Competitive sorption of metal ions on titanium phosphate sorbent

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Competitive sorption of metal ions on titanium phosphate sorbent (TiP1) in fixed-bed columns: a closed-mine waters study Mylène Trublet, Edvards Scukins, Ivan Carabante, and Daniela Rusanova ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05971 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 24, 2019

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ACS Sustainable Chemistry & Engineering

Competitive sorption of metal ions on titanium phosphate sorbent (TiP1) in fixed-bed columns: a closed-mine waters study Mylène Trubleta*, Edvards Scukinsb, Ivan Carabantec and Daniela Rusanovaa

a

Chemistry of Interfaces, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden, b Aeronautics, Department of Flight Data and Navigation, SAAB, SE-581 88 Linköping, Sweden, c Waste Science and Technology, Department of Civil, Environmental and Natural Resources Engineering, Luleå University of Technology, SE-971 87 Luleå, Sweden. * Correspondence to: [email protected], +46 (0) 920-493448

ABSTRACT Sorption fixed-bed column experiments were performed using a titanium phosphate ion-exchanger composed of –H2PO4 units [TiO(OH)(H2PO4)•H2O]. Model mine water containing five divalent metal ions (Cu2+, Zn2+, Mn2+, Ni2+ and Co2+) and a few closed-mine water samples were treated to evaluate the sorption preference of the material. For the first time, dynamic ion-exchange capacities (estimated to be between 3.2 and 4.2 meq.g-1) and static ion-exchange uptakes (calculated between 3.1 and 3.5 meq.g-1) were obtained for the same TiP1 sorbent and data were discussed in terms of sorption behavior. It was found that sorption processes on TiP1 in model and closed-mine waters during a column experiment could be accurately predicted from the corresponding batch experiment (including the sorbent’s effectiveness in different types of waters). A competitive sorption phenomenon in favor of Cu2+ on TiP1 was established for all cases, pointing towards the possibility of isolating pure copper concentrate from closed-mine waters. The relatively high amounts of calcium and magnesium ions present in mine waters did not appear to considerably affect the selectivity of TiP1 material. Exploratory experiments for sorbent regeneration and desorption using a low concentration of nitric acid were demonstrated.

KEYWORDS Titanium Phosphate; Water Treatment; Heavy Metals; Column; Sorption

INTRODUCTION Sweden has a long history of mining activities—in the late 1910’s there were about 500 active mines for base and precious metals while today there are only ca. 20 operational sites left. The most common methods for preventing the appearance of acidic and metal-rich leachate have been to cover the mining waste with earth or water. The environmental problems related to mining are 1 ACS Paragon Plus Environment

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often the result of both the historic waste heaps that were left uncovered, and uncontrolled metal ions leaching into the water cycle from the covered and uncovered mineral waste. The management of non-active mine waste and after-closure mine waste has been of immense importance for society 1,2. Thus, there is great interest and need for developing long-term waste technologies to control the mining water impact on the environment. In practice, a conventional step for removal of heavy metals in mining waters is chemical precipitation based on lime treatment. Under these conditions (pH 8-11) most dissolved heavy metal ions are precipitated as insoluble hydroxides, which are subsequently coagulated and removed from the water. However, this process can only minimize the metal concentrations to the levels of their corresponding solubility product constants, which for most dissolved heavy metal ions remain above the limitations set by the European directives for water. It has also been reported that high concentrations of calcium and potentially of magnesium ions (as a consequence of the lime treatment) could also be an obstacle in the removal of lower amounts of heavy metal ions in the waters 3. Therefore, additional finer water treatments such as ion-exchange, membrane filtration, electro-dialysis processes, reverse osmosis, etc. are recommended 4,5. The choice of treatment for waters with lower metal concentrations is case-specific and yet governed by technical and economic issues. Sorption onto environmentally friendly and costeffective materials would be economically favorable and straightforward, and ultimately the best scenario. Undeniably the ion-exchangers based on group (IV) phosphate/phosphonates types, such as alpha and gamma, have been widely studied since the 1990’s for separation of lanthanides and w.r.t. removal of heavy metal ions from various polluted waters 6–9. In particular, titanium phosphate (TiP) sorbents have been selected due to their high exchange capacity, chemical stability and high selectivity towards divalent metal ions 10–13. The highest exchange capacity of TiP sorbents has been recently reported for the TiP1 type sorbent (TiO(OH)H2PO4•H2O), where the presence of solely –H2PO4 ion-exchange units played a major role in improving and controlling the sorption properties of the material 14. This TiP1 sorbent has shown a very high affinity towards transition metal ions (Cu2+, Ni2+, Mn2+, Co2+ and Zn2+ ions) with an ion-exchange capacity estimated to be between 2.9 and 3.8 meq.g-1 under static conditions 15. The sorption performances of TiP sorbents can be compared based on the (i) experimental ionexchange capacity (IEC); (ii) the theoretical exchange capacity (TEC) and (iii) the capacity towards sodium ions 16. For a complete assessment, these values need to be estimated for batch and column modes under analogous experimental conditions and further compared to each other. A number of studies have demonstrated that the sodium form of TiP (Na-TiP) showed a higher sorption capacity, in batch and column set ups, towards divalent metal ions than the proton form (H-TiP) 7,15. Other ionic forms such as calcium (Ca-TiP) and magnesium (Mg-TiP) have also been tested and it was found that the sorption capacity of the sorbent increases following the order: CaTiP < Mg-TiP < Na-TiP 16. Another major advantage in using the alkaline form of TiP sorbents relates to the fact that the pH of treated waters could not be altered during the exchange of metal ions with alkaline ions and hence no new pH-related chemical processes (like hydrolyses or 2 ACS Paragon Plus Environment

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dissolution) could be initiated. In general, once the sorbents had been converted to their sodium forms, they could further exchange the alkali ions with heavy metal ions. The amount of metal ions sorbed per gram of sorbent determines the IEC of TiP/Na-TiP materials, while TEC is defined as the theoretical number of protons that can be exchanged per gram of TiP sorbent. A summary of these parameters, IEC, TEC, Na+ uptake and reported surface areas for most of TiP sorbents that have been tested in batch and/or column set-up are shown in Table 1. Table 1. Sorption performance of TiP ion-exchangers. Data of IEC (Ion Exchange Capacity), TEC (Theoretical Exchange Capacity), Na+ uptake and surface area. Na+ uptake (meq.g-1)

