Removal of Zinc Ions from Aqueous Solutions with the Use of Lignin and Biomass Part II

Lignin fractions were separated from black liquor, which was received as a by-product of the chemical treatment of wood. The lignin from the black liquor was precipitated with sulfuric acid. 1 M H₂SO₄ was added to the black liquor until a pH of 2.0. The resulting sediment was then centrifuged using a MPW-351 laboratory centrifuge (4200 rpm for 30 minutes). The lignin precipitate was redissolved in 0.1 M NaOH solution, and then the lignin was precipitated with 1 M H₂SO₄ solution. The precipitate was filtered off, centrifuged, and washed with

0.1 M H₂SO₄ and water. The washed and filtered lignin was dried by stirring at 50°C [17].

GPC/SEC analysis was performed in a solution of 0.5% LiCl in N,N-dimethylacetamide (DMAc) at a flow rate of 0.5 ml/min. A GPC/SEC system (Agilent Technologies, USA) with a three-column configuration of PLgel Mixed-A, 300mm, 1260 Iso Pump, and 1260 ALS autosampler equipped with an Optilab T-rEX refractometric detector (Wyatt Technology, USA) was used at 80°C. The volume of the injected test sample was 100 µl. Column calibration was performed using polystyrene standards. Results of the GPC/SEC analysis, such as the molar mass distribution function (MMD), mean values of the molar mass (Mn¯,Mw¯)$$({\overline {Mn} ,\overline {Mw}})$$ and polydispersion (Mw¯,Mn¯)$$({\overline {Mw} ,\overline {Mn} })$$ were calculated using the conventional calibration method, presenting the results in units of calibration standards.

FTIR spectra of the adsorbents analysed enabled to define characteristic functional groups on the surface of the samples analysed. A Nicolet iS50 spectrometer (Thermo Scientific, USA) equipped with a DTGS ATR detector was used with the following parameters: range 4000-400 cm⁻¹, resolution 4.0 cm⁻¹, number of scans with background collection [7,18].

Chemical composition analysis of the selected biosorbents: rice husk, oat bran, pine bark and coconut fiber was carried out in accordance with the PN-92/P-50092 standard [19].

Zinc nitrate hexahydrate Zn(NO₃)₂ · 6 H₂O p.a. (Chempur, PL) was used as a precursor of zinc ions. In a series of tests, the sorption capacity of all the sorbent materials was examined at a constant temperature of 25°C, for 120 minutes, at a pH of 4 and 7, which was obtained by adding an appropriate amount of HNO₃ (2M) or NaOH (2M).

In the sorption tests about 1 g of sorbent was placed in 250 cm³ conical flasks and 100 cm³ of an aqueous zinc solution with an ion concentration of 10 mg/cm³ and appropriate pH (4 and 7) was added. The glass-stoppered flasks were incubated on a shaker for 120 minutes at a constant temperature of 25°C and rotation speed of 155 rpm. After the set time had elapsed, the mixture obtained was filtered under reduced pressure. Blank tests containing the initial concentration of zinc ions at pH 4 and 7 were also subjected to filtration. The concentration of zinc ions in the filtrate acquired was determined by atomic absorption spectrometry (AAS).

The zinc content in the filtrate was determined by flame atomic absorption spectrometry using a ContraAA 700 (Jena Analytik) high resolution atomic absorption spectrometer.

Certified IONEX (Chem-Lab) of Zn²⁺ concentration 1000±2 μg/dm³ was used as the standard. Before the determination, the filtrates and standards were acidified with 1% hydrochloric acid.

Basic parameters of the analysis were as follows: wavelength - 213.857 nm, flame - acetylene/air, spectral buffer - 0.1% KCl, characteristic concentration corresponding to absorption of 1% of lamp radiation (absorbance 0.0044) - 0.005 mg Zn/dm³.

