Polymer-based Retention Technique Research Articles

Removal of lithium ions from aqueous solutions by an ultrafiltration membrane coupled to soluble functional polymer

In this research, the separation and concentration of lithium ions (Li+) from the aqueous solution was studied by the liquid-phase polymer-based retention technique (LPR) using poly(sodium styrene sulfonate) (PSSNa), poly(potassium sulfopropylacrylate) (PSPAK) and poly(sodium acrylate) (PAS). These polymers were characterized by Fourier transform spectroscopy and thermogravimetric analysis, corroborating their correct formation. In addition, the ability to remove Li+ from water was studied, as a function of the amount of polymer, the pH of the solution and the initial concentration of Li+. The study of isotherms and the removal of Li+ in the presence of interferents was also carried out. Finally, the reuse of the polymer was evaluated in consecutive adsorption–desorption cycles.Based on the obtained results, it was determined that a polymer: metal molar ratio of 20: 1 at pH 5.7, was the most suitable for use in the LPR process. A Li+ removal efficiency (% R) of more than 80% was obtained. Among the polymers studied, PSPAK is the water-soluble polymer with the highest % R of Li+ and it achieves adequate permeation flux during LPR experiments (43.2 L h-1m−2). The experimental data obtained from the removal of Li+ were better adjusted to the Freundlich isothermal model. On the other hand, using 1:20Li+:interfering ion molar ratio, the Li+ removal efficiency of the LPR process decreases to 40% for Na+ or K+ and decreased to 20% for Mg2+ interfering ions. The polymer can be reused up to the fourth consecutive cycle, reaching a removal efficiency of 46%.

BIOPOLYMERS APPLIED TO REMOVE METAL IONS THROUGH ULTRAFILTRATION. A REVIEW

This manuscript is an overview of the versatile biopolymer materials alginic acid, carboxymethyl cellulose, and quaternized chitosan which contain different functional groups to remove hazardous inorganic species. These bioadsorbents are completely water-soluble and have a high versatility to achieve a higher sorption capacity and efficiency under different experimental conditions in combination with ultrafiltration membranes through the liquid-phase polymer-based retention (LPR) technique. In general, thesebioadsorbents are friendly with the environment and have a higher biodegradability compared with those more employed synthetic polymers.

N-Alkylated chitosan coupled to the liquid-phase polymer-based retention (LPR) technique to remove arsenic (V) from aqueous systems

Water-soluble polymer based on alkylated chitosan with a quaternary ammonium group (Ch-QAG) was prepared, characterized, and applied to remove arsenate ions from aqueous solution by LPR technique. The arsenic removal was performed by the washing method (WM) and enrichment method (EM). Through the WM, studies of the pH and variation in the concentrations of interferents and arsenate ions were carried out. The effect of the removal of arsenate ions in simulated water was determined from the Camarones River in northern Chile. Ch-QAG showed high affinity for binding arsenate species (99% of removal) at pH 11.0 at a molar ratio of 20:1 polymer: As(V). High selectivity was also observed in the presence of interfering ions such as Cl−, SO42-, and PO43-, resulting in a removal rate over 80% at percentages over 95% for a concentration of 100 mg L-1 of As (V). The maximum retention capacity obtained was 112, 105, and 98 mg g-1 for three load cycles. The retention percentage for simulated water was 46.3% at a concentration of 1300 μ g L-1. In conclusion, the results presented in this study show that using Ch-QAG with ultrafiltration membranes is a great alternative to remove As (V) at high removal rates.

Removal of chromium ions by functional polymers in conjunction with ultrafiltration membranes

