and the oxidation state of As species (Litter et al. 2010; Chiban et al. 2012). The primary coagulating agents are iron and aluminum salts, as well as lime (Mohan & Pittman 2007). However, some problems exist with this technique for secure separation, filtration, handling, and disposal of sewage sludge (Chiban et al. 2012).

The use of filtration membranes is considered to be a promising technology for As removal from drinking water (Mondal et al. 2013). Four process types have been used: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and inverse osmosis (IO). The process type depends on membrane pore size, while membrane selectivity increases when the pressure rises (Mondal et al. 2013).

MF and UF membranes are able to remove only As particulates; these membranes do not remove dissolved As. NF and IO can remove dissolved As and particulates, but these techniques are expensive and consume large amounts of energy.

Adsorption is another process employed in water treatment, which consists of the use of solids to remove compounds that are found in a particular solution and fix them to a solid surface (Giles et al. 2011; Chiban et al. 2012).

Research on As removal by adsorption aims to obtain new adsorbent materials that include zeolites, clays, kaolin, goethite, charcoal, coconut shell, alumina, zirconium oxide, iron oxide, red mud, waste oil, husk rice, human hair, sawdust, and manganese greensand, among others (Choong et al. 2007). Some materials have been modified to increase their adsorption capacity. For example, clinoptilolite was modified with FeCl3 (Baskan & Pala 2011) and the As adsorption capacity was increased from 1.5 μg g−1 to 9.2 μg g−1. Li et al. (2011) determined that clinoptilolite doped with Fe(III) does not provide a strong capacity for adsorption using water with high organic matter content because the oxidation–reduction potential is decreased and the As is reduced to As(III) with a neutral charge.

Zeolites treated with surfactants containing quaternary amines modify their surface to a structure bilayer and change the surface charge from negative to positive; this treatment gives them better adsorption properties than CrO4, AsO43−, and NO3 species (Misaelides 2011).

Other studies have shown that MgO and Mg(OH)2 are excellent adsorbents of As present in water (Moore et al. 2002). This property has been used to treat natural zeolites such as chabazite with MgO to remove As(V). The results obtained are better than 90% (Mejía et al. 2009).

The objective of this study is to modify a dacitic tuff from the municipality of Zimapan Hgo. with MgO to be used in the process of decreasing As content in aqueous solutions. The study includes the characterization of adsorbent material using spectroscopic techniques, as well as experiments in batch for their adsorption ability, kinetics, and pH effect.

METHODS

Materials

The stone material used was a dacitic tuff with light green coloration from the town of Aguas Blancas in the Zimapan municipality of Hidalgo, Mexico. The As working solutions employed in the adsorption tests were prepared from a certified standard solution of Solutions Plus Inc. The MgO supersaturated solution was prepared with 2M of HCl for the stone material treatment and 0.1 M of HCl and NaOH were prepared to adjust the experiment's pH.

The pH control was carried out by potentiometric measurements; for these, a glass electrode from Cole Parmer Company and a Thermo Scientific model Orion 5 Star potentiometer were employed.

Adsorbent preparation

The natural dacitic tuff was ground to reduce particle size to the range 0.24–0.424 mm. The material obtained was washed with distilled water. The MgO supersaturated solution was filtered to remove MgO excess from solution. Then 20 g of tuff were mixed with 200 mL of MgO solution and it was maintained in magnetic stirring for 20 h. Afterwards, the material was filtered and dried in an oven at 110 °C (Mejía et al. 2009) and the following analyses were performed in triplicate.

Adsorbent characterization

The tuff characterization was carried out by atomic emission spectroscopy with inductively coupled plasma (ICP). Before that, the sample was fused with lithium tetraborate and metaborate in an induction furnace and the molten salt was dissolved in nitric acid containing an internal standard. It was analyzed using a sequential/simultaneous system on Jarrel-Ash ENVIRO II ICP equipment. The concentrations were converted to percentages of their respective oxides. The main phases were determined by X-ray diffraction (XRD) with INEL equipment: 2000 Equinox model at 35 kV and 25 mA. Material morphology was determined in a sample previously coated with Au using scanning electronic microscopy (SEM) equipment: Jeol brand, 6300 JSM model at 30 kV.

