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Foundry sands as low-cost adsorbent material for Cr (VI) removal a
a
a
a
I. Campos , J. A. Álvarez , P. Villar , A. Pascual & L. Herrero
a
a
Department of Environment , AIMEN Technology Centre , Porriño , Spain Accepted author version posted online: 26 Nov 2012.Published online: 29 Nov 2012.
To cite this article: I. Campos , J. A. Álvarez , P. Villar , A. Pascual & L. Herrero (2013) Foundry sands as low-cost adsorbent material for Cr (VI) removal, Environmental Technology, 34:10, 1267-1281, DOI: 10.1080/09593330.2012.745620 To link to this article: http://dx.doi.org/10.1080/09593330.2012.745620
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 10, 1267–1281, http://dx.doi.org/10.1080/09593330.2012.745620
Foundry sands as low-cost adsorbent material for Cr (VI) removal I. Campos, J.A. Álvarez∗ , P. Villar, A. Pascual and L. Herrero Department of Environment, AIMEN Technology Centre, Porriño, Spain
Downloaded by [University of Saskatchewan Library] at 22:37 08 January 2015
(Received 31 July 2012; final version received 25 October 2012 ) The potential of foundry sands, industrial waste from the iron foundry industry, was evaluated for the removal of Cr (VI) using discontinuous assays. Chemical foundry sands are composed of fine silica sand, furanic resins as binder, chemical catalyst and residual iron particles. The influence of pH, agitation rate and metal concentration on the removal process was investigated. Kinetic and equilibrium tests were conducted to determine Cr (VI) removal from aqueous solutions at a temperature range of 25–55◦ C. Cr (VI) removal of 40–100% for a range of pH 6–1.6 was obtained. This removal was attributed to the presence of a large number of protonated silanol and aluminol groups. Cr (VI) adsorption in foundry sands follows a pseudo-second-order kinetic reaction (Ho model, r 2 > 0.999) reaching kinetic constants of 0.341, 0.551, 0.775 and 0.920 g/mg h at 25, 35, 45 and 55◦ C, respectively. The adsorption data were fitted to the Langmuir adsorption isotherm model (r 2 > 0.99) obtaining adsorption capacities (qmax ) of 1.99, 2.40, 2.50, and 3.14 mg Cr (VI)/g sand at 25, 35, 45 and 55◦ C, respectively. Calculated Gibbs free energy change (G 0 ), adsorption energy (E) and activation energy (Ea ) values indicate that a physisorption mechanism governs Cr (VI) adsorption process in foundry sands. Keywords: foundry sands; Cr (VI) removal; batch assays; kinetic studies; adsorption studies
1. Introduction Large quantities of hexavalent chromium are discharged into the environment as a consequence of its wide use in modern industries: electroplating, leather tanning, cement, dyeing, metal processing, wood preservatives, paint and pigments, textile, steel fabrication and canning industries [1]. These industries produce large quantities of toxic wastewater effluents. Cr (VI) concentrations in industrial wastewater vary from 0.5 mg/L to 270 mg/L [2]. The maximum permissible limit of Cr (VI) for discharge into inland surface water is 0.5 mg/L and in potable water is 0.05 mg/L [3,4]. The hexavalent form of chromium, usually present in the form of chromate (CrO−2 4 ) and dichromate (Cr 2 O−2 ), possesses significantly higher levels of toxicity 7 than other valence states [5]. Several treatment methods for Cr (VI) removal from industrial wastewater have been established, i.e. electrochemical precipitation, ion exchange, membrane ultrafiltration, reverse osmosis and reduction [6–10]. However, most of these methods are costly due to operational, treatment and sludge disposal costs. Adsorption is another treatment alternative for the removal of heavy metals from aqueous solutions. The main properties of the adsorbents for metal removal are strong affinity and high loading capacity. Zero-valent iron has been widely used to treat heavy metals via redox and precipitation reactions [11,12]. Activated carbon is generally used ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
in wastewater treatment as an excellent adsorption material for heavy metal removal [13]. However, these commercial products remain expensive materials for heavy metal removal. Therefore, low-cost adsorbents with high metalbinding affinity need to be researched. Studies of low-cost industrial by-products and waste as adsorbent media have been performed [14–16]. Foundry sands are a waste from the production of both ferrous and non-ferrous metal castings. Foundry activities use high-quality size-specific silica sands in their moulding and casting operations. In the casting process, moulding sands are recycled and reused multiple times. Eventually, however, the recycled sands are degraded to the point that they can no longer be reused. The unsuitable sands typically are landfilled at appreciable cost. Approximately 800,000 tons of foundry sands are landfilled in Wisconsin (USA), with an annual cost of $18 million to the foundry industry [17]. Therefore, the recovery of foundry sand waste would save valuable landfill space and accrue savings through reduced disposal costs. Chemical sands are used both in core making, where high strengths are necessary to withstand the heat of molten metal, and in mould making. Chemical sands consist of 93–99% silica and 1–3% chemical binder. Silica sands are thoroughly mixed with chemicals; a catalyst initiates the reaction that cures and hardens the mass [18]. There are various types of chemical binder systems used in the foundry
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I. Campos et al.
