SIMULTANEOUS REMOVAL OF CU II, NI II, AND ZN II BY A GRANULAR MIXTURE OF ZVI AND PUMICE IN COLUMN SYSTEMS

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1 Proceedings of the 13 th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013 SIMULTANEOUS REMOVAL OF CU II, NI II, AND ZN II BY A GRANULAR MIXTURE OF ZVI AND PUMICE IN COLUMN SYSTEMS S.BILARDI 1, P.S. CALABRO 1 and N. MORACI 1 1 Università Mediterranea di Reggio Calabria, Dipartimento di Ingegneria Civile, dell'energia, dell'ambiente e dei Materiali (DICEAM), Via Graziella loc. Feo di Vito 89122, Reggio Calabria, Italy paolo.calabro@unirc.it EXTENDED ABSTRACT Zero Valent Iron (ZVI) is a widely used reactive medium for water treatment (e.g. contaminated groundwater remediation by permeable reactive barriers (PRBs), stormwater runoff treatment, potabilisation systems at household level). Despite the high removal efficiency of ZVI, the major issue related to its use is the long-term behaviour. In several published cases the hydraulic conductivity and removal efficiency were progressively reduced during operation, potentially compromising the functionality of the system. Following the results of a recent research activity, the granular mixtures between ZVI and other materials (e.g. sand, pumice) are now an established way to overcome this problem. This paper presents the results of the research activity carried out by column tests aimed at comparing the performance of a granular mixture between ZVI and Pumice (weight ratio 30:70) and of pure ZVI for the individual and combined removal of dissolved Cu II, Ni II and Zn II and at verifying the occurrence of phenomena of mutual interaction and/or competition. Although metals removal by ZVI has been extensively documented in the past, the great majority of studies examined either very simple systems (i.e. single metal solutions) or very complicated ones (e.g. real acid mine drainage), in both cases, for different reasons, the occurrence of mutual interactions (positive or negative) among the chemical species present in water is impossible to detect. For these reasons in this paper are compared column experiments carried out in similar conditions using both monocontaminant and pluricontaminant solutions. Experimental results show that ZVI and the granular mixture ZVI/pumice are able to remove copper, nickel and zinc from single-metal solutions with the following removal sequence: Cu>Zn>Ni. In pluricontaminant solutions it is observed that i) the removal efficiency of copper is unaffected by the presence of the other metals, ii) zinc removal significantly decreases from single metal solutions to three-contaminant solutions and iii) this fact is less evident for nickel and in the short term the removal of this metal increases in three of the four experiments carried out with the three-contaminant solutions. Therefore contaminants seem to be removed not by individual reactions but during a complex process of interaction with the other contaminants. Keywords: Pumice, Zerovalent iron, hydraulic conductivity, heavy metals. 1. INTRODUCTION Zero Valent Iron (ZVI) is a widely used reactive medium for water treatment (e.g. contaminated groundwater remediation by permeable reactive barriers (PRBs), stormwater runoff treatment, potabilisation systems at household level). ZVI has been studied in depth to verify its suitability for the removal of dissolved metals from groundwater by PRBs (Blowes et al., 2000) and in particular from groundwater impacted

2 by mine drainage (Fiore and Zanetti, 2009; Wilkin et al., 2003; Shokes et al., 1999) containing redox-sensitive metals such as hexavalent chromium (e.g. Wilkin et al., 2005; Blowes et al., 1997), uranium (Noubactep et al., 2005) and metalloids such as arsenic (Bang et al., 2005; Lackovic et al., 2000). Its use for runoff treatment (Rangsivek and Jekel, 2005) or for drinking water systems at household level (Noubactep et al., 2012) is more recent. From previous research it is now clear that ZVI can activate additional removal mechanisms respect to the well known oxidation-reduction allowing the removal of metals that cannot be reduced by ZVI (e.g. Zn) by coprecipitation and sorption. Reduction may be driven by four different reaction paths: Fe 0 (direct reduction), reduction by aqueous Fe II, reduction by adsorbed or structural Fe II, reduction by molecular (H 2) or atomic (H) hydrogen (Noubactep and