Use of Humic Sorbent from Sapropel for Extraction of Palladium Ions from Chloride Solutions

1 Introduction

One of the most pressing environmental problems is the pollution of the environment with industrial residues of heavy metals. In accordance with current trends, sorption technologies are preferred to purify wastewater of different nature. Despite a large variety of different sorption materials, the search continues for efficient and cost-effective sorbents. A sorbent obtained from sapropel ( abundantly present in the all-pervasive lake silt) can be one of these materials. Sapropel is a type of deposits found in fresh-water reservoirs with the content of organic and mineral substance of 15-96% wt. and 4-85% wt. respectively [1].

Due to a rich chemical composition of sapropel, it is a high-potential source of raw materials for a wide range of products. The sorbents can be obtained by sapropel carbonization [2] or by thermal treatment with access of air at the temperature range of 300-350°C [3]. The surface of thermally treated sapropel consists of mineral and carbon fragments, which can be modified. Modification of sorbents for expanding their sorption properties is one of the promising directions being actively developed at present [4]. It should be noted that silica is widely used as a substrate for modification [5, 6]. Silica sorbents have a number of advantages, such as high capacity and the presence of active groups, which allows us to fix the modifying organic compounds on its surface. Since the chemical properties of the synthesized material are determined mainly by the nature of the grafted functional group, the correct choice of this material is the most crucial moment in the process of designing a surface-modified material [7].

2 Experimental Part

For the production of a sorbent, we used the sapropel from Lake Puchai of Omsk region (with an organic matter content - 54%, mineral - 46%) [8]. The humic acids (HA) were isolated according to a standard procedure by leaching with an alkali solution [9].

The sample of the sapropel was treated with a 3.5% alkali solution for 2 hours at 50°C. The obtained extract was separated from the insoluble mineral by filtration. The deposition of HA from a filtrate was carried out at pH 1÷2 with a 20% HCl solution; then the precipitate was separated from the solution by centrifugation and dried in a drying oven at 80°C. The filtered and dried mineral residue was treated with a solution of 3% HCl to pH 1÷2, decanted, washed with water to pH 6÷7, and dried. The dried mineral residue was placed in a muffle furnace and kept at 300-350°C for 30 minutes. It should be noted that the carbonization process was conducted in air. The carbided mineral substrate was first modified with polyhexamethyleneguanidine (PHMG) [10, 11] and then with HA [12]. Before the thermal treatment of the mineral residue of sapropel, the humic acids were isolated and then deposited on a mineral substrate through a layer of PHMG. This allowed us to make efficient use of sapropel, the renewable raw material, as in the synthesis of a new sorbent both the mineral constituent of sapropel and HA, containing a large number of functional groups and being part of sapropel, were used.

A solution of palladium ions was prepared by dissolving palladium chloride in a solution of hydrochloric acid at elevated temperature. A solution of nickel ions was prepared from nickel sulfate, dissolving in distilled water at ambient temperature.

The sorbent capacity was determined under static and dynamic conditions. In static conditions, a sorbent with a mass of 0.5000±0.0002 g was placed in flasks with ground stoppers, and 20.0 ml of the solution of the component was added. The flasks were then shaken with low intensity. After a certain period, the sorbent was separated from the solution by decantation and then the residual content of the component was measured by the photometric method.

The method of spectrophotometric determination of palladium was based on the formation of a colored complex [PdCl₄]^2– with nitroso-R-salt. The complex was formed by boiling in an aqueous solution with a large excess of reagent [13].

Photometric determination of nickel was performed with dimethylglyoxime. The reaction was carried out in the alkaline solution in the presence of an oxidant (bromine, iodine and persulfate) and a water-soluble complex compound [Ni(C₄H₆N₂O₂)₃]^2– of a brownish-red color was formed [14].

The thermal analysis was performed on a Shimadzu DTG-60 thermal analyzer with free air access to the oven space. The rate of temperature was +10°C per minute.

The IR-spectroscopy was performed on the FK-801 FT-IR spectrometer of “Simex”. The samples were finely ground in the agate mortar with potassium bromide, pressed into pellets with a diameter of 3 mm, and the spectra were recorded in the wave number range of 5700-470 cm^–1 with a resolution of 8 cm^–1 and the number of scans of 32.

3 Results and Discussion

It is well known that the presence of primary amino groups in the guanidine grouping allows the polyhexamethyleneguanidine to attach to any hydroxylated surface without any chemical processes [15].

The structural modification of the sapropel mineral surface with PHMG and pre-isolated HA is shown in Fig. 1.

Figure 1

Structural modification of the surface of a mineral part of sapropel with PHMG and pre-isolated HA.