Adsorption studies

Tuff samples treated with MgO were placed in 125 mL Erlenmeyer flasks with 25 mL As solution at different concentrations (0.1 to 20 mg L−1). The samples were stirred using an orbital shaker at 120 rpm at ambient temperature (about 20 °C).

Contact time influence

Contact time influence on As adsorption was evaluated with 2 g samples of natural or treated tuff with 0.5 mg L−1 As solution adjusted to pH 7 with HCl or NaOH. Flasks with each solution were removed from the agitation system every 30 min up to 2.5 h. The solutions were filtered using Whatman 41 filter paper to subsequently determine As concentration, employing the hydride generation atomic absorption technique.

Adsorption isotherms

Solutions of As with concentrations from 0.1 to 20 mg L−1 mixed with 1 g of treated tuff at pH 7 were kept in constant agitation for 24 h. After that, the solutions were filtered and analyzed by the hydride generation atomic absorption technique and, with the data, the isotherm adsorption of As was determined.

pH influence

Treated tuff samples each of 1 g were added to flasks with As solutions of 0.5 mg L−1 and were adjusted from pH 2 to 10. The solutions were kept under constant agitation stirring for 24 h. However, pH was set again with NaOH or HCl 0.1 M 10 hours, because of the different species of As that are dependent on this property. Then, the solutions were filtered and analyzed by the hydride generation atomic absorption technique. The data obtained were employed to evaluate the pH influence of As adsorption on the treated tuff.

Analytical methods

The analyses of the solutions in each test were completed using the hydride generation atomic absorption technique. Prior to analysis, the As in solutions was pre-reduced to form As(III) with 5% KI and ascorbic acid and acidulated with concentrated HCl. The solutions were left to stand for 20 min before they were analyzed (Davidowski 1993) with an atomic absorption spectrometer, Varian 860 model with current lamp of 10 mA, flow rate sample of 10 and 1.0 ml for 7 M HCl and reducer (0.5% NaBH4 and stabilized with 0.05% NaOH).

RESULTS AND DISCUSSION

Material characterization

The data for the main elements from the material were obtained by elemental analysis with ICP equipment; these were converted to percentages from their respective oxides, which are shown in Table 1.

Table 1

Natural tuff composition from Aguas Blancas, Zimapán, Hidalgo, Mexico

OxidesConcentration (wt. %)
SiO2 69.73 ± 3.16 
Al2O3 13.14 ± 0.42 
Fe2O3(T) 2.84 ± 0.17 
MnO 0.03 ± 0.001 
MgO 0.96 ± 0.06 
CaO 0.27 ± 0.004 
Na20.14 ± 0.006 
K24.23 ± 0.19 
TiO2 0.63 ± 0.02 
P2O5 0.19 ± 0.01 
*LOI 6.59 ± 0.27 
OxidesConcentration (wt. %)
SiO2 69.73 ± 3.16 
Al2O3 13.14 ± 0.42 
Fe2O3(T) 2.84 ± 0.17 
MnO 0.03 ± 0.001 
MgO 0.96 ± 0.06 
CaO 0.27 ± 0.004 
Na20.14 ± 0.006 
K24.23 ± 0.19 
TiO2 0.63 ± 0.02 
P2O5 0.19 ± 0.01 
*LOI 6.59 ± 0.27 

*LOI = loss on ignition at 950 °C; (T) = total iron.

The content of silicon and oxides of potassium and sodium were used to carry out the material classification with the help of the total alkali–silica diagram of LeMaitre et al. (1989), which turned out to be a dacitic tuff with chlorite that gives a slightly greenish coloration.

Crystalline phases of tuff were detected by XRD and correspond to quartz and orthoclase (Figure 1); the latter provides ion exchange properties to the tuff.

Figure 1

Diffractogram of natural dacitic tuff by XRD showing quartz and orthoclase as main phases.

Figure 1

Diffractogram of natural dacitic tuff by XRD showing quartz and orthoclase as main phases.