Schöner, 2009). Despite the high removal efficiency, the major issue related to the use of ZVI is its longterm behaviour. In several published cases the hydraulic conductivity and removal efficiency were progressively reduced during operation, potentially compromising the functionality of the system (Morrison et al. 2006; Li et al. 2005). The granular mixtures between ZVI and other materials (e.g. sand, pumice) are now an established way to overcome these problems (Bilardi et al., 2013; Carè et al., 2012; Ruhl et al., 2012; Mak et al., 2011; Moraci and Calabrò, 2010; Komnitsas et al., 2007). In this study the individual and combined removal of copper, nickel and zinc, was evaluated through column tests using respectively a granular mixture of ZVI and Pumice in weight ratio 30:70 and ZVI alone. Literature presents studies on the use of ZVI to remove Cu from acid mine drainage (Shokes and Möller, 1999; Wilkin and McNeil, 2003; Bartzas et al., 2006), from stormwater runoff (Rangsivek and Jekel, 2005) or from syntetic mono-contaminant solutions (Moraci and Calabrò, 2010; Komnitsas et al., 2007a). Copper removal was mainly attributed to the cementation process ( i.e. reduction of the oxidized form of the contaminant, Cu II, and subsequent deposition of Cu 0 onto the iron surface) but also to adsorption and co-precipitation on iron corrosion products. The experiments aimed at evaluating nickel removal by ZVI (Moraci and Calabrò, 2010; Bartzas et al., 2006; Dries et al., 2005) underlined that the possibility of a spontaneous electrochemical cementation process between Ni and ZVI is less favoured than in the case of copper, because the standard redox potential of the couple Ni 2+ /Ni 0 is only slightly higher than that of Fe 2+ /Fe 0, consequently quantitative removal has been mainly attributed to adsorption, co-precipitation and adsorptive size-exclusion. Regarding zinc, reduction by ZVI is excluded since the standard redox potential of the couple Zn 2+ /Zn 0 is lower than that of Fe 2+ /Fe 0 and therefore its removal is due to the other mechanisms activated by ZVI (Bartzas et al., 2006; Dries et al., 2005). 2. MATERIALS AND METHODS The ZVI used in the experiments is of the type FERBLAST RI 850/3.5, distributed by Pometon S.p.A., Mestre - Italy. The material contains mainly iron (> %) identified impurities include Mn (0.26 %) and traces of O, S and C. The pumice comes from the quarries of Lipari (Aeolian Islands, Sicily Italy). Its mineralogical composition was determined as follows: SiO 2: %; Al 2O 3: %; K 2O: 4.47 %; Na 2O: 3.59 %; Fe 2O 3: 1.98 %; moreover it contains about 4 % of bound water entrapped in the pumice structure during the sudden cooling of magma and traces of other compounds (e.g. CaO, SO 3, MgO, TiO 2, FeO, MnO, P 2O 5). The two materials are characterized by uniform grain size distribution; coefficient of uniformity (U) is 2 and 1.4 for the Fe 0 and pumice respectively. The mean grain size (d 50) is about 0.5 mm and 0.3 mm for ZVI and pumice respectively. The solutions, used to feed the columns, were obtained by dissolving individually or in combination copper nitrate, nickel nitrate and zinc nitrate in distilled water. Copper(II) nitrate hydrate (purity ), nickel(ii) nitrate hexahydrate (purity ) and zinc(ii)

3 nitrate hexahydrate (purity ) were obtained from Sigma-Aldrich. The concentration values used for the three contaminants were of 500 mg/l or 50 mg/l for copper and 50 mg/l for nickel and zinc respectively. Ten column tests, five with columns containing a granular mixture ZVI/Pumice with a weight ratio 30:70 and five containing pure ZVI, were carried out using solutions contaminated by either Cu, Ni or Zn or solutions where the three metals were simultaneously present (Table 1). To allow a direct comparison of the different column tests the ZVI amount was set at 240 g. In the columns where ZVI was used alone, since it did not fill all the space available (in fact the reactive layer was of about 3 cm), washed quartz gravel was used as filling material. Polymethyl methacrylate (Plexiglas) columns, 50 cm long with 5 cm internal diameter, equipped with several sampling ports, were used in this study. Table 1: Experimental conditions of column tests ID Reactive medium ZVI Pumice Contaminant/Concentration Test duration [g] [g] [mg/l] [h] A ZVI/pum 30: Cu/ B ZVI/pum 30: Ni/ C ZVI/pum 30: Zn/ D ZVI/pum 30: Cu/500 Ni/50 Zn/ E