Volcanic ash from Puyehue Cordon Caulle Volcanic Complex (Chile), emitted on June 4, 2011, and deposited in Villa La Angostura at ~40 km of the source, was collected and analyzed by Raman spectroscopy, optical and scanning electron microscopy (SEM-EDS), X-ray diffraction (XRD), surface area (BET), and chemical analysis (ICP-AES-MS technique). The mineralogical and physicochemical study revealed that the pyroclastic mixture contains iron oxides in the form of magnetite and hematite as well as pyroxene and plagioclase mineral species and amorphous pumiceous shards. Carbonaceous material was also identified. Physicochemical techniques allow us to select two representative samples (average composition and Fe-rich materials) which were used to analyze their performances in the adsorption process to remove arsenic from water. Additional iron activation by means of ferric salts was performed under original sample. Results showed that the low-cost feedstock exhibited a good adsorption capacity to remove the contaminant, depending on the iron content and the water pH.
The south-andes Cordillera is one of the world regions with intense volcanic and tectonic activity. The last eruption, started on June 4, 2011, was associated with the activity of Puyehue Cordon Caulle Volcanic Complex (PCCVC), Chile, located at 40°34′57′′S-72°06′53′′W, emitting more than 5 × 106 m3 of pyroclastic material [
Although the preliminary characterization of the ash showed a composition predominantly rhyolitic, the ejected material was composed of different types of particles. Eruption stages and distance from the volcanic source affected the chemical composition of the mixture, but, in general, the material was nearly amorphous to the XRD, with a texture characterized by the presence of vesicles. However, in the early stages of the volcanic event a dark (brownish-black) and heavy particulate matter, with variable size and hardness, was emitted [
The application of the volcanic ash depends on several factors such as the mineralogy and chemical composition [
As for arsenic removal from contaminated aqueous medium, different iron-rich minerals can be used [
The aim of this work is to study the physicochemical behavior of the ash and tephras deposited in Villa La Angostura (40°45′48′′S-71°38′46′′W) and collected five months after the eruption and the need to approach the potential application (in the original form and activated with ferric phases) to remove arsenic from groundwater.
Scanning electron microscopy (SEM-EDS), optical measurements, surface (BET) and chemical analysis (by inductively coupled plasma (ICP) technique for major and trace elements), X-ray diffraction, and Raman spectroscopy were directed to the mineralogical and chemical characterization of the material.
The material from Villa La Angostura (about 40 km from the volcanic source), having particle sizes between 10 and 3000
PT and ST materials were washed with ethanol by the ultrasonic technique to facilitate the separation of very thin adhered particles and then dried at 80°C.
Scanning electron microscopy and electron diffraction spectroscopy (SEM-EDS) measurements were performed in an ESEM (FEI Quanta 200), with tungsten filament and an ETD (high vacuum secondary electron) detector. Microanalysis was carried out with an EDAX Detector Apollo 40. Chemical results were expressed as % oxides.
Chemical analysis was performed by ICP-AES for major elements (expressed as % oxides) and ICP-MS for trace Rare Earth Elements (REE) (in ppm) (ALS Chemex Lab., Canada). The geochemical behavior was analyzed from variance diagrams of REE normalized to chondrite igneous system.
The BET surface area was measured by N2 adsorption using a Micromeritics ASAP 2020 Automated Brunauer-Emmett-Teller Sorptometer.
X-ray diffraction patterns for crystalline phase analysis were collected with a Philips PW 1710 diffractometer, Cu K
Raman spectroscopic analyses were carried out with inVia Renishaw micro-Raman spectrometer equipped with an air-cooled CCD detector and edge filters. A 785.0 nm emission line from a diode laser was focused on the sample by a Leica DLML microscope, using 5x or 20x objectives. The power of the incident beam is about 5 mW. Five 10 s accumulations were generally acquired for each sample. The resolution was 2 cm−1, and the spectra were calibrated using the 520.5 cm−1 line of a silicon wafer. Spectral analysis was done by background subtraction and curve fitting.