Sorption efficiency was calculated from the following formula: 1 Yield=(C0−C)/C0 × 100% $${\rm{Yield}} = ({{\rm{C}}_0} - {\rm{C}})/{{\rm{C}}_0} \times 100\% $$ Where: C_o – initial concentration of Zn ions [mg/dm³], C – concentration of Zn ions after the set time of the adsorption process [mg/dm³]

The adsorption test was performed using a glass column, inside which selected biomass and lignin raw materials were placed in layers and model aqueous solution of zinc ions was passed through the column. The aim was to determine the influence of the process parameters: the contact time of the ion with the adsorbent, the pH of the aqueous solution, the mass of the sorbent, the concentration of the ion, and the process temperature, on the efficiency of Zn ion removal from the aqueous solutions.

The effect of the processing time was studied at a concentration of 10 mg/dm³ and 40 mg/dm³ of zinc at pH 4 & 7 and temperature of 25ºC. The aqueous zinc solution was passed through a suitably prepared column (Fig. 13). The ratio of the sorbent mass to the adsorbate volume was 0.5 g per 50 cm³ of the zinc solution. The time of one passage through the column was about 4 minutes. The process was carried out in a circulation system for 4 hours. After 30 min, 60 min, 120 min and 240 min, about 10 cm³ of the adsorbate was collected, filtered through a filter set, and the amount of zinc ion that was not adsorbed on the bed was determined. The blank test zinc ion aqueous solution was filtered in the same way.

The effect of the sorbent mass was studied at a concentration of 10 mg/dm³ of zinc at pH 4 & pH 7 and temperature of 25°C. The ratio of the sorbent mass to the adsorbate volume was 1 g per 50 cm³ of zinc solution and 2 g/50 cm³ of zinc solution. The aqueous zinc solution was passed through the prepared column. The process was carried out in a circulation system for 1 hour. After this time, 10 cm³ of the filtrate was withdrawn and the metal content was determined by the AAS technique.

The influence of temperature on the sorption process at a concentration of 10 mg/dm³ of zinc at pH 4 and 7 was tested. The ratio of the sorbent mass to the adsorbate volume was 0.5 g per 50 cm³ of the zinc solution. The prepared column with the receiving flask was placed in a water bath at 40°C. The duration of one full flow through the column was 1.5 minutes. The process was carried out in a circulation system for 1 hour. After this time, 10 cm³ of the filtrate was withdrawn and the metal content was determined by the AAS technique. Similar tests were performed at a temperature of 25°C.

A two-stage process of lignin separation from black liquor aimed at cleaning it of non-lignin deposits. Lignin fractions were obtained with an average yield of 45 g/dm³.

Using the GPC/SEC technique, the structure of the separated lignin was investigated. The average molecular weight determined was 5.900 g/ mol according to PS standards, and polydispersion (Mw/Mn) was 2.20. Based on literature data, the molecular weight of lignin is within the range of 4600-8000 g/mol, which is close to the values obtained in the analysis of molar masses of lignin in softwood [20,21]. The peak of the sample tested overlaps with that of the eluent used (0.5% LiCl/ DMAc), as evidenced by the asymmetric MMD plot. Results of the GPC analysis are presented in Tab. 1. and Fig. 2.

Fig. 2.

A study of the structure of the surface groups: oat bran, rice husk, coconut fiber, sodium alginate, chitosan, pine bark, and pectin was performed with FTIR-ATR. The sorbents analyzed contain such substances as lignin, cellulose, hemicellulose, and other polysaccharides, as well as many different aliphatic and aromatic substances [22].

It was found that the spectra for lignin fractions are characteristic of lignin structures. In the samples tested, there are characteristic absorption bands at wavelengths (3400 cm⁻¹) corresponding to hydroxyl groups (-OH) and bonds C-H corresponding to the lignin in the aliphatic chain - 3372 cm⁻¹ (Fig. 3). The bands around 2930 cm⁻¹ correspond to the methyl, methylene, and methoxy groups in the guaiacylsringil units of lignin. The characteristic stretching bands for lignin occur at wavelengths around 1600 cm⁻¹ and 1500 cm⁻¹. They correspond to the -C-C- bonds in the aromatic ring. The 1701 cm⁻¹ band corresponds to the carboxyl group (-C=O), while the 1460 and 1420 cm⁻¹ bands are related to the presence of asymmetric bending deformations of the C–H groups in guaiacyl propane and guaiacyl syrigil units in the samples tested. The 1208 cm⁻¹ wavelength corresponds to the aromatic guaiacyl rings that are characteristic of lignin. On the other hand, the 1122 and 1029 cm⁻¹ bands confirm the presence of syringyl groups [23,24].