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industry. The most common chemical binder systems used are phenolic-urethanes, epoxy resins, furfyl alcohol and sodium silicates. In this study, the capacity of chemical foundry sands to remove Cr (VI) from aqueous solution was evaluated using discontinuous assays. Kinetic and adsorption behaviour was studied; pH, agitation and initial Cr (VI) concentration effect on foundry sand adsorption capacity were also examined.
2. Materials and methods 2.1. Foundry sand sample Chemical foundry sands were used in this study. It was obtained from the Fundiciones Rey factory sited in Villagarcia de Arousa (Galicia, North West of Spain). This factory produces mainly iron metallic pieces, but also it works with bronze and aluminium. Used foundry chemical sands were sampled from Fundiciones Rey using plastic containers of 10 L. 2.2. Chemicals and analytical methods Foundry sand characterization Foundry sand physico-chemical parameters were characterized. Sand humidity was measured by drying at 105◦ C to a constant weight [19]; density relative to water was measured following the method described in [20] using a density measuring device [20]; particle size distribution was determined using sieves for the largest particles [21] followed by the Stoke law for fine particles [22]. From the particle size distribution, median particle size (D50 ) (below this value are 50% of particles) was calculated. Total iron, total aluminium and total chromium concentrations in sands were determined by digestion of sands using microwave equipment (CEM MDS-2000) [23] and subsequently measured with an ICP-OES spectrometer (VISTA-MPX, Varian Inc.) [24]. Total silicon concentration and the microscopy study of foundry sands were determined by a scanning electron microscope (SEM) (Hitachi S4800) with micro-analysis system by energy-dispersive X-ray spectrometer (EDS Bruker Quantax 400). Total organic carbon (TOC) was determined using a Shimadzu 5050A TOC analyzer coupled to a Shimadzu 5000A solid samplers module [25]. An X-ray diffraction analysis was carried out to determine the main composition of foundry sands using a Siemens diffractometer type D5000. Fourier transform infrared (FT-IR) studies were also carried out to determine the type of functional group responsible for Cr (VI) adsorption. A JASCO 4100 FT-IR spectrometer was used. An accessory for attenuated total reflectance measurement (ATR model PIKE MIRacle) with glass diamond/ZnSe plates, measuring in a range between 4000 and 600 1/cm was attached to the FT-IR equipment.