Photomicrographs obtained with SEM of both natural dacitic tuff (Figure 2(a)) and that treated with MgO (Figure 2(b)) display clear differences, since natural tuff has a compact flake structure with small pore size, while that treated with MgO presents a round structure with greater pore size than natural tuff.

Figure 2

Photomicrographs of dacitic tuff: (a) natural and (b) treated with MgO.

Figure 2

Photomicrographs of dacitic tuff: (a) natural and (b) treated with MgO.

Adsorption of As on dacitic tuff

The effects of contact time on the adsorption equilibrium of As on natural and treated tuff are shown in Figure 3. Natural tuff reached a retention maximum of less than 5%, while treated tuff increased retention percentage up to approximately 76% during the first 30 min and reaching 90% after 2.5 h of contact. MgO in dacitic tuff modifies its crystalline structure and increases intra-crystalline spaces, so As retention is improved significantly. These results are similar to those obtained by Mejía et al. (2009) with MgO in chabazite.

Figure 3

Contact time effect in retention percentage of As on natural and treated tuff with MgO. Working conditions: 5 mg L−1 of As, 2.0 g of tuff, pH = 7, and T = 20 °C.

Figure 3

Contact time effect in retention percentage of As on natural and treated tuff with MgO. Working conditions: 5 mg L−1 of As, 2.0 g of tuff, pH = 7, and T = 20 °C.

As adsorption on the treated tuff depends on its concentration at equilibrium in a working concentration range of 100 to 20,000 μg L−1. When initial concentration increases, the equilibrium concentration increases, as well as the content of As in the tuff, as shown in Figure 4.

Figure 4

Adsorption isotherm in dacitic tuff treated with MgO. Working conditions: initial concentration range of 100 to 20,000 μg L−1 As, 1 g of tuff, 24 h of contact time, 120 rpm, pH = 7 and 20 °C. Ce is As concentration in equilibrium, and qe is quantity of As adsorbed into tuff.

Figure 4

Adsorption isotherm in dacitic tuff treated with MgO. Working conditions: initial concentration range of 100 to 20,000 μg L−1 As, 1 g of tuff, 24 h of contact time, 120 rpm, pH = 7 and 20 °C. Ce is As concentration in equilibrium, and qe is quantity of As adsorbed into tuff.

Data obtained from As adsorption were employed with Langmuir, Freundlich and Kaganer–Dubinin–Radushkevich (KDR) models to determine their behavior. The Langmuir model explains monolayer adsorption behavior, represented by Equation (1) (Ramesh et al. 2007; Chutia et al. 2009; Li et al. 2012) 
formula
1
where qe and Ce are the As concentrations in adsorbent (μg g−1) and liquid phase (μg L−1) in equilibrium, respectively, Qm is the maximum adsorption capacity (μg g−1) on the monolayer of adsorbent covered by the sorbent, and b implies the adsorption enthalpy that varies with temperature (L μg−1). Qm and b can be determined from the graph of 1/qe = f(1/Ce); for this tuff Qm is 1,897.34 μg g−1 and b is 8.94 × 10−5 (L μg−1) with correlation R2 of 0.996, as given in Table 2.
Table 2

Parameters of Langmuir and Freundlich models for As adsorption on tuff treated with MgO

