Adsorptive removal of noxious cadmium from aqueous solutions using poly urea-formaldehyde: A novel polymer adsorbent

Graphical abstract


Reagents/tools
The Cd +2 concentration measurement was performed by an Atomic Absorption Spectrophotometer (Shimadzu AA-670 model).
A digital pH meter (Basic 20 Crison) was used for solution pH measurement.

Experimental design
Measuring of Cd +2 concentrations under various levels of initial Cd +2 concentration, solution pH, and contact time to obtain optimal Cd +2 removal from aqueous solution using a novel adsorbent provided from Urea Formaldehyde.

Trial registration No applicable Ethics No applicable
Protocol data Preparation a new highly efficient polymer urea-formaldehyde for the removal of cadmium ions from aqueous solutions. Maximum adsorption capacity of cadmium was 76.3 mg/g. Suitable application of urea-formaldehyde adsorbent in removing cadmium ions from wastewater is shown.

Chemicals and materials
All chemical materials used in the present research were in analytical grade. Chemicals including cadmium nitrate, urea, formaldehyde, acetic acid (CH 3 COOH) (purity degree of 100%), nitric acid (purity degree of 65%), acetate sodium (C 2 H 3 NaO 2 ), sodium dihydrogen phosphate (NaH 2 PO 4), di-sodium hydrogen phosphate (Na 2 HPO 4 ) and sodiumhydroxide (NaOH) wereprovided fromMerck company,Germany.Astock solution of 500 mg/L of cadmiumwas prepared by dissolving a certain amount of cadmium nitrate salt in deionized water. Different concentrations of cadmium solution were prepared by diluting the stock solution.

Urea-formaldehyde preparation
In order to prepare urea-formaldehyde adsorbent, firstly, 6 mL of formaldehyde was poured into volumetric flask and NaOH 0.1 M was added to it drop by drop for pH of the solution reach to 8. Then, 3 gr of urea was added to the solution and it was kept in a 50 C hot water bath to be dissolved uniformly. After that, the solution was kept at 70 C to be completely colorless. In the next step, 2 mL of cadmium with a concentration of 500 mg/L was added to the obtained solution and then 0.5 mL acetic acid 100% was added to it. After some time, obtained solution was stiff as a polymer. Then, it was ground as a powder. Then the polymer was washed three times by 50 mL nitric acid 0.1 M, each time for half an hour and the solution pH was adjusted to 6. Then the polymer was percolated and dried to remove any moisture. Finally, prepared adsorbent was used to perform cadmium adsorption tests ( Figs. 1 and 2).

Adsorption tests
All cadmium adsorption tests on urea formaldehyde polymeric adsorbent were performed using 50 mL of cadmium solution in 100 mL Erlenmeyer at a temperature range 22-24 C on a 300 rpm shaker. In this study, the effects of initial pH , 10, 20, 30, 60, 90 min) and initial cadmium concentrations (5, 10, 20, 40, 60, 80 mg/L) parameters were studied. Before adsorption process, in order to prepare buffers with pH ranging from 3.5 to 6.5, acetate sodium (C 2 H 3 NaO 2 ) 0.01 M and acetic acid 0.01 M (CH 3 COOH) were used and to prepare buffers with a pH ranging from 6.5 to 8.5, sodium dihydrogen phosphate (NaH 2 PO 4) 0.01 M was used. pH of the solutions was measured by a pH-meter (model Basic 20 Crison). After adsorption tests, adsorbents were removed from solution by membrane filter and the concentration of filtered cadmium solution was measured by an Atomic Absorption Spectrophotometer (Shimadzu AA-670 model) equipped with GFA-4B graphite furnace atomizer and D2 lamp for background correction. Cadmium hollow cathode lamp was applied as radiation source at 4 mA. An atomic absorption signal at 228.8 nm line was recorded on a graphic printed PR-4 with peak height and gas stop mode for quantification.

Cadmium removal in real samples
Efficiency of urea formaldehyde adsorbent for removal of cadmium from real sample (wastewater) was also studied. The sample was collected from effluent of a wastewater treatment plant located in south of Tehran in Ray. First, concentration of cadmium ion was measured in real sample which was 0.078 mg/L. Then, 2 mg/L cadmium was added to the real sample and they were put next to 1 gr adsorbent for 15 min under optimum pH conditions at 22-24 C and the amount of cadmium adsorption was determined by atomic adsorption spectroscopy method. To improve accuracy and validity, all adsorption tests were repeated in triple.

Data analysis
In surveying efficiency of synthesized polymer on real samples equations (1) and (2) were used to calculate percentage of standard deviation and relative standard deviation, respectively: X: Residual concentration after adsorption process in the solution in mg/L X : Mean residual concentration after adsorption process in mg/L SD: Standard deviation RSD (%): Relative standard deviation N: Number of times that test performed In order to calculate the efficiency of cadmium removal by urea-formaldehyde equation (3) was used [1][2][3].
Where, RE is removal efficiency (%), C i is initial concentration of pollutant in the solution (mg/L) and C t is final concentration of pollutant in the solution (mg/L).