Abstract In the current research water-soluble functional polymers (WSFP) were prepared via radical polymerization and purified by fractionation through ultrafiltration membranes with different molecular weights cut off (MWCO) of 30 and 100 kDa. The WSFPs were poly(3-acrylamide propyl) trimethyl ammonium chloride, P(ClAPTA), poly(2-acrylamido-2-methyl-1-propane sodium sulfonate, P(AMPSNa), and poly(3-methacrylamino propyl) dimethyl 3-sulfopropyl ammonium hydroxide, P(HMPDSPA). These polymers were characterized by Fourier transformed infrared spectroscopy (FT-IR) and thermogravimetry analysis (TGA). Using liquid-phase polymer-based retention technique (LPR), chromium [Cr(III) and Cr(VI)] retention was studied as a function of pH, polymer and chromium concentration, selectivity, maximum retention capacity, chromium elution capacity, and polymer regeneration through sorption and desorption studies. Results of FT-IR showed the characteristic absorption bands of the synthesized polymers. The decomposition temperatures of P(ClAPTA) were at 303.1 °C, and for P(AMPSNa) three decompositions temperatures were registered at 190.5 °C, 223.2 °C, and 304.8 °C. P(HMPDSPA) presented two important decomposition temperatures at 292.4 °C and 391.7 °C, respectively. Concerning to the retention of Cr(VI), it was maximal (100 %) when P(ClAPTA) was studied at pH 6. The maximum retention of Cr(III) (100 %) was achieved by P(AMPSNa) at pH 3. The optimum polymer:Cr mole ratio obtained was 10:1 for both Cr(VI) and Cr(III). The retention of Cr(VI) decreased due to the presence of interfering ions, and the hydrodynamic flow was almost constant during the ultrafiltration of polymer-Cr macromolecule.

Tetracycline removal by polyelectrolyte copolymers in conjunction with ultrafiltration membranes through liquid-phase polymer-based retention

In this study, we used a liquid-phase polymer-based retention technique assisted by polyelectrolyte copolymers containing quaternary ammonium and sulfonate groups that are capable of removing the antibiotic tetracycline (TC) through electrostatic interactions. The polymers were synthesized using zwitterionic, anionic, and cationic monomers with the aim of obtaining copolymers with different charge balances at the ratios of 1:1, 2:1, and 1:2 (negative: positive). The parameters investigated for each copolymer included the pH, ionic strength, concentration of polymer, maximum retention capacity, and sorption–elution process at pH 11.0 and 3.0. The copolymers with a charge ratio of 1:2 achieved the highest retention (80.0%) at alkaline pH, while the copolymers with charge ratios of 2:1 and 1:1 exhibited the maximum retention (72.0%) at acidic pH. Based on these results, the pH and charge of the polyelectrolyte copolymers play important roles in the TC removal processes. Additionally, the maximum retention capacity (MRC) recorded was 731.2, 176.8, and 214.8 mg TC/g of copolymer in the first charge for the three copolymer polyelectrolytes, and the second charge of the MRC process did not improve compared with the first load.

Application of the liquid-phase polymer-based retention technique to the sorption of molybdenum(VI) and vanadium(V)

This research is focused on molybdate and vanadate removal from aqueous solutions by the liquid-phase polymer-based retention (LPR) technique. In this technique, a cationic water-soluble polymer based on the ammonium quaternary group poly(diallyldimethylammonium chloride), PDDA, was used as a polymeric sorbent, and a regenerated cellulose ultrafiltration membrane was used as a filter. The removal of molybdenum and vanadium using the LPR technique was analyzed by both the washing and enrichment methods. The removal experiments using the washing method were conducted by varying the pH, polymer/metal molar ratio, concentration of metal ions, pressure and presence of interfering ions. In addition, the enrichment method was used to determine the maximum retention capacity of PDDA. The obtained results showed the highest retention capacity of molybdate (100%, using 30 mg/L in the feed) for PDDA at pH 3 and 9. The optimum molar ratio was 20:1 polymer/Mo(VI). The efficient vanadate retention (approximately 90%) was obtained at pH 6 with 10:1 polymer/V(V) molar ratio. The effect of metal concentration on the retention showed a higher retention of Mo(VI) independent of its concentrations. In the case of V(V), the maximum retention was reached when 60 mg/L of vanadium was used in the feed. The enrichment method showed the maximum retention capacities of 370 mg Mo(VI)/g PDDA and 520 mg V(V)/g PDDA. The presence of interfering ions did not affect the retention capacity of PDDA. These results showed that the combination of PDDA with ultrafiltration membranes could represent an alternative method for the removal of molybdate and vanadate from water.

