Potentiometric detection and removal of copper using porphyrins
© Vlascici et al.; licensee Chemistry Central Ltd. 2013
Received: 26 April 2013
Accepted: 2 July 2013
Published: 6 July 2013
Copper is an essential trace element with a great importance in industry, environment and biological systems. The great advantage of ion-selective sensors in comparison with other proposed techniques is that they are measuring the free metal ion activity which is responsible for their toxicity. Porphyrins are known to be among the best ionophores in formulation of ion-selective sensors.
A symmetrically substituted meso-porphyrin, namely: 5,10,15,20-tetrakis(4-allyloxyphenyl)porphyrin (TAPP) was used in the construction of a new copper selective-sensor and was also tested for the removal of copper from waste waters. The potentiometric response characteristics (slope and selectivity) of copper-selective electrodes based on TAPP in o-nitrophenyloctylether (o-NPOE), dioctyl phtalate (DOP) and dioctyl sebacate (DOS) plasticized with poly(vinyl chloride) membranes are compared.
The best results were obtained for the membrane plasticized with DOP. The sensor has linear response in the range 1x10-7 – 1x10-1 M with 28.4 ± 0.4 mV/decade near-Nernstian slope towards copper ions and presents good selectivity. Due to its chelating nature, the same porphyrin was also tested for the retention of copper from synthetic copper samples, showing a maximum adsorption capacity of 280 mg/g.
Trace metals are toxic for many life forms when their concentration exceeds a certain limit. This is the reason why their presence in the environment is an important problem and must be precisely monitored. Copper is an essential trace element with a great importance in industry, environment and biological systems. Considered to be the second toxic metal to aquatic life, copper appears in waters and wastewaters from mining industries, refineries, paper and dyeing. Besides, in the recycling process of Li-batteries technical developments regarding Ni, Co and Mn recovery implies copper monitoring in synthetic leach liquor resulted from reductive leaching. Due to the increased interest in environmental protection, both its detection and removal are very important and many methods were used during the time for this purpose.
Several techniques were used for copper monitoring such as: atomic absorption spectrometry (AAS), UV–Vis spectrometry and inductively coupled plasma atomic emission spectrometry (ICP-AES), high performance liquid chromatography (HPLC), anodic stripping voltammetry, cyclic voltammetry [1–5]. Generally, these methods requires expensive instruments, qualified personnel, sample pretreatment and are hard to use in environmental conditions.
The potentiometric method with ion-selective sensors was widely developed in the last years due to its simplicity, low cost and fast analysis and a lot of the reported sensors were used in the environmental analysis . The great advantage of ion-selective sensors is that they are measuring the free metal ion activity which is responsible for their toxicity. This is the reason why a lot of copper-selective sensors based on different ionophores were reported. Several organic compounds, such as: 1-(2-hydroxybenzylidene) thiosemicarbazide , Schiff bases [8, 9], 2-mercaptobenzoxazole , bezo-substituted macrocyclic diamide , 6-methyl-4-(1-phenylmethylidene)amino-3-thioxo-1,2,4-triazin-5-one , porphyrin derivatives , cyclic tetrapeptide derivatives , 7-hydroxy-3-(2-methyl-2,3-dihydrobenzo[d]thiazol-2-yl)-2H-chromen-2-one , tetraazacyclotetradecane derivative , dimethyl 4,4’-(o-phenylene)bis(3-thioallophanate) , succinimide derivative , polyindole  were tested as ionophores.
For copper removal, many materials and waste materials were reported in the literature [20–26]. The efficiency of removing the metals from wastewaters by adsorption method depends on the physical and chemical composition of the adsorbents.