LangmuirFreundlichKDR
Qm = 1,897.34 ± 160.30 (μg g−1KF = 0.51 ± 0.41 (μg g−1E = 11.32 ± 0.83 kJ mol−1 
b = 8.94 × 10−5 ± 8.5 × 10−6 (L μg−1n = 1.35 ± 0.14 
R2 = 0.996 R2 = 0.980 R2 = 0.993 
LangmuirFreundlichKDR
Qm = 1,897.34 ± 160.30 (μg g−1KF = 0.51 ± 0.41 (μg g−1E = 11.32 ± 0.83 kJ mol−1 
b = 8.94 × 10−5 ± 8.5 × 10−6 (L μg−1n = 1.35 ± 0.14 
R2 = 0.996 R2 = 0.980 R2 = 0.993 
Conversely, the Freundlich model explains adsorption on a heterogeneous surface with uniform energy, represented by Equation (2) (Ramesh et al. 2007; Li et al. 2012): 
formula
2
qe and Ce are the same as in the Langmuir model, KF (μg g−1) and n are the Freundlich constant, which is related to the adsorption ability and intensity, respectively. Freundlich constant values are obtained from the graph of log qe = f (log Ce). For this tuff, KF is 0.51 (μg g−1) and n is 1.3535 with a correlation R2 of 0.980; although this is a high correlation value, the experimental data best fit the Langmuir model.
Langmuir and Freundlich isotherms do not provide information about the rate of adsorption that takes place between adsorbent and ionic species of As, so the data were analyzed using the KDR model (Chutia et al. 2009), which is represented by Equation (3): 
formula
3
where Q is the amount of ions adsorbed per unit weight of adsorbent (mol g−1), Xm is the maximum adsorption (mol g−1) and ε is the Polanyi potential that is calculated with Equation (4): 
formula
4
R is the gas constant (8.31447 × 10−3 kJ K−1 mol−1), T is absolute temperature (293 K), and Cf is As concentration in the equilibrium liquid phase (mol L−1).
The activity coefficient β (mol2 J−2) is used to assess the nature of the interaction between As and link sites of the dacitic tuff in the process of adsorption. Also, it relates adsorption energy E with Equation (5) (Chutia et al. 2009): 
formula
5

The E value in this adsorption process is 11.322 kJ mol−1 and indicates a mechanism of ion exchange, which is in the range of energies of 8–16 kJ mol−1.

pH influence of As adsorption on dacitic tuff

The pH of the solution has an influence on As(V) adsorption on materials due to ionization of the compounds at different pH values (H2AsO4, HAsO42−, AsO43−). As(V) species predominate at pH greater than 2.5, while H3AsO4 species predominate at pH less than 2.5. Conversely, the tuff composition changed the pH set at the beginning of the adsorption process, and it was continuously adjusted to maintain the pH. In Figure 5, the tendencies of the graphs are near to 2.22 and 6.96 pK of H2AsO4 and HAsO42− respectively from 4 to 7 h.

Figure 5

Variation of pH in the As adsorption process for dacitic tuff in the experiments with initial range of pH 3–10.

Figure 5

Variation of pH in the As adsorption process for dacitic tuff in the experiments with initial range of pH 3–10.

The surface charge of the tuff is determined by protons transferred from solution to the tuff surface (Table 3, mechanisms 1 and 2) and the interactions between adsorbent and ionic species (Jiménez-Cedillo et al. 2009; Li et al. 2012) are presented as 3 and 4 of the same table, the water being associated to them.

Table 3

Mechanisms involved in As removal with treated tuff

No.Mechanisms
 
 
 
 
No.Mechanisms
 
 
 
 

At alkaline pH, the adsorbent surface is negative, and the coulomb repulsion forces between the negative charge of the ions and the negative charge of the surface significantly reduces the adsorption rate.

The results of pH effect on As adsorption of treated tuff at 0.5 mg L−1 concentration in the pH range 2 to 9 are presented in Figure 6. The best adsorption takes place between pH 3 to 7 with a greater than 90% retention percentage and a contact time of 24 h.

Figure 6

Retention of As depending on pH. Initial concentration of 500 μg L−1 of As, 1 g of tuff, 24 h of contact, 120 rpm stirring, 2 to 9 pH and 20 °C.

Figure 6

Retention of As depending on pH. Initial concentration of 500 μg L−1 of As, 1 g of tuff, 24 h of contact, 120 rpm stirring, 2 to 9 pH and 20 °C.

CONCLUSIONS

The adsorbent material from Zimapan municipality, Hgo employed for this study was characterized by its chemical composition, surface morphology, and main stages as a dacitic tuff with a small amount of chlorite mineral.

The natural material does not present significant As retention. However, when it is treated with MgO, its morphology improves and increases its adsorption capacity up to 87%.

The data obtained from As adsorption by the treated tuff are closer to the Langmuir model of a monolayer at pH 7 and ambient temperature.