Liquid-Phase Polymer-Based Retention: Theory, Modeling, and Application for the Removal of Pollutant Inorganic Ions

The aim of this work is to give the theoretical background of liquid-phase polymer-based retention (LPR) technique, provide a complete model of separation process, and evaluate its application for the removal of pollutant inorganic ions (PIIs) from aqueous solution. Thus, fundaments of LPR modeling by washing method from the ion distribution in active and passive zones are specified and its application was evaluated for cadmium removal using poly(sodium styrene sulfonate) as water-soluble polymer (WSP). LPR simulations were performed using a semiempirical description from data of concentration, filtration factorm, and retention coefficients. It is concluded that semiempirical simulation of PII retention by WSP can be performed from a small amount of experimental data using the described modeling. In addition, new approximations associated with molecular structure of polymer should be performed and linked to parameters of LPR experiments.

Microwave assisted synthesis of Jeffamine cored PAMAM dendrimers

This study reports the use of a new microwave assisted amidation method for the synthesis of Jeffamine cored poly (amidoamine) (PAMAM) dendrimers up to fourth generations. Reaction proceeding is monitored by attenuated total reflectance (ATR) without final purification. It was found that current method decreases the aminolysis time, and minimizes the excessive use of reagents and solvents. Synthesized dendrimers were purified by liquid-phase polymer-based retention (LPR) technique and characterized by 1H-NMR, 13C-NMR, EA, GPC and UV–VIS spectroscopy.

WATER-SOLUBLE CATIONIC CELLULOSE COUPLED TO A ULTRAFILTRATION MEMBRANE FOR THE REMOVAL OF ARSENIC AND CHROMIUM

This article describes the retention properties of arsenic (V) and chromium (VI) using a water-soluble polymer based on cationic hydroxyethylcellulose and combined with an ultrafiltration membrane. The polymer was purchased commercially and was characterized by Fourier transform infrared spectroscopy (FTIR). The study of polymer removal capacity of As(V) and Cr(VI) were performed by liquid-phase polymer-based retention technique (LPR). By means of a washing method studied retention as a function of pH, the amount of polymer, and the concentration of As(V) and Cr(VI). The results showed that the retention capacity of both oxy-anions increases with the solution pH. Due to the presence of quaternary ammonium groups in the polymer structure, the retention of As(V) and Cr(VI) was higher at a pH of 9 than that at acidic pH levels, reaching values of 65% for Cr(VI) and 43% for As(V). The study in function of amount of polymer showed that using 75 mg the retention of As(V) was 65%. In the case of Cr(VI) using 50 mg of polymer, 67% retention was obtained. Using low concentrations for arsenic and chromium, maximum retention was found at 65% and 95%, respectively. The maximum retention capacity (MRC) of the polymer was determined by adding As(V) and Cr(VI) to the polymer solution and concentrating in successive steps. FTIR spectra before and after of retention showed differences by the interaction with oxy-anions.

Removal of As(V) using liquid-phase polymer-based retention (LPR) technique with regenerated cellulose membrane as a filter

In this study, regenerated cellulose membrane was used as a filter in liquid-phase polymer-based retention technique. The poly(4-vinyl-1-methylpyridinium bromide), P(BrVMP), was used as extracting reagent of As(V). The role of pH, polymer:As(V) molar ratio, and influence of regenerated cellulose membrane were investigated by washing method. It was observed that the efficient retention was obtained at pH 9 with 20:1 polymer:As molar ratio and it was about 100 % at Z = 10 for P(BrVMP). Experimental data showed that the regenerated cellulose membrane, compared to poly(ethersulfone) membrane, has a capacity to interact with As(V). The maximum retention capacity of P(BrVMP) was determined by enrichment method, and then, using alternately washing and enrichment methods, the charge–discharge process and recovery of P(BrVMP) were performed.

Removal of arsenate from ionic mixture by anion exchanger water-soluble polymers combined with ultrafiltration membranes

This study shows the influence of Cl−, SO4 2−, NO3 −, SiO3 2−, Na+, and Ca2+ on arsenate removal by anion exchanger polymers using the liquid-phase polymer-based retention (LPR) technique. The LPR was carried out in the presence of anion exchanger soluble polymers containing quaternary ammonium salts. These polymers were characterized by NMR. Compared with As(V) removal from deionized water, the results showed that in the presence of ionic mixture, the As(V) removal capacity decreased. However, P(ClVBTA) showed As(V) removal ability of 91 % when the ionic mixture was used. Polymers with chloride exchanger groups showed a higher ability to remove arsenate than the polymer that contains methyl sulfate as anion exchanger group. At higher arsenate concentration (47.6 mg L−1), arsenate retention by the water-soluble polymers ranged between 58 and 91 %. This removal capacity increased gradually reaching 100 % retention when the arsenate concentration in the cell was minimum (5.5 mg L−1). The values of maximum retention capacity were 264 mg g−1 for P(ClMPTA), 260 mg g−1 for P(ClVBTA), and 200 mg g−1 for P(ClAPTA) at the total filtrate volume of 300 mL. The charge–discharge process found to be suitable for saturate the polymer with As(V) and then eluting As(V) for regenerating the extracting capacity of polymer.