IEC (meq.g-1)

TEC (meq.g-1)

SA (m2.g-1)

Batch Data Ti(HPO4)2·H2O Ti(H2PO4)(PO4)·2H2O Ti2O3(H2PO4)·2H2O Ti(OH)1.36(HPO4)1.32·2.3H2O TiO1.11(OH)0.58(H2PO4)0.8(HPO4)0.2·0.64H2O TiP1 (TiO(OH)(H2PO4)H2O)

3.9-7.7 7.9 2.1-3.3 2.5-3.2 5.1 6.3

0.6-5.3* 0.2* 0.1-3.3* 0.8-1.9 1.7 2.9-3.8

7.8 7.8 10.7 5.7 9.8 10.2

3.7-122 n.a n.a 73.1-94.4 n.a 26.4-114.0

Column Data TiO1.25(OH)0.47(H2PO4)0.77(HPO4)0.13·2.3H2O TiP1-Ti(HPO4)2·H2O TiP1 (TiO(OH)(H2PO4)H2O)

4-2-4.7** 2.9 6.2

2.6-2.8 1.7 3.2-4.2

8.2 8.9 10.2

n.a n.a 60.4

TiP ion-exchangers

Ref 17–19 18 11,20 12,13,21 18 14,15,22

7,16 23

This work

*Values were obtained from sorption experiments using the proton form ** The TiP ion-exchanger was converted to its potassium form

Data in Table 1 reveals that an average of ca. 50% conversion from proton to sodium forms (calculated from the TEC) is achieved for most TiP sorbents. Apart from a recently published paper 18 , the Na+ uptake for α-TiP has been determined experimentally and 75% conversion was reported only at very high pH. This has been explained by the coexistence of different exchange units (Ti(HPO4)2, Ti(NaPO4)(HPO4) and Ti(NaPO4)2), geometry restrictions and possible partial hydrolysis of the material 17,19. Data also show that IEC values for all TiP sorbents are lower than the maximum exchange capacities based on sodium uptake values, which could be related to the properties of hydrated metal ions and/or the pH of the waters. For example, the reported IEC of αTiP (0.55 meq.g-1) towards scandium ions referred to sorption at pH ~ 2 18, while another study of α-TiP showed an IEC of 5.3 meq.g-1 towards lead ions at pH > 4. The latter could also be related to formation of lead precipitates in addition to the exchange processes. The surface areas reported for these materials vary considerably and can be considered as low or average. An interesting detail is that in all cases the entire surface area is comprised of external surface area, which would correlate well with effective sorption on reachable sites 14,15. The table reveals some of the main challenges in comparing the sorption performances of TiP sorbents— the general insufficiency of 3 ACS Paragon Plus Environment


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systematic sorption data, analogous experimental conditions utilized, widespread values for IEC and Na-uptake for seemingly similar sorbents reported. The combination of factors (related to the chemistry, physics and porous characteristics of sorbents) that may account for particular sorption data is somewhat undetermined. In other words, although there are many studies on the syntheses and characterization of TiPs (see Table 1), there is also an absence of methodically obtained data for their sorption performances in continuous flow (column) modes, sorbent regeneration, selectivity towards metal ions, etc., and almost no comparison between the sorption behavior in batch and column modes have been reported. The latter raises another important aspect of inorganic sorbent performances. Substantial differences between the sorption capacities estimated from a static experiment (batch set-up) and from a dynamic one (column set-up) have been reported 24. A few examples are the sorption of lead(II) ions onto activated carbon or chromium(VI) ions onto pomace, where the amount sorbed decreased more than 70 %, in a continuous flow set-up 25,26. Therefore, it appears that the main challenge in using the ion-exchange technique for metal ion removal is transposing to the continuous flow mode the selective property of the sorbent towards heavy metal ions (observed in batch mode) while keeping the sorption capacity high. This work focuses on details of the sorption properties of TiP1 sorbent in continuous flow mode, towards a mixture of heavy metals ions. Model mine waters and after-closure mine waters were treated systematically in a column set-up, and data are compared to each other and to the EU water regulations. The dynamic sorption experiments were performed using the sodium form of TiP1 (Na-TiP1) under a broad range of working conditions and a case of competitive sorption and its potential uses is revealed.

EXPERIMENTAL SECTION Synthesis The TiP1 ion-exchanger, in the proton form, TiO(OH)H2PO4•H2O, was synthesized in a manner similar to that described by Trublet et al. 15. In brief, 50 mL of a TiOSO4•(H2SO4)x solution from Sigma Aldrich (CAS number: 123334-00-9, 27-31% H2SO4 and ~ 5 % Ti basis) was heated to 80 °C for 30 min. Concentrated phosphoric acid (85% H3PO4) was added to the solution to achieve a molar ratio of TiO2:P2O5=1:1. The resulting mixture was heated to 80 °C for an additional 30 minutes and stirred for 5 hours. The formed precipitate was allowed to settle for about 10 hours and dried at 60 °C. Post-synthesis treatments were performed using first diluted HCl acid (0.05 M and 0.01 M) and then distilled water. The final white solid was dried at 60 °C. To obtain the sodium form of TiP1, Na-TiP1, the solid was placed in a 0.5 M Na2CO3 solution (m:V = 1:50) for 24 hours, rinsed with deionized water until the pH was 5.5- 6.0 and dried at 60 °C. The sodium exchange for TiP1 system was estimated to be ca. 6.3 meq.g-1 corresponding to an ionexchange capacity towards divalent metal ions (IEC) of ca. 3.1 meq.g-1 (for batch set up). 4 ACS Paragon Plus Environment