Sand leaching characterization Due to the composition of the foundry sands, which contain toxic substances such as heavy metals, phenol, etc., sand leaching assays were conducted to determine the mobilization and release of these compounds to water bodies and to confirm their environmental innocuous behaviour [26] (sand concentration 0.1 kg/L, agitation for 24 h). pH and conductivity were measured in aqueous solution (ratio of sand:water 1:2.5) using Crison GLP 22 and Crison GLP 32, respectively. COD was measured by carrying out a digestion at 150◦ C with potassium dichromate, follow by UV-Vis spectrophotometry measurement at 620 nm with a spectrophotometer (Hatch DR/2000). Aqueous iron, chromium, nickel, aluminium and manganese concentrations were analyzed through ICP-OES spectrometry (VISTA-MPX, Varian Inc.) [24]. Anionic and cationic compounds were measured by suppressed ion chromatography using a Methron 882 compact IC plus device. Finally, scanning of phenols was carried out by gas chromatography–mass spectrometry (GC-MS) using a Perkin Elmer Turbomass chromatographer. Discontinuous assay monitoring Aqueous concentrations of Cr (VI) were measured using a UV-Vis spectrophotometer (CAY 100 BIO, Varian Inc.) following standard methods [27]. A coloured complex with 1,5 diphenylcarbazide reagent grade (Scharlab S.L.) at acid pH was measured at 540 nm. Aqueous Cr (VI) solutions and UV-Vis spectrometric standards were prepared by diluting K2 Cr2 O7 PA-ACS-ISO (Panreac Química SAU) in distilled water. All glassware used for experimental purposes was washed in 10% nitric acid and subsequently rinsed with de-ionized water to remove any possible interference by other metals. pH monitoring during kinetic batch tests was conducted to study the variations of pH through the assays, but as the fluctuation of pH was lower than 1 unit for all temperatures, the data has not been shown. Control assays without sands were carried out to estimate Cr (VI) removal due to other mechanisms (reduction, glass adsorption, etc.) As the fluctuation of Cr (VI) concentration in these control assays was lower than 5% for all temperatures, Cr (VI) removal through other mechanisms has been rejected (data not shown). 2.3. Discontinuous assay methods pH effect studies In an adsorption process, aqueous phase pH affects the speciation of a metal to be adsorbed, as well as the dissociation and/or activation of active groups of the adsorbent material. Therefore, metallic compound adsorption will be significantly influenced by medium pH. Batch tests were carried out in duplicate to study the effect of initial solution pH on Cr (VI) removal. The tests
Environmental Technology were conducted in 100 mL polypropylene bottles at 20◦ C containing 50 mL of 50 mg Cr/L solution and 2 g of sands at initial pH of 1.6, 2.2, 2.5, 4.0, 6.0 and 8.0. The initial pH of the solution was adjusted with 0.1 M nitric acid or 0.1 and 0.25 M sodium hydroxide. Bottles were shaken at 15 rpm using a rotary shaker (Stuart STR4) and aqueous Cr (VI) concentration was measured at the end of the assay (24 h).
Downloaded by [University of Saskatchewan Library] at 22:37 08 January 2015
Agitation effect studies Due to energy demands and the effect on adsorption process efficiency, it is important to determine the optimal agitation rate. Batch tests in duplicate were carried out in 100 mL Erlenmeyer flasks with agitation from 60 to 220 rpm using an orbital shaker (IKA KS 4000ic). The operating conditions of these tests were: 20◦ C, pH 2.5, 50 mg/L of initial Cr (VI) concentration and 40 g/L sand concentration. Cr (VI) removal efficiency was calculated after 24 h. Initial Cr (VI) concentration effect studies Batch tests in duplicate were carried out to study the effect of initial Cr (VI) concentration on removal efficiency of foundry sands. The initial Cr (VI) concentration varied from 50 to 400 mg/L and each test was monitored at 25, 35, 45 and 55◦ C. The assays were conducted under the following operating conditions: 180 rpm, pH 2.5 and 40 g/L sand concentration. Cr (VI) removal efficiency was calculated after 24 h. Kinetic assays: temperature effect Discontinuous kinetic assays were carried out in duplicate to determine the adsorption kinetic of Cr (VI) in the presence of chemical foundry sands at 25, 35, 45 and 55◦ C. Two grams of chemical sands were placed in 100 mL Erlenmeyer flasks containing 50 mL of 50 mg Cr/L solution at a pH of 2.5. The Erlenmeyer flasks were shaken at 180 rpm, and Cr (VI) concentration was measured through 48 h of kinetic assay time. The liquid and solid phases were separated using a centrifuge at 3600 rpm and aqueous Cr (VI) concentration was measured as indicated above after filtering the sample through a 0.45 μm filter. The quantity of Cr (VI) adsorbed by sands at time t was calculated according the following equation: qt =
(C0 − Ct )V ms
(1)
where qt (mg/g) is mg of Cr (VI) adsorbed in a gram of sand at time t; C0 (mg/L) is the initial Cr (VI) concentration; Ct (mg/L) is the Cr (VI) concentration at time t; V (mL) is the assay liquid volume; ms (g) is the weight of sand used in the assay. Cr (VI) removal is calculated according the following equation: Cr (VI) removal efficiency (%) =
(C0 − Ct ) × 100 (2) C0
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Adsorption isotherms: temperature effect Batch adsorption assays were conducted in duplicate, in a similar method to the kinetic tests: 100 mL Erlenmeyer flask at 180 rpm, a constant dry mass of sands of 2 g and using 50 mL of dissolved Cr (VI) at initial concentrations of 50, 100, 150, 200, 300 and 400 mg/L. The initial pH value was adjusted to 2.5 and the assays were conducted at 25, 35, 45 and 55◦ C. Assay time was set at 24 h, which was found to be enough to ensure equilibrium (data not showed). Adsorption assay duration was 24 h to guarantee that adsorption process equilibrium was achieved at all temperatures. 2.4.