The adsorption process between As and the treated dacitic tuff occurs through ion-exchange agreement balances of the KDR model.

The range of optimum pH for As adsorption to take place on this doped material is between 3 and 7.

Finally, it is concluded that the dacitic tuff treated with MgO is an efficient and economical material to use for As removal in aqueous solution. This is recommended for treatment of natural water with As outside allowable limits, as in the case of water from Zimapán Hgo, where the material was obtained.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge Hidalgo State University and financial support from CONACYT through the Master of Chemistry programme.

REFERENCES

REFERENCES
Chiban
M.
Zerbet
M.
Carja
G.
Sinan
F.
2012
Application of low-cost adsorbents for arsenic removal: a review
.
Journal of Environmental Chemistry and Ecotoxicology
4
(
5
),
91
102
.
Choong
T. S.
Chuah
T.
Robiaha
Y.
Gregory Koaya
F.
Aznib
I.
2007
Arsenic toxicity, health hazards and removal techniques from water: an overview
.
Desalination
217
,
139
166
.
Chutia
P.
Kato
S.
Kojima
T.
Satokawa
S.
2009
Arsenic adsorption from aqueous solution on synthetic zeolites
.
Journal of Hazardous Materials
162
,
440
447
.
Davidowski
L.
1993
A Simple Continuous Flow Hydride Generator for ICP-OES
.
ICP Application Study 67
,
Perkin Elmer
,
Norwalk
,
CT, USA
.
Giles
D. E.
Mohapatra
M.
Issa
T.
Anand
S.
Singh
P.
2011
Iron and aluminium based adsorption strategies for removing arsenic from water: review
.
Journal of Environmental Management
92
,
3011
3022
.
LeMaitre
R.
Bateman
P.
Dudek
A.
Keller
J.
Lameyre
J.
Le Bas
M.
Zanettin
B.
1989
A Classification of Igneous Rocks and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks
.
Blackwell Scientific Publications
,
Oxford
.
Li
Q.
Xu
X.
Cui
H.
Pang
J.
Wei
Z.
Sun
Z.
Zhai
J.
2012
Comparison of two adsorbents for the removal of pentavalent arsenic from aqueous solutions
.
Journal of Environmental Management
98
,
98
106
.
Li
Z.
Jean
J.-S.
Jiang
W.-T.
Chang
P.-H.
Chen
C.-J.
Liao
L.
2011
Removal of arsenic from water using Fe-exchanged natural zeolite
.
Journal of Hazardous Materials
187
,
318
323
.
Litter
M. I.
Morgada
M.
Bundschuh
J.
2010
Possible treatments for arsenic removal in Latin American waters for human consumption
.
Environmental Pollution
158
,
1105
1118
.
Mejía
F.
Valenzuela García
J. L.
Aguayo Salinas
S.
Meza Figueroa
D.
2009
Adsorción de arsénico en zeolita natural y pretratada con óxidos de magnesio
.
Revista Internacional de Contaminación Ambiental
25
(
4
),
217
227
.
Misaelides
P.
2011
Application of natural zeolites in environmental remediation: a short review
.
Microporous and Mesoporous Materials
144
,
15
18
.
Mohan
D.
Pittman
C. U.
Jr.
2007
Arsenic removal from water/wastewater using adsorbents: a critical review
.
Journal of Hazardous Materials
142
,
1
53
.
Moore
R.
Holt
K.
Zhao
H.
Salas
F.
Hasan
A.
Lucero
D.
2002
Sorption of Arsenic from Drinking Water to Mg(OH)2, Sorrel's Cements, and Zirconium Doped Materials
.
Sand Report, Sandia National Laboratories, Albuquerque, NM, USA.
Ramesh
A.
Hasegawa
H.
Maki
T.
Ueda
K.
2007
Adsorption of inorganic and organic arsenic from aqueous solutions by polymeric Al/Fe modified montmorillonite
.
Separation and Purification Technology
56
,
90
100
.
Vaclavikova
M.
2008
Removal of arsenic from water streams: an overview of available techniques
.
Clean Technologies & Environmental Policy
10
,
89
95
.