Fixation of a PHMG layer to the surface of a carbided mineral part of sapropel occurs due to interaction between the free silanol groups of sapropel and the amino groups of PHMG (Fig. 1).

A strong hydrogen bond between the hydrogen atom of the silanol group and the nitrogen atom of the amino group is formed. As a result, the equilibrium between the molecular and ionic forms of the chemisorption fixation of the amine on the surface is established.

The fixation of humic substances on the surface of the modified sapropel occurs due to the formation of a large number of hydrogen bonds between the free amino group of PHMG and hydroxyl groups of humic acids. The main contribution to the fixation of the organic reagent is provided by electrostatic interaction between carboxylate ions of humic acids and protonated amino groups of polyhexamethyleneguanidine, which do not participate in the bond formation with silanol groups on the surface of a mineral part of sapropel [16, 17].

We determined the number of modifiers attached to the mineral substrate, using the method of chemical oxygen demand (COD) [14]. It is established that 12% of polyhexamethyleneguanidine and 3% of humic acids were attached to the mineral substrate obtained from sapropel.

Before applying the modified humic sorbent for purification of water of different nature, it is necessary to determine the stability of the modifiers at various pH [18]. According to the obtained COD values, it was established that leaching of PHMG and HA from the surface of the sorbent does not occur in the pH range between 2 and 9 units indicating a strong interaction of the substrate with the modifiers.

We used thermal analysis to study the conversion of a humic sorbent from sapropel in air at the temperature range between 20 and 1000°C (Fig. 2). In the low temperature region (up to 150°C), an endothermic effect caused by the removal of adsorption water was observed. The mass loss was 2 mg (7%).

Figure 2

Curves of thermal analysis of a humic sorbent: 1 - TG; 2 - DTG; 3 – DTA.

With a further increase in temperature, the exothermic effects with a significant mass loss of 12 mg (36%) were observed. This phenomenon occurs due to the intensive decay of organic matter.

In the temperature range of 150-330°C, the exothermic effect was weakly expressed and it was due to the destruction and oxidation of peripheral chains in the humic sorbent. On the DTG curve, this effect corresponded to a distinct peak at 210°C. The mass loss was 7%.

In the temperature range of 330-380°C, an intensive decomposition of organic matter, in particular, the humic acids and nonhydrolyzable residue [19] began. In the region of 380-520°C, the DTA and DTG curves show an endothermic effect responsible for the thermal decomposition of organomineral and mineral structures. The main processes occurring in this interval are the reactions of decarboxylation and dehydrogenation.

The exothermic effect at the range of 520-560°C was due to the carbonization of organic matter, and at the range of 780-1000°C, no change in mass was observed that can be attributed to the decomposition of clay minerals [20].

The IR-spectroscopic study of the obtained humic sorbent allows us to determine the presence of functional groups which will define the areas of its application. The IR-spectrum of the humic sorbent is presented in Fig. 3.

Figure 3

IR-spectra of humic sorbent.

In the IR spectrum of humic sorbent modified with polyhexamethyleneguanidine and humic acids, an absorption band in the region of 3364 cm^–1 caused by the presence of –OH and –NH-NH₂ groups was observed. The absorption in the region of 1431 and 1632 cm^–1 was associated with oscillations of C=C bonds of the aromatic ring conjugated to carboxyl groups [21, 22]. The absorption in the region of 1871 and 2372 cm^–1 can be attributed to the valence vibrations of the C=O carboxyl groups.

The intense peaks in the region of ⩽ 1000 cm^–1 were observed, indicating the presence of both the C-O bonds in polysaccharide fragments and Si-O-Si and Si-O-C [23].

Thus, the IR-spectroscopic studies confirmed the presence of various functional groups characteristic of PHMG and HA on the surface of a mineral substrate from sapropel modified with the pre-isolated HA through a layer of PHMG.

In the research, we have studied the sorption characteristics of the sorbent with respect to metals in the form of anions, using palladium as an example, and in the form of cations, using nickel ions as an example.

We experimentally determined the influence of the pH, concentration and duration of phase contact on the sorption value on a humic sorbent from sapropel under static conditions. The maximum sorption value was also determined.

We performed experiment for determination of the palladium sorption from hydrochloric acid solutions in which palladium was in form of [PdCl₄]^2–. The concentration of the initial solutions of palladium ranged between 0.2 and 3.7 mg/mL, pH 1, and the duration of sorption was 24 hours (Fig. 4).

Figure 4

Isotherm of palladium sorption by a humic sorbent.