Adsorption process description and thermodynamic parameters
Activation energy (Ea ), adsorption energy (E) and Gibbs free energy change (G 0 ) for Cr (VI) adsorption process in foundry sands were calculated by Arrhenius equation, Dubinin–Radushkevich (D–R) isotherm model and thermodynamic equilibrium constant (Kc0 ). These parameters are used to describe the mechanism of adsorption process: physisorption or chemisorption. Moreover, Gibbs free energy change (G 0 ), enthalpy change (H 0 ) and entropy change (S 0 ) were calculated to describe the degree of spontaneity, type of reaction (endothermic or exothermic) and randomness of the adsorption process.
3.
Results and discussion
3.1. Foundry sand characterization Table 1 shows the physico-chemical characterization and leaching results of the foundry sands used in this work, while particle size distribution and foundry sand X-ray diffractogram are shown in Figure 1(a) and 1(b), respectively. The high TOC content was due to organic matter from the resins used in the moulds. Relative density values were slightly lower (2.39 to 2.55) than those indicated by Siddique and Noumowe [18]. Median particle size (D50 ) of used sands was also lower than green foundry sands utilized by Lee et al. [15,17] (0.19 to 0.20 mm). Regarding metals, foundry sands contain 0.9% Fe, 30.4% Si and 0.3% Al. These metals will directly affect Cr (VI) removal. Zero iron has been used for metal removal through redox and precipitation reactions (as hydroxides at high pH) [15,28]. In addition, previous studies have proved the efficiency of Si and Al groups in Cr (VI) removal. The main negative − species of Cr (VI) (CrO− 4 and CrO7 ) will be joined through electrostatic interactions to silanol (SiOH+ 2 ) and aluminol (AlOH+ ) groups at low pH [16]. In the foundry sands 2 used in this work, four main crystalline species have been identified by X-ray diffractogram: SiO2 (α-quartz); Fe2 O3 (maghemite); Al2 O3 (corundum) and TiO2 ; SiO2 being predominant. These data fit with the results obtained from the
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I. Campos et al. Physico-chemical characterization of foundry sands.
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Parameter Sand characterization Humidity (%) Relative density ∗ (mm) D50 TOC (% dm† ) Total Fe (mg Fe/g dm) Total Si (mg Si/g dm) Total Al (mg Al/g dm) Total Cr (mg Cr/g dm) Sand leaching characterization‡ pH Conductivity (μS/cm) COD Fe Total Cr Cr (VI) Ni Al As Ba Cd Co Mo Cu Ni Pb Sb Se F− Cl− NO− 3 NO− 2 PO−3 4 SO−3 4 Li+ Na+ NH+ 4 K+ Ca2+ Mg2+ Phenol 2-chlorophenol 2-metilphenol 3-metilphenol + 4-metilphenol 2,4-dichlorophenol 2,6-dichlorophenol 4-chloro-3-metilphenol 2,4,5-trichlorophenol 2,4,6-trichlorophenol Tetrachlorophenol Pentachlorophenol
Value 1.9 ± 0.0 2.0 ± 0.5 0.13 ± 0.05 13.1 ± 0.3 9.4 ± 2.5 303.6 ± 47.2 3.1 ± 1.4 2.8 ± 0.5 3.86 ± 0.02 915 ± 8 166.0 ± 0.0 91.8 ± 2.0 0.02 ± 0.00
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Foundry sands as low-cost adsorbent material for Cr (VI) removal a
a
a
a
I. Campos , J. A. Álvarez , P. Villar , A. Pascual & L. Herrero
a
a
Department of Environment , AIMEN Technology Centre , Porriño , Spain Accepted author version posted online: 26 Nov 2012.Published online: 29 Nov 2012.