ZVI/pum 30: Cu/50 Ni/50 Zn/ A1 ZVI Cu/ * B1 ZVI Ni/ C1 ZVI Zn/ * D1 ZVI Cu/500 Ni/50 Zn/50 600* E1 ZVI Cu/50 Ni/50 Zn/50 432* * Test stopped after column clogging. The solution was pumped, in up-flow mode, from a single PE bottle for each solution using a precision peristaltic pump (Ismatec, ISM930), the flow rate was maintained constant at a value of 0.5 ml/min. In order of evaluating the evolution of the clogging for the ZVI/pumice mixture and for ZVI alone, during column tests, the hydraulic conductivity was measured using either constant-head (k >10 6 m/s) or variable-head (k < 10-6 m/s) permeability methods (Head and Keeton, 2008). In order to characterize the magnitude of contaminant removal, the removal efficiency (E) was calculated using Eq. 1. E = Mrem/Min*100 [%] (1) where Min is the mass of contaminant flowed into the column and Mrem is the mass of removed contaminant. The aqueous concentrations of Cu II, Ni II, Zn II were measured by Atomic Absorption Spectrophotometry (AAS - Shimadzu AA 6701F) using conventional Standard Methods (APHA 2005). All chemicals used for experiments and analysis were of analytical grade. 3. Results and discussion 3.1. Removal efficiency of Ni II, Cu II and Zn II Figure 1 shows the normalized concentration (C/C 0) of the contaminants for the column tests carried out using single-metal solutions, where C is the effluent concentration measured at the outlet and C 0 is the influent concentration. The experiments carried out using copper solution demonstrates that (Tests A and A1) it is easily removed by both the granular mixture and ZVI already the first sampling port (3 cm from inlet, data not shown) and for the whole duration of the test, without any sign of medium exhaustion. Nickel removal is significantly less effective especially if ZVI alone is

4 used, this fact has been attributed (Calabrò et al., 2012) by a slower reaction kinetic requiring a higher residence time in the column that is better guaranteed in the system containing the ZVI/Pumice mixture (14.6 h of residence time against 0.7 h) C/C elapsed time(h) Test A Test B Test C Test A1 Test B1 Test C1 Figure 1. Breakthrough curve at the outlet for tests A, B, C, A1, B1, C1. Zinc removal is more effective than nickel one but breakthrough is nevertheless quite fast. The better removal of Zn respect to Ni is attributable to higher sorption affinity that iron oxides have for Zn than for Ni (Cornell and Schwertmann, 1996; Stumm and Morgan, 1996).Therefore in mono-contaminant systems, using either the granular mixture between ZVI and Pumice or ZVI only the removal sequence is Cu>Zn>Ni. In the column tests with the granular mixture ZVI/Pumice and three-contaminant solutions copper breakthrough was observed only in Test D and at the first sampling port (L = 3 cm). Instead for Test E copper is completely removed from solution already at the first sampling port. In order of comparing directly the removal for all the experiments carried out (mono and three contaminant solutions, ZVI/Pumice granular mixture and pure ZVI), figures 2 and 3 show Ni and Zn concentrations versus time measured at the outlet of the column for all the investigated systems. Cu data have not been reported since, as already mentioned, this metal is always completely removed below instrument s detection limit. As observed for single-metal-solution nickel needs a longer residence time to be removed from solution and breakthrough occurs already at the first sampling. When using the three-contaminant-solution containing 500 mg/l Cu (test D and D1), zinc removal is similar to nickel one especially in the short and medium term. In the experiment carried out using the three contaminant solutions containing 50 mg/l Cu (test E and E1), zinc removal is significantly higher than nickel one. In order to make possible a direct comparison of the long term performance, Table 2 reports the removal efficiency and the contaminants mass removed calculated after 1440 h from the beginning of the experiments for tests A-E and B1-C1 while for test A1 the presented data refer to the last sampling available (1404 h), experiments D1 and E1 have not been reported due to their limited duration. Table 3 presents the removal efficiency and the contaminants mass removed calculated after 432 hours from the beginning of the experiment in order to have an equal amount of contaminants in input to all the columns.