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Sorbent characterization The TiP1 ion-exchanger, TiO(OH)H2PO4•H2O, has been previously characterized using XRD, 31P MAS NMR, SEM, elemental analyses and the results are described in Trublet et al. 14,15. In brief, the white solid showed a great degree of amorphousness that further increased during the conversion to Na-form. The presence of only one type of P-exchange units was confirmed by the narrow resonance line (at -7.2 ppm) observed in the 31P NMR spectrum of TiO(OH)H2PO4•H2O. The sodium form of the sorbent displayed a complex 31P resonance line due to the multiple overlapping sites, as expected 15. The SEM images of the sorbent (that can be found in Ref.14 and the Supporting Information of Ref.22) revealed agglomerates of particles together with some grains of 500-1000 nm in size. The BET characterization of TiO(OH)H2PO4•H2O sorbents revealed relatively low surface areas between 25 and 60 m2.g-1 depending on the source of titanium, which consisted of almost entirely external surface areas; the average pore diameter was about 15 nm in all cases 15. The elemental analyses (performed by ALS Scandinavia AB, Luleå) confirmed that Ti/P = 1 in the sorbent. Static (batch) experiments The affinity of Na-TiP1 sorbent towards divalent metal ions in the presence of calcium ions was initially probed in batch experiments. The sorbent was mixed with a model mine-water solution consisting of ZnSO4•7H2O, CuSO4•5H2O, MnSO4•H2O, CoSO4•7H2O, NiSO4•7H2O and CaCl2•2H2O with a concentration of 2 mmol.L-1 each. The primary pH of the solution was found to be 5.0 and no large alterations of the pH were observed during any sorption experiments. The mass of Na-TiP1 (g) to volume (mL) ratio was m:V = 1:650. The suspensions were stirred for 24 hours and then filtered. The metal ion concentrations in solution were determined using ICP. The amounts of sorbed metal ions (qeq) were determined using equation 1: qeq = (Co – Ce) V/m

(1)

where C0 is the initial metal ion concentration in solution (mg.L-1), Ce is the metal ion concentration at equilibrium (mg.L-1) and V/m (L.g-1) is the volume-to-mass ratio. The selectivity of the sorbent was assessed based on the ratios of adsorbed species as discussed by Lv et al and Inglezakis et al. 27,28. For a metal ion couple A/B, the ratio of qA(t)/qB(t) defines selectivity (S) 28, S = qA(t)/ qB(t)

(2)

where qA(t) and qB(t) refer to the metal uptake of ions with different (stronger or weaker) affinity to the sorbent at time t, respectively. In the case of a multicomponent system different ratios can be considered. We have used this ratio to estimate the selectivity factors at the time of equilibrium and all starting concentrations of ions were equal (2 mmol.L-1 each).

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Column design and dynamic (column) experiments The column set-up was designed on batch data for the Na-TiP1 sorbent obtained earlier 22. The sorption isotherms carried out using this sorbent revealed that the Temkin model best describes the sorption performances in batch mode w.r.t. to adsorbed divalent metal ions (when compared to the Langmuir and Freundlich models). High-affinity-type sorption curves were obtained revealing full metal uptake at low initial metal concentration. The kinetics of ion-exchange processes was also analyzed in this study and the overall kinetics of sorption on Na-TiP1 (in batch) was found to be relatively fast (5-10 min) and of pseudo-second order, indicating that the chemisorption process was the rate-limiting step 22. These were the basis to build a lab-scale column, which ensures saturated and fast adsorption. Moreover, the goal was to compare Na-TiP1 sorption performances in batch mode to the sorption capability in column mode and not only using model waters but real close-mine effluents. The column experiments were conducted with Na-TiP1 solid using a glass rod with 30-cm length and an inner diameter of 1 cm. The column was packed as follows: 3-cm layer of glass beads (with 1-mm diameter), filter paper, a thin layer of glass wool and the active sorbent, Na-TiP1 in required amounts. The same filling order was kept on the top of the sorbent (a glass wool, filter paper and glass beads). Experiments with two different amounts of Na-TiP1 were performed. About 4.5 g of Na-TiP1 were used for the column trials with model mine water, which corresponds to a bed volume (BV) of about 7 mL, while 1.5 g of Na-TiP1 were used for the experiments with closed-mine waters, corresponding to a BV of 3 mL. To enable comparison between these two experimental set-ups the volume of water treated through the columns was expressed in BV units (calculated as the volume of solution treated divided by the volume of sorbent used). The model mine water used was a mixture of five different salts (ZnSO4•7H2O, CuSO4•5H2O, MnSO4•H2O, CoSO4•7H2O and NiSO4•7H2O) with a concentration of 2 mmol.L-1 each. The initial pH of water was 4.1 and the final pH (after sorption) was about 4.2. The flow rate (Q) was set to 130 mL.h-1. The second column set up involved a closed-mine water A with an initial composition as follows: [Ca]=216.6 mg.L-1, [K]=19.8 mg.L-1, [Mg]=86.4 mg.L-1, [Al]=8.32 mg.L-1, [Cd]= 0.046 mg.L-1, [Cu] = 4.66 mg.L-1, [Mn] = 5.83 mg.L-1, [Ni] = 0.165 mg.L-1 and [Zn] = 14.44 mg.L-1. Two flow rates were tested, Q = 130 mL.h-1 and Q = 65 mL.h-1, in order to evaluate the contact time conditions suitable for this water. Between each sorption cycle, desorption experiments were performed with 0.05 M HNO3 and the sorbent was converted to its sodium form using 0.05 M Na2CO3 (for 1.5 h). It was then rinsed with deionized water until the pH at the outlet and the inlet of the column was in the same range (pH 5.5-6.0). A closed-mine water sample B containing larger amounts of Ca2+ (ca. 900-1000 mg.L-1) and Mg2+ (ca.7.7mg.L-1) ions and low amounts of transition metal ions was also tested on the Na-TiP1 column.