Chelating water-soluble polymers associated with ultrafiltration membranes for metal ion removal

This article describes the retention properties of commercial chelating water-soluble polymers, for different metal ions in aqueous solution using a liquid-phase polymer-based retention (LPR) technique. The polymers studied were poly(ethyleneimine) or P(EI) (water-free and a 50 % aqueous solution) and poly(ethyleneimine epichlorohydrin) or P(EIE) (a 17 % aqueous solution). These commercial polymers were fractionated by ultrafiltration membranes and then characterized by Fourier-transformed infrared spectroscopy. The extraction process was performed using the following metal ions: Cu2+, Cd2+, Co2+, Ni2+, Zn2+, Pb2+ and Cr3+. In the washing studies, we varied the pH (3, 5 and 7) and retention time. The results showed that P(EI) showed high retention for all the metal ions at pH 7 and for selective retention of Cu2+ at pH 5, while P(EIE) showed selective retention of Cu2+ ions at pH 7. Using the enrichment method, the maximum retention capacity of Cu2+ and Cd2+ was achieved using a 50 % aqueous solution of P(EI) at pH 5 and 7, respectively. Finally, charge–discharge experiments for Cu2+ were analysed by changing the pH from basic to acidic over three cycles. These results showed that it is possible to remove metal ions and regenerate the removal capacity of the polychelatogens using the LPR technique.

Polymer-enhanced ultrafiltration: counterion distribution and its relation with the divalent metal-ion retention properties by sulfonic acid polyelectrolytes

The distribution of divalent counterions and its relation with the retention properties of sulfonic polyelectrolytes were analyzed, and the results were interpreted under the light of the counterion condensation theory using two-zone model and Donnan’s equilibria. In this regard, poly(vinyl sulfonic acid) and sodium poly(styrene sulfonate) solutions at pH 6.0 were diafiltered by the liquid-phase polymer-based retention technique. Metal-ion retention and distribution around polyelectrolyte chains were obtained from the diafiltration experiment as well as the maximum retention capacity (MRC) for each polymer. The MRC values obtained were 0.30, 0.15, 0.21, and 0.26 mmol of metal-ion per mmol of polymer for PVSA–Cu2+, PVSA–Cd2+, PSS–Cu2+ and PSS–Cd2+, respectively. The excess of ions added, the volume in the inside cell, and the order of addition of the ions contacted with the polymer were important factors defining the form of the retention profile.

Poly(l-lysine) as a polychelatogen to remove toxic metals using ultrafiltration and bactericide properties of poly(l-lysine)–Cu2+ complexes

Poly(l-lysine) is a water-soluble, synthetic polypeptide containing functional amine groups that help remove di- and trivalent metal ions from aqueous solutions. This polymer’s removal properties were studied under different experimental conditions: (1) competitive and non-competitive conditions; (2) different pHs; and (3) filtration factors. Under the conditions of the Liquid-Phase Polymer-Based Retention (LPR) technique, the cooper (II) ion interaction was found to be selective and efficient when compared to other divalent metal ions studied. However, interestingly, this selectivity disappeared when trivalent metal ions were present. The polymer–metal ion interactions are based on the amino groups of the side-chains as well as the polypeptide backbone chain. The removal of metal ions was strongly dependent on the pH. By structural characterization with FT-IR and EPR spectroscopy, participation of the amide and mainly amine groups was found to take place for the coordination. For the Cu2+, coordination through four amine nitrogen donor atoms in the primary coordination sphere was detected. Antibacterial activity tests were conducted with the poly(l-lysine)–Cu2+ complex and showed a higher activity in comparison with the precursors Cu2+ and poly(l-lysine) at the same concentrations for E. coli (6538P), a Gram-negative bacterium, and S. aureus (ATCC), a Gram-positive bacterium.