Fig. 3.

In all spectra analyzed, Fig. 3-11, there is a system of bands in the region of 3250-3400 cm⁻¹ corresponding to the vibrations of υ(O-H) bonds that may come from alcohols, phenols, and carboxylic acids, as well as intramolecular hydrogen bonds of cellulose [22]. The width of these bands is characteristic for carbohydrates due to the presence of many hydroxyl groups in their structure [25]. The bands in the wavenumber range of 2950-2850 cm⁻¹ can be attributed to the vibrations υ(C-H), 2950-2962 cm⁻¹, coming from asymmetric stretching vibrations, and 2850 cm⁻¹, from symmetrical stretching vibrations.

Fig. 4.

Fig. 5.

Fig. 6.

Fig. 7.

Fig. 8.

Fig. 9.

Fig. 10.

Fig. 11.

The bands characteristic for δ(O-H) and δ(C-H) vibrations occur in the range of 1200-1500 cm⁻¹, with the O-H bands coming from bending vibrations occurring in first and second order alcohols. For the C-H bond band, bending vibrations occur in this range, symmetrically at approx. 1375 cm⁻¹ and asymmetrically at approx. 1450 cm⁻¹. There are many bands in the 900-1200 cm⁻¹ region, which can be mainly attributed to the stretching vibration of bonds υ(C-O) and υ(C-C) [26]. For all compounds tested, the bands in the range 1000-1035 cm⁻¹ represent vibrations of the glycosidic bond originating from the polysaccharide backbone, including vibrations υ(C-O) and C-O-C [27]. Moreover, the band at 1158 cm^–1 corresponds to vibrations of the C-O-C groups in the anomeric regions of hemicelluloses. Additionally, the intense band at 1030 cm^–1 may be related to the vibrations bending in the plane of the C-H groups in the aromatic syringyl and guaiacyl units of lignin [22]. In the spectrum of chitosan (Fig. 6), there are also two characteristic bands for this polysaccharide, the first one at a wavenumber equal to 1588 cm⁻¹, resulting from vibrations of the N-H bond (amide II) [27], and the band in the region of 1645 cm⁻¹, corresponding to vibrations of the carbonyl group C=O of the amide group CONHR (amide I). The 1320cm⁻¹ range characteristic of the C-N bond (amide III) is also noticeable [28]. The spectrum of alginate (Fig. 11) shows an intense band from asymmetric vibrations at 1593 cm⁻¹and symmetrical vibrations of 1405 cm⁻¹, as well as stretching of the -CO bonds in the -COO- group at 1021 cm⁻¹, related to asymmetric vibrations of -COC, and at 814 cm⁻¹, related to mannuronic acid residues [28,29]. In the case of pectins (Fig. 10), the most characteristic bands which distinguish them from other polysaccharides are 1323 cm⁻¹ and 831 cm⁻¹,resulting from vibrations of the pyranose ring, as well as 944 cm⁻¹, representing bending vibrations of the C-O bond. Moreover, in the pectin spectrum at a wavenumber of 1737 cm⁻¹, there is a characteristic band for vibration υ(C=O) for carbonyl esters and at 1592 cm⁻¹ for the stretching vibration of COO-carboxyl ions [30–33].

FTIR examination confirmed that the structures of the sorbents analysed are a mixture of various polysaccharides dominated by hydroxyl, carboxyl, and carbonyl groups, and they can act as active centers involved in the binding of various harmful compounds, e.g., zinc ions. Therefore, on the basis of the FTIR analysis, it was confirmed that the selected natural wastes are suitable for the development of an ecological biosorption system [22].