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The metal ion concentrations in all waters used in the column experiments were determined with an Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). COLUMN DATA ANALYSIS AND MODELING The sorption data for each metal ion present in the multicomponent water systems were analyzed as described below. The column capacity qsorbed,i (mmol) represents the number of moles of the metal ion, i, that is retained by the column. It is calculated using the equation below: 𝑡=𝑡

𝑞𝑠𝑜𝑟𝑏𝑒𝑑,𝑖 = 𝑄 ∫𝑡=0 𝑒𝑥ℎ (𝐶0,𝑖 − 𝐶𝑡,𝑖 ) 𝑑𝑡

(3)

With Q the flow rate (L.h-1), C0,i the initial concentration of the metal ion i in the feed (mmol.L-1), Ct,i the concentration of the metal ion i at the outlet of the column at time t (mmol.L-1) and texh the time at the exhaustion point. The ion-exchange capacity (IEC) of TiP1, expressed in meq.g-1, is calculated using the following equation: 𝐼𝐸𝐶 =

∑𝑖 𝑞𝑠𝑜𝑟𝑏𝑒𝑑,𝑖 ∗𝑧𝑖

(4)

𝑚

With zi being the charge of the metal ion and m the mass of TiP1 used in the column experiments. The breakthrough profiles for the lower flow rate (Q = 65 mL.h-1) of sorption of the closed-mine water on TiP1 were modeled using the Thomas approach. The Thomas model was developed on the assumptions that the axial and radial dispersion in the column are negligible and that the rate driving force is governed by second order reversible kinetics. It is derived from the equation of mass conservation in a flowing system 29. It can be expressed as follows: 𝐶𝑡,𝑖 𝐶0,𝑖

=

1

(5)

𝑘 𝑞 𝑚 exp( 𝑇ℎ 𝑇ℎ − 𝑘𝑇ℎ 𝐶0 𝑡)+1 𝑄

Where kTh is the Thomas rate constant in L.mmol-1.h-1 and qTh the ion-exchange capacity of the sorbent in mmol.g-1. The Thomas model parameters were determined using the non-linear least square approach to fit a sigmoid function. Python's scipy.optimize.curve_fit function was used in this process 30. The Thomas model could not be applied to the breakthrough curves for the higher flow rate (Q = 130 mL.h-1), as the sorption equilibrium could not be reached under these column conditions.

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Column sorption experiments with a model mine water solution Na⁺

[Na+]

0

-1 (mmol.L )

5

Cu²⁺

Time, h 10 15 20

25

30

C/C0

20

1.0

15 10

0.5

(a)

5

C/C0 0

5

0.0 100 200 300 400 500 Volume of treated water, BV

600

Ni²⁺

Time, h 10 15 20

Zn²⁺

25

Co²⁺

30

1.2 0.8

(b)

0.4

0 0

Mn²⁺

Thomas model

0.0 0

100 200 300 400 500 Volume of treated water, BV

600

Figure 1. Breakthrough curves for sorption of a model mine water solution of Co2+, Cu2+, Mn2+, Ni2+, Zn2+ ions on Na-TiP1. Flow rate 130 mL.h-1. C is the metal ion concentration at time t and C0 is the initial concentration. The dashed lines are guidence for the eye. Curve fitting following the Thomas model is shown for Cu2+ ions (solid line). Figure 1 displays the breakthrough curves for the sorption of five metal ions present in the model mine water solution, on Na-TiP1. In order to better describe the ion-exchange processes, the data are separated into two sub-figures: the release of sodium ions in solution (in mmol.L-1) is shown in Figure 1a together with the corresponding exchanged amount of Cu2+ ions, while the sorption behavior of other transition metal ions is depicted in Figure 1b. The release of sodium ions in water during ion-exchange occurs as an inverted S-shaped curve. At the beginning of the experiment, the sodium ions are exchanged with the divalent metal ions and released into the water, which is confirmed by the high concentration of Na+ ions determined at the column outlet. At the end of the experiment, when all exchange sites are nearly saturated, no more sodium ions can be exchanged and the Na+ concentration at the outlet is very low 31,32. On the other hand, the sorption behavior of copper ions follows a very well–shaped sigmoidal curve which is characteristic of breakthrough. The data are well-simulated with the Thomas model pointing towards the presence of solely second-order kinetics processes. According to the simulations, the sorption capacity qTh is equal to 1.057 mmol.g-1, which is very close to the experimental value obtained (qsorbed = 1.163 mmol.g-1). All simulations together with Thomas model parameters are shown in the Supporting Information (Figure S1 and Table S1). The secondorder governed kinetics sorption processes have been associated with the sorption of metal ions on Na-TiP1 in static mode 22 and it is reassuring to see that similar behavior could be suggested for the sorption performances of this sorbent in dynamic mode. The sorption profiles of four other divalent metal ions, Ni2+, Co2+, Mn2+ and Zn2+, can be described as rather irregular S-shaped curves (sigmoidal functions). In the initial stage of the experiment, the concentration of individual metal ions in water is very low as sorption/exchange processes are taking place in the Na-TiP1 column (C/C0 ~ 0). At a certain point (the breakthrough point), the 8 ACS Paragon Plus Environment