Water-soluble copolymers in conjunction with ultrafiltration membranes to remove arsenate ions

Water-soluble polymer poly[3-methacryloylamine)propyl)trimethyl ammonium chloride, P(ClMPTA) and the copolymer with 4-vinyl pyridine, poly[(3-methacryloylamine)propyl) trimethylammonium chloride-co-4-vinyl pyridine], P(ClMPTA-co-4VP) were synthesized by radical polymerization, at different feed mole ratios ClMPTA:4VP 1:1, 1:2, and 2:1. The copolymer compositions were determined by FT-IR and H-NMR spectroscopy and analyzed by TG-DSC. The liquid-phase polymer-based retention (LPR) technique was used to study the water-soluble polymers’ arsenic removal properties. The solution’s conductivity properties were evaluated at different pH. The copolymers can bind more selectively divalent anionic arsenic species from an aqueous solution (pH 8 ≥ pH 6 > pH 4). Assays for the mol ratio copolymer: As(V) 75:1, 37.5:1, 20:1, 10:1, and 5:1 at arsenic concentrations of 10 and 37.5 ppm were carried out. Apparently, the behavior of the copolymers with the solution’s pH was similar to pure cationic homopolymer; however, when the retention capacity was expressed as real mass of quaternary ammonium comonomer, the retention values were enhanced for lowest mol ratio 10:1 and 5:1. The retention capacity of exchanger with quaternary ammonium group was improved in presence of a weak base 4-vinyl pyridine comonomer, differently to the behavior showed by those copolymers of ClMPTA with acrylic acid groups as comonomer.

Swelling behavior and metal ion retention from aqueous solution of hydrogels based onN-1-vinyl-2-pyrrolidone andN-hydroxymethylacrylamide

Hydrophilic copolymer gels were prepared by free-radical copolymerization of 1-vinyl-2-pyrrolidone (NVP) and N-hydroxymethyl acrylamide (HMAm). The ratios of both monomers and crosslinker agent (CA) were varied, and the effect of external stimuli such as pH of the swelling media and inorganic ion interaction were investigated. The gels showed a pH-responsive behavior. The interactions of inorganic ions with the hydrophilic hydrogels were determined as a function of pH, copolymer composition, and variation of the composition crosslinking degree. Additionally, a decrease of the pH of the solution caused a decrease in the equilibrium degree of swelling of the copolymer gels. The capacity to remove several metal ions such as Cu(II), Cr(III), Co(II), Zn(II), Ni(II), Cd(II), and Fe(III) in the aqueous phase was determined by using the liquid-phase polymer-based retention (LPR) technique. In addition, the copolymers and their complexes with transition metal ions were characterized by the monomer reactivity ratios by using the Kelen-Tudos method, elemental analysis, FTIR and 1H-NMR spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. Based on the results of swelling and retention capacity of metal ions of poly(NVP-co-HMAm), it was concluded that the swelling and retention capacity properties of the hydrogels depends on the reaction environment's pH. This is regarded as a functional hydrogel, which could be employed as biodegradable materials for applications in agriculture chemicals and as drug delivery systems. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Study of polymer–metal ion–membrane interactions in liquid-phase polymer-based retention (LPR) by continuous diafiltration

In this work were studied polymer–metal ion–membrane interactions during continuous diafiltration of divalent metal ions (Co 2+, Ni 2+, Cu 2+, Zn 2+, Cd 2+, and Pb 2+) using poly(acrylic acid) (PAA) as water soluble polymer by liquid-phase polymer-based retention (LPR) technique at pH 6. Polyethersulfone (PES) membranes were used and characterized by different methods (solute rejection test, force atomic microscopy (AFM) and contact angle). Changes on fouled membrane roughness (22.91 ± 2.2) with respect to virgin membrane (7.05 ± 4.6) and changes on contact angle measures (76.0 ± 0.9 and 85.0 ± 2.34 with water) suggest an increasing in surface hydrophobicity of PES membrane associated mainly with deposited forming on the surface as results of enhanced in the polymer–membrane interaction throughout proceed filtration. From metal ion concentration data in the permeate throughout filtration experiment polymer–metal interactions were studied and analyzed by equations based on the combination of molar binding ratio and the consideration of chemical equilibrium between the polymer and metal ions. Membrane–metal ion interaction was around 10% for all cations studied, with the following order in the interactions: Ni 2+ ≈ Zn 2+ > Pb 2+ > Co 2+ > Cu 2+ > Cd 2+. A relatively high and specific interaction of the carboxylate groups of PAA with metal ions was observed. This interaction showed the order: Cu 2+ > Cd 2+ > Co 2+ > Ni 2+ ≈ Zn 2+.