From the presented data, it was observed that at the initial concentration of palladium exceeding 2.9 mg/mL, the equilibrium concentration was 1.6 mg/mL. The sorption capacity reached its maximum value of 38.0±2.7 mg/g.

The equilibrium of the palladium sorption is described by the equation of the Freundlich isotherm, n = 0.60, k = 28.8 (Fig. 5).

Figure 5

Isotherm of palladium sorption by a modified humic sorbent in logarithmic coordinates.

The isotherm equation has the form: a = 28.8·C^0.6, R²_table = 0.75, R²_calc = 0.99.

When studying the effect of sorption duration on the sorption capacity, the solution concentration was 3.0 mg/mL, and the sorption time varied from 0.25 to 9 hours. The results are presented in Table 1.

The system reached equilibrium after 8 hours of sorption and its maximum value of 38.0±2.7 mg/g under the given conditions (Table 1).

With the sorption under dynamic conditions, the concentration of palladium was 1.0 mg/mL, pH 1, the sorbent loading weight in the column was 6.7 g, and the solution loading rate in the column was 0.30±0.05 mL/min (Fig. 6).

Figure 6

Dependence of palladium concentration on the volume of the solution passed through the humic sorbent.

The dynamic (working) exchange capacity (DEC) was 8.7 mg/g and the full dynamic exchange capacity (FDEC) was 29.7 mg/g.

In order to determine the mechanism of palladium sorption on the modified humic sorbent, the IR-spectra of the sorption products were recorded (Fig. 7).

Figure 7

IR-spectrum of humic sorbent: 1 - before sorption; 2 - after sorption of palladium.

When the sorbent is saturated with palladium, a shift in the region of 3352 cm^–1 to the long-wavelength region (associated with the stretching vibrations of the NH-NH₂ group) was observed in the IR-spectrum of the modified humic sorbent. The appearance of the peaks in the region of 2338 and 2361 cm^–1 was due to presence of the N-Me bond and indicated that under these conditions, in addition to ion exchange, sorption proceeded by additional complexation. At the same time, changes were observed in the region of 1617 cm^–1 with the N-Me shift to the long-wavelength region. Thus, the changes in the IR-spectrum showed that the palladium sorption occurs with the participation of NH-groups.

In addition to the amino groups, carboxyl and hydroxyl groups can be found on the surface of a sorbent, which allows one to use it for the sorption of heavy metal ions in the form of cations.

We performed the sorption of nickel ions from the solutions with concentration from 0.6 to 4.2 mg/mL. The static capacity of the sorbent was 32.0±1.8 mg/g at pH 4-5. The equilibrium in the system was reached within 4 hours of sorption. The experimental data is sufficiently well described by the equation of the Freundlich isotherm: a = 21.9·C^0.4.

The IR-spectroscopy method has shown that the sorption of nickel ions on the humic sorbent occurs with the participation of carboxyl and hydroxyl groups (Fig. 8).

Figure 8

IR-spectrum of humic sorbent: 1 - before sorption; 2 - after sorption of nickel ions.

In the IR-spectrum after the sorption of nickel ions by the humic sorbent from sapropel, a noticeable shift of peaks in the region of 3300-3600 cm^–1 pertaining to -OH groups was observed. A shift in the region of 1392-1620 cm^–1 was due to the vibrations of the C=C bonds of the aromatic ring conjugated to the C=O bond of the carboxyl group.

4 Conclusion

1. We synthesized a sorbent based on the integrated use of the renewable raw materials, sapropel, by modifying the carbided mineral part of the sapropel with pre-isolated humic acids through a layer of polyhexamethyleneguanidine.

2. The synthesized sorbent shows high sorption capacity in relation to metals both in the form of anions [PdCl₄]^2– and in the form of cations Ni²⁺. The sorption is described by the equation of the Freundlich isotherm a = 28.8·C^0.6 and a = 21.9·C^0.4. The maximum static capacity of a humic sorbent reaches 38.0±2.7 and 32±2 mg/g, respectively.

3. The sorption of palladium from chloride solutions on the modified humic sorbent occurs with the participation of amino groups, and the sorption of nickel ions occurs with the participation of carboxyl and hydroxyl groups.

4. Based on the obtained values of sorption capacities it is recommended to use the humic sorbent for wastewater treatment from heavy metals, and the extraction of precious metals from technological waste solutions.

5. For further use of a synthesized humic sorbent, additional research should be conducted on desorption of platinum metals and investigate prospect of a sorbent reuse. In addition, some further work should beperformed to increase the sorption capacity by developing specific surface area and porosity.

5 Summary

The synthesis of a humic sorbent on the basis of the complex waste-free use of sapropel will solve a number of environmental problems and to suspend waterlogging of lakes.