5 Ni (mg/l) Test B Test D Test E Test B1 Test D1 Test E elapsed time (h) Figure 2. Time-dependant evolution of Ni concentration in the column effluent for test B- D-E-B1-D1-E Test C Test D Test E Test C1 Test D1 Test E1 Zn (mg/l) elapsed time (h) Figure 3. Time-dependant evolution of Zn concentration in the column effluent for test C- D-E-C1-D1-E1. Table 2: Removal efficiency (E) calculated for a fixed value of mass of contaminant in input into the column equivalent to 1404 h for test A1 and 1440 h for tests A-E and B1- C1. ID mi [g] E Mass removed [mol] Cu Ni Zn Cu Ni Zn Cu Ni Zn Ni+Zn A % E B % E E-02 C % E E-02 D % 37.2% 33.8% 3.30E-01 1,37E E E-02 E % 26.0% 43.2% 3.30E E E E-02 A % E B % E E-02 C % E E-02 For 1440 h duration (Table 2), in the tests carried out using a three-contaminant solution and a granular mixture of ZVI and Pumice (tests D and E), copper removal is unaffected by the presence of the other metals while the removal efficiencies of Ni and Zn decrease respect to mono contaminant tests. It is nevertheless interesting that the removal efficiency reduction is higher for Zn (about 58% respect to the experiment carried out with the solution containing Zn only) than for Ni (reduction of about 33%) leading to very similar removal efficiencies for Ni and Zn and, for test D, to a change in the sequence that in this case is Cu>Ni>Zn.

6 Data presented in Table 3 confirm that Ni removal is enhanced in three-contaminant systems in fact in three cases over four the removal of this metal is higher than in corresponding mono-contaminant ones. In the short term in the experiments carried out with ZVI (experiments D1 and E1) the sequence Cu>Zn>Ni is maintained but in these cases the removal efficiencies for Ni did not diminish but increased by about 40%. Zn removal efficiencies are reduced respect to the experiments carried out with monocontaminated solutions. The comparison of the performance in Ni removal for the column systems D E and D1 E1 brings the convincement that most probably copper presence influences positively Ni removal, it is possible that the significant enrichment of the column medium in copper due to the fast cementation of this latter leads to the formation of a bimetallic system between ZVI and Cu that enhances Ni removal. The fact that in systems D1 and E1 the sequence Cu>Zn>Ni was maintained is attributable to the limited duration of the experiments and to the fact, already mentioned, that due to the limited hydraulic residence time, Ni removal is less favoured in ZVI systems respect to ZVI/Pumice ones. Table 3: Removal efficiency (E) calculated at equal contaminant mass in input to the column equivalent to 432 h. ID mi [g] E Mass removed [mol] Cu Ni Zn Cu Ni Zn Cu Ni Zn Ni+Zn A % E B % E E-03 C % E E-03 D % 64.7% 54.9% 1.02E E E E-02 E % 51.3% 67.5% 5.68E E E-02 A % E B % E E-03 C % E E-03 D % 44.1% 59.0% 1.02E E E E-02 E % 49.5% 65.3% 1.02E E E E-02 The simultaneous removal of the three contaminants (Zn 100 mg/l, Cu 50 mg/l and Ni 50 mg/l) was studied by Komnitsas et al. (2007b) and Bartzas et al. (2006) during their column experiments for the treatment of artificial acid mine drainage solutions (also other compounds were therefore present in columns influent). In these studies, column tests results basically confirmed the findings of the experiments presented in this paper for copper and nickel but showed a better zinc removal that could be related to the different test conditions (e.g. flow rate). The removal efficiency of copper and zinc observed in Tests D and E (Table 3) is comparable to the results obtained by Rangsivek and Jekel (2011) during their study on the removal of these metals from roof runoff. Finally the higher zinc removal efficiency respect to nickel is further confirmed by the results of the batch tests carried out by Dries et