Metal ions recovery with alginic acid coupled to ultrafiltration membrane

Alginic acid (AA) is a natural polysaccharide derived from brown algae. Naturally AA is present in cellular wall forming insoluble complexes with ions as calcium, magnesium, and sodium. This polymer is composed of uronic acids as d-manuronic acid and l-guloronic acid (units differing in C5 configuration) which are disposed in blocks or alternating on principal chain due its spatial configuration. In its structure only hydroxy and carboxylic acid are present, with a p K a alginic acid = 3.45. At pH = 4.3 this polymer is completely soluble in water. Metal ion retention was evaluated using liquid-phase polymer-based retention (LPR) technique elution method, and metal ions studied were Ag +, Co 2+, Ni 2+, Cu 2+, Zn 2+, Cd 2+, and Pb 2+ at different pH and filtration factor. A high efficiency for all metal ions at all pH was reveled with a maximum at pH = 4.5 of 100% of majority of metal ions. To evaluate the maximum retention capacity (MRC) of AA, LPR technique concentration method was used. Metal ion/polymer ratio from 48 to 325 mg/g for Zn 2+ and Ag + were studied, respectively. Homopolymer and polymer–metal ion complexes were characterized using FT-IR, Far-IR spectroscopy, dynamic light scattering (DLS), and thermogravimetric analysis. FT-IR revealed relevant shifts between AA and PMC, which involve carboxylic acid, hydroxy, and ether groups. DLS shows non-pH-dependent sizes of alginic acid–silver complexes.

Removal enrichment and recovery of Ni(II), Zn(II) and phosphate from phosphation rinsing waters with liquid-phase polymer-based retention technique

The removal enrichment and recovery of Ni(II), Zn(II) and phosphate with liquid-phase polymer-based retention (LPR) was studied on a laboratory level and on a pilot scale with real wastewater. Hyperbranched poly(ethylenimine) (HBPEI) was used, in favour, as complexing and ion exchange agent. Sorption behaviour and loading capacities of the polymer with respect to Ni(II), Zn(II), Mn(II) and phosphate were studied under various conditions in order to implement the system into a pilot unit for the treatment of industrial phosphation rinsing waters. The recovery of Ni(II) and Zn(II), accomplished in that pilot unit, constantly reached values over 99%. Starting from an initial metal concentration of around 20 mg l −1 enrichment up to 1000 mg l −1 for Ni(II) and Zn(II) was achieved. At the same time the heavy metal concentration of the discharge water always remained below the German discharge limits of 0.5 mg l −1 Ni(II) and 2 mg l −1 Zn(II), respectively. Phosphate anions were also enriched and partially recovered.

Poly(2-acrylamido glycolic acid- co-acryloyl morpholine) and poly(2-acrylamido glycolic acid- co-acrylamide): Synthesis, characterization, and retention properties for environmentally impacting metal ions

Poly(2-acrylamido glycolic acid- co-acrylamide), P(AGA- co-AAm), and poly(2-acrylamido glycolic acid- co-4-acryloylmorpholine), P(AGA- co-AMo), were synthesized by radical polymerization. The water-soluble polymers containing tertiary amine, amide, hydroxyl, and carboxylic acid groups were investigated as polychelatogen, in view of their metal ion binding properties by using the liquid-phase polymer-based retention technique under different experimental conditions. The retention properties for the following metal ions were investigated: Ag +, Co 2+, Ni 2+, Cu 2+, Zn 2+, Cd 2+, Pb 2+, Al 2+, Cr 3+ and Fe 3+. P(AGA- co-AMo) showed a selective retention for tri-valent cation Al 3+ at pH 3, but no retention at higher pH. P(AGA- co-AAm) showed the highest metal ion retention capability, specially at pH 5 and pH 7 with values close to 100% to di-valent cations.