To cite this article: I. Campos , J. A. Álvarez , P. Villar , A. Pascual & L. Herrero (2013) Foundry sands as low-cost adsorbent material for Cr (VI) removal, Environmental Technology, 34:10, 1267-1281, DOI: 10.1080/09593330.2012.745620 To link to this article: http://dx.doi.org/10.1080/09593330.2012.745620
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http:// www.tandfonline.com/page/terms-and-conditions
Environmental Technology, 2013 Vol. 34, No. 10, 1267–1281, http://dx.doi.org/10.1080/09593330.2012.745620
Foundry sands as low-cost adsorbent material for Cr (VI) removal I. Campos, J.A. Álvarez∗ , P. Villar, A. Pascual and L. Herrero Department of Environment, AIMEN Technology Centre, Porriño, Spain
Downloaded by [University of Saskatchewan Library] at 22:37 08 January 2015
(Received 31 July 2012; final version received 25 October 2012 ) The potential of foundry sands, industrial waste from the iron foundry industry, was evaluated for the removal of Cr (VI) using discontinuous assays. Chemical foundry sands are composed of fine silica sand, furanic resins as binder, chemical catalyst and residual iron particles. The influence of pH, agitation rate and metal concentration on the removal process was investigated. Kinetic and equilibrium tests were conducted to determine Cr (VI) removal from aqueous solutions at a temperature range of 25–55◦ C. Cr (VI) removal of 40–100% for a range of pH 6–1.6 was obtained. This removal was attributed to the presence of a large number of protonated silanol and aluminol groups. Cr (VI) adsorption in foundry sands follows a pseudo-second-order kinetic reaction (Ho model, r 2 > 0.999) reaching kinetic constants of 0.341, 0.551, 0.775 and 0.920 g/mg h at 25, 35, 45 and 55◦ C, respectively. The adsorption data were fitted to the Langmuir adsorption isotherm model (r 2 > 0.99) obtaining adsorption capacities (qmax ) of 1.99, 2.40, 2.50, and 3.14 mg Cr (VI)/g sand at 25, 35, 45 and 55◦ C, respectively. Calculated Gibbs free energy change (G 0 ), adsorption energy (E) and activation energy (Ea ) values indicate that a physisorption mechanism governs Cr (VI) adsorption process in foundry sands. Keywords: foundry sands; Cr (VI) removal; batch assays; kinetic studies; adsorption studies
1. Introduction Large quantities of hexavalent chromium are discharged into the environment as a consequence of its wide use in modern industries: electroplating, leather tanning, cement, dyeing, metal processing, wood preservatives, paint and pigments, textile, steel fabrication and canning industries [1]. These industries produce large quantities of toxic wastewater effluents. Cr (VI) concentrations in industrial wastewater vary from 0.5 mg/L to 270 mg/L [2]. The maximum permissible limit of Cr (VI) for discharge into inland surface water is 0.5 mg/L and in potable water is 0.05 mg/L [3,4]. The hexavalent form of chromium, usually present in the form of chromate (CrO−2 4 ) and dichromate (Cr 2 O−2 ), possesses significantly higher levels of toxicity 7 than other valence states [5]. Several treatment methods for Cr (VI) removal from industrial wastewater have been established, i.e. electrochemical precipitation, ion exchange, membrane ultrafiltration, reverse osmosis and reduction [6–10]. However, most of these methods are costly due to operational, treatment and sludge disposal costs. Adsorption is another treatment alternative for the removal of heavy metals from aqueous solutions. The main properties of the adsorbents for metal removal are strong affinity and high loading capacity. Zero-valent iron has been widely used to treat heavy metals via redox and precipitation reactions [11,12]. Activated carbon is generally used ∗ Corresponding
author. Email: [email protected]
© 2013 Taylor & Francis
in wastewater treatment as an excellent adsorption material for heavy metal removal [13]. However, these commercial products remain expensive materials for heavy metal removal. Therefore, low-cost adsorbents with high metalbinding affinity need to be researched. Studies of low-cost industrial by-products and waste as adsorbent media have been performed [14–16]. Foundry sands are a waste from the production of both ferrous and non-ferrous metal castings. Foundry activities use high-quality size-specific silica sands in their moulding and casting operations. In the casting process, moulding sands are recycled and reused multiple times. Eventually, however, the recycled sands are degraded to the point that they can no longer be reused. The unsuitable sands typically are landfilled at appreciable cost. Approximately 800,000 tons of foundry sands are landfilled in Wisconsin (USA), with an annual cost of $18 million to the foundry industry [17]. Therefore, the recovery of foundry sand waste would save valuable landfill space and accrue savings through reduced disposal costs. Chemical sands are used both in core making, where high strengths are necessary to withstand the heat of molten metal, and in mould making. Chemical sands consist of 93–99% silica and 1–3% chemical binder. Silica sands are thoroughly mixed with chemicals; a catalyst initiates the reaction that cures and hardens the mass [18]. There are various types of chemical binder systems used in the foundry