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concentration of each metal ion starts to increase as the number of ion-exchange sites on Na-TiP1 surface decreases, until it reaches satruration (the exhaustion point). At the end of sorption/exchange processes, the concentrations of metal ions at the column inlet and outlet are nearly identical (C/C0 ~ 1). A closer look at the data (and their simulations) shows that before reaching the exhaustion point for each metal ion, a leap in the sorption curve (C/C0 > 1) is observed, which indicates subnormal sorption and/or multiprocess phenomenon. Hence, the probability of displacement of a heavy metal ion upon loading of other metal ions is considered. The fact that the C/C0 ratio continues rising above 1 and that substantial differences in the BV parameters are observed (the ratio first increases for Ni2+ and Co2+ ions followed by Mn2+ and finally for Zn2+ ions) point towards possible displacement of these ions taking place simultaneously with further sorption of Cu2+ ions. In other words, a competitive sorption phenomenon is observed in favor of copper ions, most likely due to a somewhat stronger affinity towards these ions. During the first phase (up to 162 BV), zinc and copper ions are displacing cobalt, nickel and managanese ions already bound to the sorbent due to the higher affinity of TiP1 for Zn2+ and Cu2+ ions. Via the second phase (until the exhaustion point for copper ions), Zn2+ ions which have previously sorbed onto TiP1 are displaced by Cu2+ ions due to a higher affinity for the latter. The most irregular Scurve is seen for nickel ions, while the closest to regular S-shape belongs to zinc ions (Ni2+ , Co2+ , Mn2+ , Zn2+). The order in which metal ions are displaced shows the affinity of the Na-TiP1 surface to these divalent ions. The TiP1 sorbent shows the highest affinity to copper ions and least preference for nickel ions. It is worth pointing out that even for nickel ions, the initial slope curve is very steep, which indicates a favorable sorption process under these experimental conditions. The same order of affinity of Na-TiP1 towards these transition metal ions was determined in closed-batch experiments earlier 22. Futhermore, in Figure 1, the volume of water treated before the breakthrough point is considerably larger in the case of copper ions (253 BV) than for zinc ions (108 BV) or managanese, nickel and cobalt ions (81 BV). The decrease of Na+ ion concentrations in the water coincides well with the breakthrough point (81 BV) of Ni2+, Co2+ and Mn2+ indicating that the number of sodium exchange sites decreases. After full breakthrough (around 108 BV), the concentrations of Ni2+ and Co2+ ions at the outlet exceed the initial feed concentration by about 40% and up to 15% for Mn2+ ions. The breakthrough point of Zn2+ ions is observed at 108 BV. After full breakthrough of zinc ions (at 180 BV), the concentration at the outlet exceeded by ca. 20% the initial concentration in the feed solution, while Cu2+ ions continue to be retained by the column for an additional 73 BV (before its breakthrough point at 253 BV). It appears that the total ion-exchange capacity of sorbent (IEC) increases or is kept in the same range, while the capacity towards individual metal ions (qsorbed) varies due to competition for available sorption sites. As the column loading proceeds it cannot be expected that metal ions would be removed from the water to the same extent, even if they are of equal concentrations. However, it can be suggested that during dynamic sorption of equal higher amounts of metal ions, more- or less-active sites would become involved in the sorption processes and this would result in sorption leaps (above 1) of different degrees. For the particular case of TiP1, functional sites such as TiO(OH)Na2PO4, TiO(OH)NaHPO4 and TiO(OH)H2PO4 can be 9 ACS Paragon Plus Environment


considered, with the latter being the least active. In other words, the affinity of the sorbent for heavy metal ions shall be considered together with other factors (concentration, ionic strength, active sites) in order to explain the column sorption profile observed. The fact that the sorption leaps mirror the selectivity series of metal ions on TiP1 sorbent unambiguously shows that the affinity of this sorbent could be considered as the main factor that determines the column sorption profile. When the concentrations at the outlet reaches the feed concentrations (C/C0 ~ 1), the column feed is in equilibrium with all five metal ions sorbed on the surface of TiP1. The feed of 575 BV results in a full loading of sorbent (4.2 meq.g-1) and complete breakthrough of the five divalent metal ions. It is worth noticing here that at ca. 250 BV, the column is entirely exchanged with copper ions. Under such conditions, if the targeted metal is copper, the column service time should be prolonged and a pure Cu2+-loaded column could be sustained. This can be further combined with desorption cycle and a neat copper concentrate isolated. Ion-exchange capacity (IEC) In order to further evaluate the affinity and selectivity ratios during the sorption performances of TiP1 in a continuous flow fixed-bed column set-up, the data have been compared to those in static batch mode (see Figure 2). Metal ion uptake, mmol.g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

1.2 Batch (without Ca²⁺) Column (without Ca²⁺)

0.8

Batch (with Ca²⁺)

0.4

0.0 Co²⁺

Cu²⁺

Mn²⁺

Ni²⁺

Zn²⁺

Ca²⁺

Metal ions Figure 2. Total sorption capacity of Na-TiP1 towards divalent ions (Mn2+, Co2+, Ni2+, Cu2+, Zn2+ and Ca2+). Sorption in batch and fixed-bed column modes with the corresponding standard deviation.


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Table 2. Selected selectivity ratios for sorption experiments with/without calcium ions under batch and column modes (corresponding to Figure 2). Batch (without Ca2+) Column (without Ca2+) Batch (with Ca2+) Ion couples Selectivity ratios Selectivity ratios Selectivity ratios Cu/Mn 6.21 5.59 9.20 Cu/Co 5.91 6.03 5.98 Cu/Ni 7.14 5.01 9.13 Cu/Zn 3.04 3.86 2.99 Cu/Ca n.a n.a 8.91 Zn/Co Zn/Mn Zn/Ni Zn/Ca n.a not applicable

5.90 4.09 6.23 n.a

1.45 1.56 1.30 n.a

3.06 2.00 3.05 2.98

The comparison of total metal uptake of Na-TiP1 towards a mixture of divalent metal ions (used as a model mine water) in static (batch) mode and in continuous flow (fixed-bed column) mode with or without the presence of calcium ions is displayed in Figure 2. The selectivity based on the metal uptake ratios obtained under the conditions of equal starting concentration and fixed equilibrium time are displayed in Table 2. The ratios calculated for the three experiments show a similar trend for the selected metal ion pairs. It is clearly revealed that the selectivity is maintained with or without the presence of calcium ions and during the dynamic experiment and that copper ions have the highest affinity to the Na-TiP1 surface. The affinity of Na-TiP1 in batch mode towards copper, zinc and manganese ions is not considerably altered by the presence of calcium ions and is only slightly lowered for the cobalt and nickel ions. Hence the affinity of TiP1 sorbent can be expressed as: Cu2+ >> Zn2+ > Mn2+ > Co2+, Ni2+, Ca2 +. This data supports the fact that the affinity of Na-TiP1 towards transition metal ions would remain in a similar order in the cases of mine waters that have undergone lime treatment. The ion-exchange capacities (IEC) in batch mode for the model waters prepared with and without calcium ions were estimated to be 3.7 and 3.5 meq.g-1, respectively. Similarly, the IEC of the column design is calculated to be 4.2 meq.g-1, which is also in relatively good agreement with the obtained data for TiP1 in batch mode, where the column experiment shows slightly higher IEC. Although batch and column modes are not entirely comparable due to saturation differences, both experiments are needed in order to evaluate sorbent performances. The batch mode is preferred when pollutants are strongly sorbed and chemistry of sorption is sought (as is the case for divalent metal ions on TiP sorbents). The column experiments ensure one-step-closer to real field view but they are often governed by complex physical/chemical processes and are time- and resource-consuming. Figure 2 also shows the amount of metal ions retained by the TiP1 ion11 ACS Paragon Plus Environment