Preliminary tests for As removal by adsorption were carried out at room temperature (20°C) using PM material (up to 3000
The pH of suspensions was adjusted between 3 and 9 by using 0.1 M solutions of HCl or NaOH. The values were monitored by means of the Denver Instrument Ultrabasic Benchtop pH meter. In order to maintain a relatively constant ionic strength, the arsenic solutions contain 0.01 M NaCl as background electrolyte. The experiments were done by triplicate.
According to the reflection microscope, the volcanic material shows a predominance (70–80%) of pumice fragments (glass blabbing). To a lesser extent (30–20%) crystals and crystal fragments of metal oxide and silicate phases were observed.
Figure
SEM micrographs of samples (a) PM, (b) PT, (c) ST, (d) STW, and (e) RB. PM: volcanic pyroclastic mixture; PT: pumiceous type; ST: scoria type; STW: washed scoria type; RB: rounded block.
Table
EDS data for volcanic pyroclastic mixture (PM) and selected typical components.
% oxide | PM | ST | STW | RB | PT | PTW |
---|---|---|---|---|---|---|
CO2 | 4.12 | 7.30 | 6.15 | 12.56 | 3.64 | 2.44 |
Na2O | 4.65 | 4.05 | 3.44 | Nd | 5.58 | 5.37 |
MgO | 2.08 | 2.82 | 3.02 | 10.07 | 1.41 | 0.57 |
Al2O3 | 14.85 | 17.01 | 17.89 | 4.72 | 13.84 | 14.52 |
SiO2 | 61.25 | 54.56 | 51.60 | 37.48 | 62.76 | 68.28 |
K2O | 1.48 | 1.21 | 0.60 | Nd | 3.19 | 2.36 |
CaO | 2.75 | 4.90 | 5.61 | 3.70 | 2.52 | 1.85 |
TiO2 | 1.01 | 0.88 | 1.57 | 4.47 | 1.07 | 1.06 |
Fe2O3 | 7.81 | 7.27 | 10.12 | 27.00 | 5.99 | 3.55 |
PM: volcanic pyroclastic mixture; ST: scoria type; PT: pumiceous type; RB: rounded block; w: washed.
According to EDS results, the SiO2/Al2O3 ratio in the ST particles is near 3, value that increases to ~5 in the PT samples. The diminution of Al content can be associated with the enrichment of Si species (crystalline or glassy). Likewise, the RB particles are characterized by the absence of alkaline elements (K and Na) and the presence of Mg and Ca. This fact suggests the existence of mixed valence iron oxides or related phases (magnetite-type) containing Fe, Mg, Al, and Ti. The presence of Ca can be associated with some silicate (probably plagioclase). It is evident that the finest particles are mobilized by the ultrasonic treatment, affecting the relative Si/Fe composition. The washing of ST samples facilitates the exposition of iron species by elimination of Si-rich particles adhered in vesicles. Finally, the presence of carbonaceous materials is observed particularly in the dark particles.
Moreover, Table
Major and minor elements (expressed as % oxides) by ICP AES.
Sample | PM | PT | ST |
---|---|---|---|
SiO2 | 67.03 | 70.30 | 60.52 |
TiO2 | 0.89 | 0.65 | 1.17 |
Al2O3 | 14.17 | 13.35 | 14.52 |
Fe2O3 | 5.58 | 4.56 | 8.92 |
MnO | 0.14 | 0.12 | 0.16 |
MgO | 1.05 | 0.56 | 2.77 |
CaO | 2.95 | 2.02 | 5.76 |
Na2O | 5.04 | 5.12 | 3.77 |
K2O | 2.37 | 2.52 | 1.49 |
P2O5 | 0.17 | 0.13 | 0.33 |
Cr2O3 | 0.01 | 0.01 | 0.01 |
BaO | 0.08 | 0.08 | 0.05 |
SrO | 0.02 | 0.02 | 0.04 |
LOI | 0.50 | 0.56 | 0.49 |
PM: volcanic pyroclastic mixture; ST: scoria type; PT: pumiceous type.