1268
I. Campos et al.
Downloaded by [University of Saskatchewan Library] at 22:37 08 January 2015
industry. The most common chemical binder systems used are phenolic-urethanes, epoxy resins, furfyl alcohol and sodium silicates. In this study, the capacity of chemical foundry sands to remove Cr (VI) from aqueous solution was evaluated using discontinuous assays. Kinetic and adsorption behaviour was studied; pH, agitation and initial Cr (VI) concentration effect on foundry sand adsorption capacity were also examined.
2. Materials and methods 2.1. Foundry sand sample Chemical foundry sands were used in this study. It was obtained from the Fundiciones Rey factory sited in Villagarcia de Arousa (Galicia, North West of Spain). This factory produces mainly iron metallic pieces, but also it works with bronze and aluminium. Used foundry chemical sands were sampled from Fundiciones Rey using plastic containers of 10 L. 2.2. Chemicals and analytical methods Foundry sand characterization Foundry sand physico-chemical parameters were characterized. Sand humidity was measured by drying at 105◦ C to a constant weight [19]; density relative to water was measured following the method described in [20] using a density measuring device [20]; particle size distribution was determined using sieves for the largest particles [21] followed by the Stoke law for fine particles [22]. From the particle size distribution, median particle size (D50 ) (below this value are 50% of particles) was calculated. Total iron, total aluminium and total chromium concentrations in sands were determined by digestion of sands using microwave equipment (CEM MDS-2000) [23] and subsequently measured with an ICP-OES spectrometer (VISTA-MPX, Varian Inc.) [24]. Total silicon concentration and the microscopy study of foundry sands were determined by a scanning electron microscope (SEM) (Hitachi S4800) with micro-analysis system by energy-dispersive X-ray spectrometer (EDS Bruker Quantax 400). Total organic carbon (TOC) was determined using a Shimadzu 5050A TOC analyzer coupled to a Shimadzu 5000A solid samplers module [25]. An X-ray diffraction analysis was carried out to determine the main composition of foundry sands using a Siemens diffractometer type D5000. Fourier transform infrared (FT-IR) studies were also carried out to determine the type of functional group responsible for Cr (VI) adsorption. A JASCO 4100 FT-IR spectrometer was used. An accessory for attenuated total reflectance measurement (ATR model PIKE MIRacle) with glass diamond/ZnSe plates, measuring in a range between 4000 and 600 1/cm was attached to the FT-IR equipment.