Raw materials of plant origin used in the research: rice husk, pine bark, coconut fiber, and oat bran were analysed for their chemical composition. The tests were carried out according to the PN-92/P-50092 standard, the results of which are presented in Tab. 2.

The values for individual fractions present in the pine bark sample are similar to those given in the literature [37]. The highest cellulose content was obtained in the samples of rice husk and coconut fibre. Lignin, on the other hand, is the fraction present in the largest percentage in pine bark and coconut fiber samples.

The sorption tests consisted in placing aliquots of about 1 g of sorbent in separate 250 cm³ conical flasks and adding 100 cm³ of an aqueous solution with a zinc ion concentration of 10 mg/dm³ at pH 4 and 7. The flasks were covered with a glass stopper and incubated in a shaker for 120 minutes at a constant temperature of 25 °C and 155 rpm. After the set time the mixture was subjected to filtration under reduced pressure. The content of zinc ions in the filtrate was determined by AAS and the adsorption yield calculated. The results for 9 samples are presented in Fig. 12. In the case of pectin and sodium alginate samples, in order to obtain a filtrate for the determination of Zn (II) by the AAS method, calcium pectinate, and calcium alginate were precipitated with the addition of about 12 g of calcium chloride to the solution and then filtered [38].

Fig. 12.

The results indicate that all the samples had the ability to absorb zinc ions. An important factor in sorption processes is the pH of the solution, which affects the efficiency of the process. In most cases, the sorption process performed better in an alkaline environment. The acidity of the solution can affect the metal ions and the surface charge of the adsorbent. At low pH, active surface groups (including hydroxyl and amine groups) are protonated, which can create a repulsive force between the positively charged metal ions and the positively charged sorbent surface. In addition, metal ions can compete with H⁺ which leads to low removal efficiency. As the pH increases, the degree of deprotonation of functional groups gradually increases, which facilitates the adsorption of metal ions, as electrostatic attraction increases [27,39]. On the other hand, at high pH, metal ion hydrocomplexes may precipitate. These complexes have large sizes and are not dissociated in solution, hence they hardly interact with free carboxyl groups in the molecules, which leads to lower binding capacity [40]. In the water system, the surface of the sorbent takes a negative charge, which is important for the adsorption of positively charged metal ions harmful to the environment from model solutions [7]. The highest affinity to zinc ions after 120 minutes is shown by oat bran, pine bark, coconut fiber and activated carbon. According to literature data, oat bran adsorbs heavy metal ions due to the high content of β-glucans [41,42]. The lowest affinity can be attributed to chitosan, pectins and sodium alginate. Lignin absorbs approximately 66% of zinc ions. The sorption process is pH dependent for pectin, chitosan, and rice husk samples. The dependence of the process efficiency on the pH is mainly determined by the characteristic functional groups on the surface of the adsorbents. In the case of chitosan and pectin, higher adsorption values were obtained at an acidic pH, while rice husk sorbed better at a neutral than at an acidic pH.

After analysis of the results of Zn(II) sorption obtained and a preliminary study of the permeability of the sorption column, samples were selected for further testing. The samples met the following criteria: they should not dissolve in water (sodium alginate, pectin), nor swell and clog the sorption column (oat bran); they sorbed zinc ions best, were of plant origin: pine bark, coconut fiber or lignin, and the sorption process was zixznot pH-dependent (chitosan, rice husk). Sample weights used for further testing reflected the tests described in section 3.5.

The distribution of individual sorbents in the column (Fig. 13) assumed the use of coconut fiber as a component that prevents the movement of looser, smaller and lighter fractions (bark and lignin) when the aqueous solution was passed through the column. During sorption, the aqueous solution affects the sorbent mixture as it flows through successive layers. On each of them, impurities are successively reduced and the substance removed from the solution remains on the sorption bed.

Fig. 13.

The concept assumed the use of lignin in the sorption column as a valuable biopolymer of practical importance, which has so far been mainly a byproduct in the pulp and paper industry. The use of such a by-product, arising in the production of cellulose pulp, is a potential source for the production of ecological sorbent and the transformation of waste into raw material. In this way, the lignin would be fully utilized.