Page 12 of 23

exchanger under continuous flow conditions for model mine water without Ca2+ ions present. It can be seen that the selectivity order is very similar to the one observed for the batch experiments. It also correlates well with the sequence of breakthrough points observed in Figure 1 where cobalt, nickel and manganese ions were the first to break through followed by zinc and finally copper ions. Similar data for granulated TiP-Na sorbent were obtained for batch and column conditions in the presence of Ca2+ ions when sorption of divalent cations (Pb2+, Cu2+, Ni2+, Mn2+, Cd2+) was studied. It was reported that all TiP materials exhibited a high affinity for both copper and lead ions and that they were able to remove these ions quantitatively in excess of Ca2+, Mg2+ and Na+ ions 16. These results also strongly suggest that the column sorption profile of Na-TiP1 towards metal ions in terms of both sorption capacity and selectivity series/affinity can be predicted based on the data obtained in batch mode. Another important point to highlight is the short residence time, estimated to be 4 min. for the given column conditions, which indicates that the ion-exchange processes in TiP1 matrices are relatively fast. It relates well with the kinetics data for single heavy metal ions exchanged in batch mode on TiP1, where an average contact time of 10 min. was found for an initial concentration of 2.5 mM 22. Column sorption experiments with closed-mine water Time, h

Time, h

[Na+] -1 (mmol.L )

0

5

10

15 Na Cd

12.0

20

25

Al Cu

8.0

(a)

4.0 0.0 0

200 400 600 Volume of treated water, BV

30

C/C0

C/C0

1.2

1.2

0.8

0.8

0.4

0.4

0.0

0.0

0

5

10

(b)

0

15

20

25

30

Ca Mn Ni Zn Mg

200 400 600 Volume of treated water, BV

Figure 3. Breakthrough curves for sorption of a closed-mine water solution containing various ions on Na-TiP1. Flow rate 65 mL.h-1. The dashed lines are guidance for the eye.

Figure 3 displays the sorption breakthrough curves of a closed-mine water solution that has undergone lime treatment, on Na-TiP1. The water contains large amounts of magnesium and calcium ions (86-216 mg.L-1) and a number of heavy metal ions present at different smaller amounts (ca. Zn-14, Cu-5, Mn-6, Ni-0.2 mg.L-1). A flow rate of 65 mL.h-1 was found to be more 12 ACS Paragon Plus Environment

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suitable for depicting the individual metal ion sorption on TiP1. Analogous to the model mine water case, data are divided into two sub-figures. Figure 3a shows the release of exchanged sodium ions and the breakthrough curves for copper, aluminum and cadmium ions, while Figure 3b displays the sorption behavior of the other heavy metal ions. In all cases, S-type curves can be seen with different degrees of irregularity. Curve fitting of the experimental data points using the Thomas model are shown in Figure 4 and the parameters are listed in Table 3. C/C0

C/C0 Al³⁺

1.0

1.0 Ca²⁺

0.5

0.5

0.0

0.0 0

10 20 Time, hours

0

30

C/C0

10 20 Time, hours

30

C/C0 Cd²⁺

1.0

Cu²⁺

1.0

0.5

0.5

0.0

0.0 0

10 20 Time, hours

30

0

C/C0

C/C0

1.0

1.0

0.5

10 20 Time, hours

30

Mn²⁺

0.5

Mg

0.0

0.0 0

10 20 Time, hours

0

30

C/C0

C/C0

1.0

1.0 Ni²⁺

0.5

10 20 Time, hours

30

Zn²⁺

0.5 0.0

0.0 0

10 20 Time, hours

30

0

10 20 Time, hours

30

Figure 4. Curve fitting using closed-mine water sorption data on TiP1. The Thomas model is used (solid lines).



Page 14 of 23

Differences in sorption data profiles are the result of competitive sorption that occurs on TiP1 surface. Again, a similar selectivity series of metal ions can be suggested: Mg2+ < Ca2+ < Ni2+ < Mn2+ < Zn2+ < Cu2+ {< Cd2+, Al3+} The latter two metal ions in the order should be considered separately due to physical/chemical reasons. Under the column pH conditions the aluminum ions are known to form the following species Al(OH)2+ and Al(OH)2+ (pH ca. 4.5 -5.5) hence the observed somewhat higher affinity to aluminum ions 33,34. For the cadmium ions, one possible explanation could be related to the size of the d10-cadmium ions being bigger than the zinc ions, which ensures easier formation of Cd(H2O)62+ and faster exchange of coordinated water molecules 35. It is noticeable that no substantial metal ion sorption leaps are observed in the case of closed-mine water, while the affinity order of metal ions is kept similar to the one established for the model water system. A sorption leap would mean a concentration outbreak inside the column that did not occur in this case. Only ion exchange of Mg2+ ions slightly crosses above 1. This is most likely due to a concentration difference, i.e., water specificity, resulting in different amounts of total ions present/ionic strength. This concentration difference shall be considered together with the established affinity of the sorbent. TiP1 showed the least preference for Mg2+ ions and hence these were the first to be replaced by other divalent ions. The total amount of transition metal ions was not high enough to produce a high concentration tide that would result in a sorption leap above 1. Similar data were reported for the sorption of Cu2+, Zn2+ and Cd2+ by S. fluitans biomass pre-loaded with Ca2+ where no concentration jump was observed if a smaller amount of Zn2+ was loaded in the column, even if the bio-mass showed high affinity to it and a high sorption leap at high zinc ion loading 36. In relation to functional site availability, it can be speculated that the Na-TiP1 becomes partially converted into Mg-TiP1 and/or Ca-TiP1 as adsorption proceeds. Hence, a more complex ionexchange process is taking place, i.e., the sorption simultaneously takes place on Na-TiP1 and on Ca/Mg- TiP1 sites and all sites are active in concentration ranges that are proportional to the amount of other metals added to the column. Nevertheless, Na-TiP1 displays high selectivity towards metal ions although relatively high amounts of calcium and magnesium ions are present in the water. In this case, the residence time of water in the column is estimated to be 3 min, which suggests that a contact time around 5-10 min might be more suitable to achieve optimal treatment of this water. Similar data were reported earlier for the sorption on Na-TiP1 in batch mode where a 5- to 10-min contact time was needed for low concentrations of individual metal ions to exchange 22. Data showing the TiP1 sorption performance of the same closed-mine water, in dynamic mode with a higher flow rate are shown in the supplementary materials (Figure S2). Although the higher flow rate means faster removal of metal ions and is economically preferred, it is not favorable in this case as the amount of metal ions in the fixed-bed column decreased due to the contact time between the water and sorbent being too short for sorption equilibrium to take place 37–39. The data at a high flow rate also reveal that the affinity trend is preserved along with the preferential sorption of metal ions on the TiP1 surface. 14 ACS Paragon Plus Environment