Sand leaching characterization Due to the composition of the foundry sands, which contain toxic substances such as heavy metals, phenol, etc., sand leaching assays were conducted to determine the mobilization and release of these compounds to water bodies and to confirm their environmental innocuous behaviour [26] (sand concentration 0.1 kg/L, agitation for 24 h). pH and conductivity were measured in aqueous solution (ratio of sand:water 1:2.5) using Crison GLP 22 and Crison GLP 32, respectively. COD was measured by carrying out a digestion at 150◦ C with potassium dichromate, follow by UV-Vis spectrophotometry measurement at 620 nm with a spectrophotometer (Hatch DR/2000). Aqueous iron, chromium, nickel, aluminium and manganese concentrations were analyzed through ICP-OES spectrometry (VISTA-MPX, Varian Inc.) [24]. Anionic and cationic compounds were measured by suppressed ion chromatography using a Methron 882 compact IC plus device. Finally, scanning of phenols was carried out by gas chromatography–mass spectrometry (GC-MS) using a Perkin Elmer Turbomass chromatographer. Discontinuous assay monitoring Aqueous concentrations of Cr (VI) were measured using a UV-Vis spectrophotometer (CAY 100 BIO, Varian Inc.) following standard methods [27]. A coloured complex with 1,5 diphenylcarbazide reagent grade (Scharlab S.L.) at acid pH was measured at 540 nm. Aqueous Cr (VI) solutions and UV-Vis spectrometric standards were prepared by diluting K2 Cr2 O7 PA-ACS-ISO (Panreac Química SAU) in distilled water. All glassware used for experimental purposes was washed in 10% nitric acid and subsequently rinsed with de-ionized water to remove any possible interference by other metals. pH monitoring during kinetic batch tests was conducted to study the variations of pH through the assays, but as the fluctuation of pH was lower than 1 unit for all temperatures, the data has not been shown. Control assays without sands were carried out to estimate Cr (VI) removal due to other mechanisms (reduction, glass adsorption, etc.) As the fluctuation of Cr (VI) concentration in these control assays was lower than 5% for all temperatures, Cr (VI) removal through other mechanisms has been rejected (data not shown). 2.3. Discontinuous assay methods pH effect studies In an adsorption process, aqueous phase pH affects the speciation of a metal to be adsorbed, as well as the dissociation and/or activation of active groups of the adsorbent material. Therefore, metallic compound adsorption will be significantly influenced by medium pH. Batch tests were carried out in duplicate to study the effect of initial solution pH on Cr (VI) removal. The tests
Environmental Technology were conducted in 100 mL polypropylene bottles at 20◦ C containing 50 mL of 50 mg Cr/L solution and 2 g of sands at initial pH of 1.6, 2.2, 2.5, 4.0, 6.0 and 8.0. The initial pH of the solution was adjusted with 0.1 M nitric acid or 0.1 and 0.25 M sodium hydroxide. Bottles were shaken at 15 rpm using a rotary shaker (Stuart STR4) and aqueous Cr (VI) concentration was measured at the end of the assay (24 h).
Downloaded by [University of Saskatchewan Library] at 22:37 08 January 2015
Agitation effect studies Due to energy demands and the effect on adsorption process efficiency, it is important to determine the optimal agitation rate. Batch tests in duplicate were carried out in 100 mL Erlenmeyer flasks with agitation from 60 to 220 rpm using an orbital shaker (IKA KS 4000ic). The operating conditions of these tests were: 20◦ C, pH 2.5, 50 mg/L of initial Cr (VI) concentration and 40 g/L sand concentration. Cr (VI) removal efficiency was calculated after 24 h. Initial Cr (VI) concentration effect studies Batch tests in duplicate were carried out to study the effect of initial Cr (VI) concentration on removal efficiency of foundry sands. The initial Cr (VI) concentration varied from 50 to 400 mg/L and each test was monitored at 25, 35, 45 and 55◦ C. The assays were conducted under the following operating conditions: 180 rpm, pH 2.5 and 40 g/L sand concentration. Cr (VI) removal efficiency was calculated after 24 h. Kinetic assays: temperature effect Discontinuous kinetic assays were carried out in duplicate to determine the adsorption kinetic of Cr (VI) in the presence of chemical foundry sands at 25, 35, 45 and 55◦ C. Two grams of chemical sands were placed in 100 mL Erlenmeyer flasks containing 50 mL of 50 mg Cr/L solution at a pH of 2.5. The Erlenmeyer flasks were shaken at 180 rpm, and Cr (VI) concentration was measured through 48 h of kinetic assay time. The liquid and solid phases were separated using a centrifuge at 3600 rpm and aqueous Cr (VI) concentration was measured as indicated above after filtering the sample through a 0.45 μm filter. The quantity of Cr (VI) adsorbed by sands at time t was calculated according the following equation: qt =
(C0 − Ct )V ms
(1)
where qt (mg/g) is mg of Cr (VI) adsorbed in a gram of sand at time t; C0 (mg/L) is the initial Cr (VI) concentration; Ct (mg/L) is the Cr (VI) concentration at time t; V (mL) is the assay liquid volume; ms (g) is the weight of sand used in the assay. Cr (VI) removal is calculated according the following equation: Cr (VI) removal efficiency (%) =
(C0 − Ct ) × 100 (2) C0
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Adsorption isotherms: temperature effect Batch adsorption assays were conducted in duplicate, in a similar method to the kinetic tests: 100 mL Erlenmeyer flask at 180 rpm, a constant dry mass of sands of 2 g and using 50 mL of dissolved Cr (VI) at initial concentrations of 50, 100, 150, 200, 300 and 400 mg/L. The initial pH value was adjusted to 2.5 and the assays were conducted at 25, 35, 45 and 55◦ C. Assay time was set at 24 h, which was found to be enough to ensure equilibrium (data not showed). Adsorption assay duration was 24 h to guarantee that adsorption process equilibrium was achieved at all temperatures. 2.4.