al. (2002) Hydraulic conductivity The hydraulic conductivity measured at the beginning of the column tests was between 1.5x10-2 and 3.0x10-2 cm/s both using ZVI only or the ZVI pumice granular mixtures. Excepted for test B1, all the experiments carried out on column systems containing ZVI only were forcedly interrupted because of tygon tubes disconnection due to the excessive pressure caused by the clogging of the reactive medium. Table 4 clearly shows how the granular mixture between ZVI and Pumice is by far more effective than ZVI alone in maintaining unvaried the hydraulic conductivity in the long term (Moraci and Calabrò, 2010). It is also evident (experiments D1 and E1) that the use of the multicontaminant solution significantly accelerates the permeability reduction in the columns filled with ZVI

7 only while negative effects have not been observed in the experiments (D and E) carried out using the ZVI/Pumice granular mixture. Table 4: Hydraulic conductivity determined at the end of column tests for all investigating systems. ID time [h] k [cm/sec] ID time [h] k [cm/sec] A E-02 A E-07 B E-02 B E-03 C E-02 C1 2016* 4.7 E-07 D E-02 D1 432* 2.3 E-06 E E-02 E1 432* 3.0 E CONCLUSIONS ZVI and the granular mixture ZVI/pumice, at w.r. 30:70, are able to remove copper, nickel and zinc from single-metal solutions with the following removal sequence: Cu>Zn>Ni, and consequently, the residence time in the system strictly necessary to guarantee metal removal decreases with the same sequence. Considering the same mass of contaminant in input to the systems (after 432 hours of test), the removal efficiency of copper is virtually unaffected by the presence of the other metals, zinc removal significantly decreases from single metal solutions to threecontaminant solutions while nickel removal increases in three of the four experiments carried out with the three-contaminant solutions. In one case, in the experiment carried out using the three contaminant solution containing 500 mg/l of copper, the removal sequence for the mixture ZVI/Pumice changes to Cu>Ni>Zn, also in the long term (duration 1440 hours) therefore contaminants seem to be removed not by individual reactions but during a complex processes of interaction with other contaminants. The better removal of nickel observed in the three-contaminant solutions and especially in the one containing the highest copper concentration and using the granular mixture as reactive medium could be due to the enrichment of the reactive material in copper. In fact the fast cementation of this latter probably leads to the formation of a bimetallic system between ZVI and Cu that can enhance Ni removal. This finding, if further confirmed, is particularly interesting since nickel removal by ZVI is quite problematic. For this reason a research program aimed at testing a granular mixture between pumice and copper coated ZVI for nickel removal is in progress. ACKNOWLEDGEMENTS The authors are grateful to Dott. Giuseppe Panzera for its essential assistance during this research activity. This research was funded by Project PON01_01869 TEMADITUTELA. REFERENCES 1. APHA, AWWA, WEF, (2005). Standard Methods for the examination of water and wastewater, 21 st 496 ed. American Public Health Association, Washington D.C. (USA). 2. Bang S., Johnson M.D., Korfiatis G.P. and Meng X. (2005) Chemical reactions between arsenic and zero-valent iron in water, Water Res., 39, Bartzas G., Komnitsas K. and Paspaliaris I. (2006). Laboratory evaluation of Fe 0 barriers to treat acidic leachates. Miner. Eng., 19, Bilardi S., Calabrò P.S., Caré S., Moraci N. and Noubactep C., (2013) Effect of pumice and sand on the sustainability of granular iron beds for the removal of Cu II, Ni II, and Zn II. Clean Soil Air Water, doi: /clen

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