Effect of pH
Solution pH is one of the most important parameters in adsorption process, because the binding of cations to active surface groups is strongly dependent on the surface charge of particles [4][5][6]. The effect of pH on the rate of adsorption of cadmium on urea formaldehyde adsorbent is shown in Fig. 3. In this research, after testing different pHs, according to Fig. 3 highest adsorption efficiency was obtained for cadmium ion by formaldehyde adsorbent at pH = 5.5 which indicates that this adsorbent acts better in weak acidic and near to neutral pH and has higher adsorption in this condition, while at pHs lower than 5.5, adsorption rate decreases and this is because at lower pHs, there is competition between H + ions and cadmium ions, but in pHs higher than 5.5 the rate of cadmium adsorption by studied adsorbent increases and this may be due to the formation of solution complexes [7][8][9].

Effect of contact time
Contact time between adsorbate and adsorbent is an important parameter in adsorption process [10][11][12]. As shown in Fig. 4, during first 5 min shaking, highest cadmium adsorption has happened by urea formaldehyde and by passing time, adsorption efficiency did not considerably change and was mostly stable until the adsorbent was saturated. This can be attributed to the availability of more vacant biding sites for cadmium adsorption during initial contact times [6].

Adsorption isotherms
In order to design an adsorption process for application in field, the isotherms for an adsorbate-adsorbent systemshouldbedeterminedexperimentally [13].Theisothermsshowthechemicalequilibriumforthespecific conditions of temperature, pH, adsorbent dosage etc. and thus indicate the maximum achievable adsorption capacity in adsorption process. To study the isotherm of cadmium adsorption by urea formaldehyde polymer, Langmuir, Freundlich and Temkin adsorption isotherm models were used in optimum conditions.

Langmuir isotherm
Langmuir isotherm is valid for single layer surface adsorption and was first performed with the aim of describing gas surface adsorption on activated carbon [14]. Assumptions of Langmuir model: 1-adsorption energy is the same and does not depend on the amount of absorbed material on the adsorbent, in other words, the adsorption capability of each site is the same as the other and the presence of absorbed material in each site has no other effect, 2-adsorption bonds are reversible and 3-absorbed material is a layer with thickness of a molecule [15,16].  The linear form of Langmuir isotherm is represented by the expression: Where, q e is the amount of contaminant per 1 gr of adsorbent, C e is the amount of remaining contaminant after equilibrium, q max is maximum amount of absorbed substance after equilibrium and b is Langmuir constant.

Freundlich isotherm
The most important multisite adsorption isotherm for heterogeneous surfaces is the Freundlich adsorption isotherm [17]. Freundlich isotherm indicates non ideal, reversible and multilayer adsorption with heterogeneous distribution of heat and adsorption on heterogeneous surface [18]. The non-linearized form of Freundlich isotherm model is given in the following equation: Freundlich isotherm can also be written as follows: LnK is y-intercept. Slope of this line is 1 n which indicates adsorption intensity and K indicates adsorption capacity.

Temkin adsorption isotherm
The Temkin isotherm is usually applied for heterogeneous surface energy systems (non-uniform distribution of sorption heat) [19]. Assumptions of Temkin model: 1-adsorption is exponential, and 2adsorption is single layer. Temkin isotherm is presented as: q e = BLnA + B LnC e (7) In the above equation, B = RT/b is a constant showing heat of sorption (J/mol) obtained from the Temkin plot (qe against lnCe), A (slope) is Temkin isotherm equilibrium binding constant (L/g), b (intercept) is Temkin isotherm constant, R is universal gas constant (8.314 J/mol.k) and T is absolute temperature (Kelvin).
In this study, three isotherm models such as Langmuir, Freundlich, and Temkin adsorption isotherms were used. Maximum adsorption capacity was 76.3 mg/g. Freundlich, Langmuir and Temkin adsorption isotherms parameters are presented in Table 1. Considering R 2 correlation coefficient, the results showed that cadmium adsorption on urea formaldehyde adsorbent best matches Freundlich isotherm model. Since Freundlich isotherm indicates multi-layer adsorption on heterogeneous surfaces, it can be concluded that cadmium adsorption by urea formaldehyde occurred in multi layers (Figs. 5-7).

Study of cadmium removal in real samples
Results of tests performed on real sample are presented in Table 2. Regarding the test performed on considered adsorbent, the value of standard deviation was obtained 2.47% that indicates low errors of tests. ConsideringTable2,therateofcadmiumremovalonrealwastewatersamplesbyureaformaldehydeadsorbent is equal to 54.6% that indicates good function of adsorbent in absorbing cadmium from aqueous solutions.