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Table 3. Thomas model parameters (kTh Thomas rate constant and qTh ion-exchange capacity of TiP1) with the coefficient of determination and qsorbed, the amount of metal sorbed on TiP1.

Thomas paramters kTh (L.mmol-1.h-1) qTh (mmol.g-1) qsorbed (mmol.g-1) R2

Cu

Zn

Mn

Ni

Cd

Al

Ca

Mg

4.468 0.060 0.057 0.983

3.619 0.142 0.132 0.992

10.078 0.065 0.052 0.965

135.193 0.0007 0.0001 0.960

862.856 0.0003 0.0003 0.996

1.386 0.276 0.248 0.997

0.149 0.896 0.714 0.982

0.399 0.508 0.329 0.994

qtotal (mmol.g-1)

1.56 1.53

The coefficients of determination (R2) for each metal ion present in the closed-mine water are all high and close to 1, which shows that the Thomas approach can be used to accurately describe the sorption behavior of a multicomponent system on TiP1 ion-exchanger. The amounts of metal ions sorbed on TiP1 determined from the Thomas model (qTh) are in very good agreement with the ones calculated from the experimental data. Similar observations were reported for another TiP system, where the Thomas model was used to describe the sorption of nickel ions 23. The breakthrough curves of model mine water could not be fitted accurately using the Thomas approach, (with the exception of copper), due to the competitive sorption phenomenon that is strongly pronounced when all metal ions are present in equal amounts and/or the sorption equilibrium is maintained. Besides, the Thomas constants’ errors were assessed by the Monte Carlo method using one thousand random simulations. The corresponding 2D graphs illustrating the correlation between the two constants’ errors can be found in the Supporting Information (Figure S5). Both parameters (kThand qTh) are dependent on each other and it is therefore necessary to show their relationship and error dependence by plotting 2D graphs. No error data related to the Thomas parameters have been published for TiP systems and almost none can be found showing the error dependence for other systems.



Page 16 of 23

Table 4. Volume of water treated in relation to European regulations for drinking water. Volume of water treated (BV units)

Model mine water (Q = 130 mL.h-1)

Cu

Zn

Mn

Ni

Co

Cd

Al

Ca

Mg

IEC (meq.g-1)

253

108

81

81

81

n.a

n.a

n.a

n.a

4.2

Closed-mine water Cycle 1 (Q = 130 mL.h-1) 283 64 ≤42 ≤42 n.a 192 ≤42 85 64 2.3 -1 143 Cycle 2 (Q = 65 mL.h ) 369 208 22 22 n.a 283 80 51 3.2 Drinking water level 2.0a 1.5b 0.05a 0.02a 0.005a 0.2a 100b 50b n.a regulations (mg.L-1) a Data collected from European directive 98/83/EC. b Data collected from European directive 80/778/EEC, the value for calcium designates the advisable level. n.a: not applicable. A short summary of the sorption data can be found in Table 4. The volumes of water treated for each metal ion that follow the European Regulations together with the ion-exchange capacity (IEC) are gathered in Table 4. For example, in the case of copper ions in model mine water, about 1.8 L water (253 BV x 7 mL.BV-1) for 4.5 g of TiP1 sorbent could be succesfully treated in order to obtain water that contains copper ions below the limits set by the EU drinking water regulations (2 mg.L-1). It can be seen that in general the ion-exchanger is selective towards copper, cadmium and zinc ions, in the presence of relatively high concentrations of calcium and magnesium ions. The ion-exchange capacity of TiP1 in the mine water treatment increases from about 2.3 meq.g-1 to 3.2 meq.g-1 for Q = 130 mL.h-1 and Q = 65 mL.h-1, respectively, due to the longer contact time. The ion-exchange capacity in cycle 2 for the mine water (3.2 meq.g-1) is in good agreement with previously reported data for TiP1 in batch mode for the same water (3.1 meq.g-1) 22. The volume of water treated below the limits of the European regulations is the highest for copper (277 BV) followed by cadmium (219 BV), zinc (156 BV) and alumium (85 BV) ions, verifying the higher affinity of TiP1 towards these metal ions. In addition, it should also be mentioned that due to the chemical exchange of sodium ions with calcium and magnesium ions, the Na-TiP1 sorbent could be used (in a parallel system) for simultaneous water softening (an example of this is shown in the Figure S3). The resulting CaTiP1 and/or Mg-TiP1 columns could also be used in another system as waste water sorbents as reported in the past 16. Exploratory column desorption experiments with 0.05M HNO3 The sorbent regeneration is equally important as its exchange properties. The following desorption experiments and sorbent recovery using Na2CO3 solution are used to illustrate the possibilities that 16 ACS Paragon Plus Environment

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utilization of TiP1 opens. A comprehensive study on the subject is underway and the data will be published elsewhere. C

C (mmol.L-1)

(mmol.L-1)

20.0

0.60 Ca

10.0

Mn

0.40 0.20

0.0 0

20

40

60

80

100

0.00 0

Volume of HNO3, BV C (mmol.L-1)