REE trace elements (expressed as ppm) by ICP MS.
Sample | La | Ce | Pr | Nd | Sm | Eu | Gd | Tb | Dy | Ho | Er | Tm | Yb | Lu |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
PM | 29 | 65.7 | 9 | 40 | 8.13 | 2 | 8 | 1 | 8 | 1.71 | 5.35 | 0.82 | 5.4 | 0.87 |
PT | 31 | 69.6 | 9 | 37 | 8.57 | 2 | 8 | 1 | 8 | 1.86 | 5.45 | 0.88 | 5.8 | 0.92 |
ST | 20 | 45.5 | 6 | 26 | 6.51 | 2 | 6 | 1 | 6 | 1.35 | 3.98 | 0.60 | 3.8 | 0.59 |
PM: volcanic pyroclastic mixture; ST: scoria type; PT: pumiceous type.
The X-ray diffraction pattern of the volcanic pyroclastic mixture (PM) indicates the presence of an absolute majority of a vitreous amorphous phase, whereas the PT samples is also characterized by a very low resolution. However, the XRD pattern of the dark-ST material, shown in Figure
XRD pattern of ST sample (see text).
According to the total alkali versus silica diagram (TAS) for the volcanic rocks, shown in Figure
Total alkali-silica (TAS) plot for PT, PM and ST samples.
Figure
Spider diagram of REE for PT, PM, and ST samples.
Surface area (BET) of the pyroclastic material (PM) is 2.23 m2 g−1, while the ST sample presented a relatively lower value (1.73 m2 g−1). The difference can be attributed to the presence of crystalline phases [
The micro-Raman spectroscopy is a useful technique to define the mineralogy of composite materials, becoming a sensor for identifying promptly the species, isolated or embedded in the matrix [
Micro-Raman spectra registered in different spots (see text).
Spectrum of Figure
Likewise, the spectrum of Figure
Spectrum of Figure
Although the volcanic ash constitutes a significant environmental hazard [
The As content in the PM, PT, and ST studied materials (12.3, 15.7, and 12.4 ppm, resp.) is comparable to that observed in the loess of Argentinean aquifers [
The literature reports of adsorption data for ferric-impregnated volcanic ash showed that an increase of ~5% as Fe2O3 (by treatment of ferric solution (FeCl3 20 g L−1)) greatly elevates the As(V) removal ability by the formation of iron (hydro)oxides [
Figure
Arsenic removal by using volcanic pyroclastic mixture (PM), scoria-type (ST) material, and activated pyroclastic mixture (PMA), at different pH.
The components of the volcanic ash emitted by PCCVC eruption were identified by means of spectroscopic and microscopic techniques, which revealed a mixture of alkali silicates (predominantly glass) and microcrystals of iron oxides (hematite, magnetite), phases of plagioclase type, pyroxene type, and carbonous material. The pyroclastic- and the scoria-type materials, with bulk iron contents of 5.58 and 8.92% Fe2O3, have not shown arsenic leaching, retaining the arsenic in solution. On the other hand, the “in situ” chemical modification with a small proportion of iron oxide (~5% as Fe2O3) led to a useful adsorbent for arsenic removal from aqueous solution, increasing also the effective range of pH. Comparatively, the good performance can be attributed to the increase of the adsorption sites through the formation of Fe-O-H groups. On the basis of physicochemical characterization, the studied ash seems to be an interesting raw material for the arsenic removal, transforming the volcanic waste in a suitable, inexpensive, and abundant adsorbent.
The work was done by financial support of ANPCyT BID 2011 PICT-2186 Argentina and CUIA (Italy-Argentina). Authors thank Mr. R. Viña (LANADI Lab., University of La Plata) and Eng. A. Kang (LIMF Lab., University of La Plata) for technical measurements.