Adsorption process description and thermodynamic parameters
Activation energy (Ea ), adsorption energy (E) and Gibbs free energy change (G 0 ) for Cr (VI) adsorption process in foundry sands were calculated by Arrhenius equation, Dubinin–Radushkevich (D–R) isotherm model and thermodynamic equilibrium constant (Kc0 ). These parameters are used to describe the mechanism of adsorption process: physisorption or chemisorption. Moreover, Gibbs free energy change (G 0 ), enthalpy change (H 0 ) and entropy change (S 0 ) were calculated to describe the degree of spontaneity, type of reaction (endothermic or exothermic) and randomness of the adsorption process.
3.
Results and discussion
3.1. Foundry sand characterization Table 1 shows the physico-chemical characterization and leaching results of the foundry sands used in this work, while particle size distribution and foundry sand X-ray diffractogram are shown in Figure 1(a) and 1(b), respectively. The high TOC content was due to organic matter from the resins used in the moulds. Relative density values were slightly lower (2.39 to 2.55) than those indicated by Siddique and Noumowe [18]. Median particle size (D50 ) of used sands was also lower than green foundry sands utilized by Lee et al. [15,17] (0.19 to 0.20 mm). Regarding metals, foundry sands contain 0.9% Fe, 30.4% Si and 0.3% Al. These metals will directly affect Cr (VI) removal. Zero iron has been used for metal removal through redox and precipitation reactions (as hydroxides at high pH) [15,28]. In addition, previous studies have proved the efficiency of Si and Al groups in Cr (VI) removal. The main negative − species of Cr (VI) (CrO− 4 and CrO7 ) will be joined through electrostatic interactions to silanol (SiOH+ 2 ) and aluminol (AlOH+ ) groups at low pH [16]. In the foundry sands 2 used in this work, four main crystalline species have been identified by X-ray diffractogram: SiO2 (α-quartz); Fe2 O3 (maghemite); Al2 O3 (corundum) and TiO2 ; SiO2 being predominant. These data fit with the results obtained from the
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I. Campos et al. Physico-chemical characterization of foundry sands.
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Parameter Sand characterization Humidity (%) Relative density ∗ (mm) D50 TOC (% dm† ) Total Fe (mg Fe/g dm) Total Si (mg Si/g dm) Total Al (mg Al/g dm) Total Cr (mg Cr/g dm) Sand leaching characterization‡ pH Conductivity (μS/cm) COD Fe Total Cr Cr (VI) Ni Al As Ba Cd Co Mo Cu Ni Pb Sb Se F− Cl− NO− 3 NO− 2 PO−3 4 SO−3 4 Li+ Na+ NH+ 4 K+ Ca2+ Mg2+ Phenol 2-chlorophenol 2-metilphenol 3-metilphenol + 4-metilphenol 2,4-dichlorophenol 2,6-dichlorophenol 4-chloro-3-metilphenol 2,4,5-trichlorophenol 2,4,6-trichlorophenol Tetrachlorophenol Pentachlorophenol
Value 1.9 ± 0.0 2.0 ± 0.5 0.13 ± 0.05 13.1 ± 0.3 9.4 ± 2.5 303.6 ± 47.2 3.1 ± 1.4 2.8 ± 0.5 3.86 ± 0.02 915 ± 8 166.0 ± 0.0 91.8 ± 2.0 0.02 ± 0.00