Results and discussion
Working concentration range and slope
The behaviour of any ion-selective sensor depends on the nature and structure of the ionophore used in the membrane composition. Due to the fact that free porphyrins have donor atoms in their structure, the expectations are to have good affinity to transition metals ions. This was proved by several papers reported in the literature [13, 27, 28] that mention different porphyrins as sensing material. Besides the critical role of the ionophore, the nature of the plasticizers having different dielectric constants influence the mobility of the ionophore in the membrane phase. The selection of the best plasticizer can improve the sensor potentiometric answer in terms of sensitivity and sometimes also of selectivity. There are some reports which recommend lower dielectric constant plasticizers for some divalent metal selective sensors  and others in which the best results were obtained by using plasticizers characterized by big dielectric constants . In this respect, this work was focused on obtaining and comparatively presentation of three sensors having the same weight percentage composition of the membrane, but using three different plasticizers: sensor A – plasticized with o-nitrophenyloctylether (o-NPOE, Ɛ = 24), sensor B – plasticized with dioctyl phtalate (DOP, Ɛ = 7) and sensor C – plasticized with dioctyl sebacate (DOS, Ɛ = 4).
Potentiometric response characteristics of the copper-selective sensors A-C
Working conc. range (M)
Detection limit (μM)
5x10-5 – 1x10-1
(26.0 ± 0.3)
1x10-7 – 1x10-1
(28.4 ± 0.4)
1x10-5 – 1x10-1
(24.8 ± 0.4)
From the obtained data it can be highlighted that there is no specific rule in choosing the plasticizer due to the fact that DOP has medium values of the dielectric constant comparatively with those of NPOE and DOS, so that selecting of one plasticizer should be made after performing tests.
Effect of interfering ions
Selectivity coefficients of the obtained sensors
Interfering ion (X)
The data presented in Table 2 put into evidence that also in the terms of selectivity the best results were obtained for sensor B, having the membrane plasticized with DOP. The sensor has very good selectivity in comparison with the other tested cations and was further used in all the determinations.
From Table 2, it results that the sensor A has Fe3+ as interfering ion, but it could not be declared as an iron-selective sensor due to the values of the slopes which are sub-Nernstian.
Effect of pH
Response time and lifetime
The average time for the copper-selective electrode to reach 95% of the final potential value after successive immersion of the electrode in a series of copper ion solutions, each having a 10-fold difference in concentration, was measured. The response time from 10-3 to 10-2 M copper solutions was of 10 s, but it became longer for diluted solutions.
One of the most important characteristics of a sensor is its lifetime. In our case, the sensor having the membrane plasticized with DOP has also the best lifetime, of 6 weeks. During this period of time no significant change of the potential was noticed. The stability and reproducibility of the best obtained sensor were also tested. The standard deviation of 15 replicate measurements made for the 1 × 10−3 M solution was ±0.4 mV.
Response characteristics of the proposed sensor comparatively to other similar electrodes presented in the literature
Response time (s)
Detection limit (M)
1x10-5 – 1x10-1
28.6 ± 0.4
1x10-6 – 1x10-1
29.8 ± 0.7
1x10-8 – 5.7x10-4
5x10-6 – 1.6x10-2
29.2 ± 2.0
1x10-7 – 1x10-1
27.9 ± 0.8
1x10-6 – 1x10-1
29.2 ± 0.4
4.4x10-6 – 1x10-1
1x10-6 – 1x10-2
30.2 – 25.9
1x10-6 – 1x10-1
29.6 ± 0.3
1x10-6 – 1x10-1
9.8x10-6 – 1x10-1
1x10-5 – 1x10-2
1x10-7 – 1x10-1
28.4 ± 0.4
Determination of copper from synthetic solutions
Determination of copper in synthetic samples
Copper sample (mg/L)
Found by electrode (mg/L)
Found by AAS (mg/L)
(19.3 ± 0.4)
(19.6 ± 0.2)
(49.5 ± 0.5)
(49.8 ± 0.3)
Determination of copper from spent lithium ion batteries
The synthetic leach liquor composition was: 50 g/L of Co, 10 g/L of Li, 7 g/L of Al, 5 g/L of Ni, 2 g/L of Fe, 3 g/L of Cu, 1.5 g/L of Mn. Due to the ionic strength of the synthetic solution, sodium nitrate was added to copper samples to brought them to the same ionic strength . The obtained results using the sensor based on three measurements were of 2.97 ± 0.05 g Cu/L.