20 40 60 80 Volume of HNO3, BV

100

C

0.0

0.5

1.0

4.0

1.5 Cu Zn

2.0 Mg Al

2.0

(mmol.L-1)

0 .015 Cd Ni

0 .010 0 .005

0.0

0 .000 0

20

40

60

80

100

0

Volume of HNO3, BV

20 40 60 80 Volume of HNO3, BV

100

Figure 5. Desorption curves (after sorption of the closed-mine water on Na-TiP1) using 0.05 M HNO3. Desorption experiments were performed with diluted HNO3 (0.05 M). The acid concentration used during the desorption experiments is considerably lower than the one used for regeneration of zirconium phosphate sorbent 40,41. A rather dilute concentration was chosen to minimize the presence of nitrite ions in circulating waters and to keep the possibility of metal complexation/chelation with other anions to a minimum. The length of time for the desorption experiment is related to the concentration of acid. A high acid concentration ensures a faster desorption process. However, usage of a higher acid concentration even if it does not cause any sorbent dissolution or structural changes results also in more acidic water streams. Furthermore, if a higher concentration of acid is used, it would in fact slow down the overall process of the desorption-regeneration-new-sorption cycle as a more extensive water washing step between the desorption and Na2CO3 regeneration of column would be needed. A balance between all these parameters needs to be kept and a possible combination of these factors is demonstrated below. The data are obtained after a cycle with closed-mine water and the results are shown in Figure 5. Due to the different concentration ranges for the divalent metal ions desorbed, all data are separated into four different plots. The desorption curves have a similar profile for all cases, where the desorbed individual metal ion concentration in the acidic solutions decreases gradually (reversed S-curves). All ions are desorbed simultaneously at the beginning of the process (up to 25 BV), while at 40 BV practically all Ca2+ and Mg2+ ions are desorbed, while other divalent metals are still 17 ACS Paragon Plus Environment


Page 18 of 23

attached to the sorbent to a different degree (although considerably lower than the initial concentration). This indicates that more than one functional site was involved in the primary sorption process. The desorption volume for all the elements with the exception of aluminum ions is estimated to be 36 BV. This results in an acidic solution 4.5 to 8 times more concentrated in copper, cadmium and zinc ions than the initial mine water. For aluminum, the desorption volume is about 70 BV, which is higher than for the other metals. Another study has also shown that it is possible to use HCl as a desorption solution instead of HNO3 23. It is advisable to examine the possibilities of complexation between the acid and the metal to determine the most suitable desorption solution to utilize. A slower flow rate or a higher concentrated acidic solution could be used to bring down the desorption volume, but all shall be performed according to industrial-case specifics. Desorption can be a valuable step to concentrate the mine-water waste that could be further treated. To be able to use the TiP1 sorbent further, a complete regeneration under these column conditions could be reached in less than 1 h (as can be seen in Figure S4, Supplementary Materials). Our exploratory data on the TiP1 and the reported data for LTP (with active TiP1 part) show that the column efficiency can be affected after the 6th regeneration cycle and that it has been kept at about 70% for the single component system 23. CONCLUSIONS •

• • • • • •

The TiP1 ion-exchanger showed a very high affinity towards transition metal ions in the presence of high concentrations of calcium and magnesium ions in closed-mine water. The selectivity order can be expressed as: Model mine water: Cu2+>> Zn2+> Mn2+> Co2+, Ni2+, Ca2+ Closed-mine water: Cu2+{Cd2+, Al3+}> Zn2+> Mn2+> Ni2+, Ca2+, Mg2+ A competitive sorption phenomenon on the TiP1 surface was observed in fixed-bed column mode, revealing the highest affinity towards copper ions. The ion-exchange capacity of TiP1 in column mode (3.2 – 4.2 meq.g-1) was in the range of or higher than the one estimated for batch conditions (3.1 – 3.5 meq.g-1) for the same waters. Relatively fast kinetics of the ion-exchange process in column set-ups (a contact time of about 5 min.) were encountered for metal ions separation in aqueous solutions. Sorption curves of multi-metal ions systems on a TiP1 fixed-bed column were accurately described using the Thomas model. Regeneration of the sorbent is achieved using dilute nitric acid. The TiP1 fixed-bed column and batch data are in excellent agreement ensuring that for new types of waters the column design could be based on the batch experimental results.

ASSOCIATED CONTENT *S Supporting Information


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Breakthrough curves from the model mine water fitted to the Thomas model for Co2+, Mn2+, Ni2+ and Zn2+ ions (Figure S1); Thomas model parameters obtained for the model mine water fittings (Table S1); sorption breakthrough profiles of closed mine water A on Na-TiP1 with a flow rate of 130 mL.h-1 (Figure S2); sorption breakthrough profiles of closed mine water B on Na-TiP1; 2D error graphs of the Thomas model constants obtained in Table 3 (Figure S4); TiP1 regeneration with sodium carbonate solution in column set-up (Figure S5). ACKNOWLEDGMENT The Swedish Research Council Formas and Boliden Mineral AB are acknowledged for their financial support of this work. The authors also thank the LTU-based Centre of Advanced Mining and Metallurgy (CAMM2) for their financial contribution. The foundation in memory of J.C. Kempe and S.M. Kempe are acknowledged for grants used to upgrade the NMR spectrometer to a Bruker Ascend Aeon WB 400 spectrometer. The Wallenberg Foundation and Åforsk Foundation are also acknowledged for financial support. REFERENCES (1)

(SGU), G. S. of S. Swedish ore mines https://www.sgu.se/en/mineral-resources/swedishore-mines/.

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Gardea-Torresdey, J. L.; Tiemann, K. J.; Gonzalez, J. H.; Henning, J. A.; Townsend, M. S. Calcium and Magnesium Interference Studies for the Binding of Heavy Metal Ions in Solution by Medicago Sativa (Alfalfa); Kansas State Univ., Manhattan, KS (United States): United States, 1996.

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Graphic Abstract

A know-how cycle for heavy metals removal from mine waters is disclosed via titanium phosphate exchanger’s performance in dynamic mode.

ACS Paragon Plus Environment

Competitive sorption of metal ions on titanium phosphate sorbent

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