Copper adsorption capacity, distribution coefficient and removal capacity
Analysing the values, it results that TAPP can also be used with good results as an efficient adsorbent for copper(II) ions.
The detection and removal of copper ions using 5,10,15,20-tetrakis(4-allyloxyphenyl)porphyrin (TAPP) was investigated. Porphyrin (TAPP) was embedded as ionophore in a PVC matrix, using dioctyl phtalate (DOP) as plasticizer, for obtaining a new copper-selective sensor. The resulted sensor is characterized by good sensitivity, very good selectivity and short response time of 10 s. The electrode was used for the potentiometric detection of copper in synthetic samples with a good precision level. The same TAPP porphyrin was tested for the retention of copper from synthetic copper samples with a maximum adsorption capacity of 280 mg/g.
The porphyrin 5,10,15,20-tetrakis(4-allyloxyphenyl)porphyrin (TAPP) was synthesized, purified and characterized in accordance with previously published procedures . For membrane preparation, poly(vinyl)chloride (PVC) high molecular weight, bis(2-ethylhexyl)sebacate (DOS), o-nitrophenyloctylether (NPOE), dioctylphtalate (DOP), sodium tetraphenylborate (NaTPB) and tetrahydrofurane (THF) were purchased from Fluka and Merck. All salts, acids and base were of analytical reagent grade. Double distilled water was used. The performance of each sensor was investigated by measuring its potential in the concentration range 10-5- 10-1 M of different cationic solutions. In the case of copper(II) the solutions were made in a concentration range up to 10-8 M. Stock solutions, 0.1 M, were prepared by dissolving metal nitrates in double distilled water and standardized if necessary. All working solutions were prepared by gradual dilution of the stock solutions.
Electrode membrane preparation and measurements
The membranes have the weight percentage composition as follows: 1% (0,005 g) ionophore, 33% (0,165 g) PVC and 66% (0,330 g) plasticizer. Sodium tetraphenylborate was used as additive (20 mol% relative to ionophore). The electroactive material and the solvent mediator were mixed together, and then the PVC and the appropriate amount of THF (3–5 mL) were added and mixed to obtain a transparent solution. This solution was transferred onto a glass plate of 20 cm2, and the THF was allowed to evaporate at room temperature leaving a tough, flexible membrane embedded in a PVC matrix. The round shape pieces of membranes (diameter = 8 mm) were cut out and assembled on the Fluka electrode body. The measurements were carried out at room temperature using a Hanna Instruments HI223 pH/mV-meter by setting up the following cell:
Ag|AgCl|KNO3 (0.1 M)|sample|ion-selective membrane|0.01 M KCl|AgCl, Ag.
The detection limit of each sensor was established at the point of intersection of the extrapolated linear mid-range and final low concentration level segments of the calibration plot. The effect of pH on the potentiometric response of the sensor was obtained by introducing the best obtained sensor in solutions of HNO3, NaOH and different buffers, in a pH range from 1 to 12.92.
Where Qe is the quantity of copper adsorbed on the porphyrin at the time if equilibrium (mg/g), C0 is the initial concentration of copper in the aqueous solution (ppm), Ce is the final cooper concentration at the time of equilibrium (ppm), m is the mass of porphyrin used as adsorbent (g), V is the volume of the solution (L).
The research leading to these results has received funding from the European Community's Seventh Framework Programme (FP7/2007-2013) under Grant Agreement 266090 (SOMABAT), by UEFISCDI Romanian co-financing EU-7FP- SOMBAT - Module III- nr. 128 EU/2011, by IPA MIS, Project RoS-NET No. 464 and is a result of collaboration between the coauthors within the project POSDRU/21/1.5/G/38347. All are gratefully acknowledged.
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