Tests of zinc ion adsorption from model water solutions with the use of a sorption column allowed to determine the influence of process parameters on the efficiency of the removal of Zn ions from water solutions. The parameters taken into account were: the contact time of the ion solution with the adsorbent, pH of the aqueous solution, sorbent mass, ion concentration, and process temperature.

3.6.1.

Effect of ion-sorbent contact time, adsorbate concentration and pH on sorption yield of zinc ions

The effect of the processing time was investigated at a concentration of 10 mg/ dm³ zinc for pH 4 and 7. The columns contained stacked beds ( each type of bed weighed 0.5 g) and an aqueous solution of zinc was circulated through a column. The time for 1 pass through the column was approximately 1.5 minutes. The ratio of the sorbent mass to the adsorbate volume was 0.5 g per 50 cm³ of the zinc solution. The process was carried out for 4 hours.

After 30 min, 60 min, 120 min, and 240 min, approx. 10 cm³ of the adsorbate was collected and filtered through the filter assembly. The content of zinc ions in the filtrate was determined by AAS, and the adsorption yield was calculated.

Analyzing the results, presented in Fig. 14, it can be stated that, initially, the adsorption process is fast and then slows down. The adsorption equilibrium was established after about 120 minutes. It was also found that the effectiveness of the process was correlated with pH. In case of pH 7, after 240 minutes, the yield was at the level of approx. 83%, while at pH 4, it was slightly lower - approx. 76%. The test at a zinc concentration of 40 mg/ dm³ (Fig. 15) shows similar tendency

Fig. 14.

Fig. 15.

The results obtained (Fig. 15) show that the adsorption yield decreases with the increasing concentration of zinc ions. This is due to the partial saturation of active sites on the sorbent surface, which are capable of capturing metal ions. The increase in the number of metal ions in the solution increased competition for active centers, which makes the adsorption process less effective [7]. In the case of pH 4 and pH 7, after 240 minutes, a similar efficiency of approx. 40% was recorded.

3.6.2.

Effect of sorbent mass and pH on the adsorption process

The effect of the sorbent mass at a zinc concentration of 10 mg/dm³ and pH 4 & 7 on the adsorption yield was studied. The ratio of the sorbent mass to the adsorbate volume was 1 g/50 cm³ and 2 g/50 cm³.

The Zn solution circulated through the column, for which the time of a single passage was 2.5 minutes and 5 minutes, respectively. The process was run for 1 hour. After this time, approx. 10 cm³ of the adsorbate was collected and filtered. The zinc ion concentration was determined by the AAS method and then the adsorption yield calculated.

The results presented in Fig. 16 show that as the mass of the sorbents used increased, so did the yield of the process. It was caused by an increase in the area of the contact surface of the adsorbent and an increase in the number of characteristic functional groups present on its surface, which determine the sorption capacity. In this case the pH parameter was not significant. For the column containing 1 and 2 g of each sorbent bed, similar yield values above 90% were obtained for pH 4 and 7, respectively.

Fig. 16.

3.6.3.

Influence of temperature and pH on the adsorption process

Temperature is considered to be one of the most important parameters influencing the efficiency of the zinc ion adsorption process [43]. A study of the influence of temperature on the sorption process at a zinc concentration of 10 mg/dm³ at pH 4 and 7 was carried out. The ratio of the weight of the sorbent to the volume of the adsorbate was 0.5 g /50 cm³ of the zinc solution.

The column was filled with sorbents and the test solution circulated through it. A flask with zinc solution and another collecting the adsorbate were placed in a 40 °C water bath. The duration of one full flow through the column was 1.5 minutes. The process was carried out for 1 hour. After this time, approx. 10 cm³ of the adsorbate was collected, filtered through a filter set, and subjected to an AAS examination. The results are presented in Fig. 17.

Fig. 17.

The increase in temperature had a positive effect on the sorption yield in the sorbents tested. According to the literature data, an increase in the temperature of the reaction system causes a decrease in the viscosity of the solution, which results in an increase in the diffusion rate of ions to the adsorbent surface, as well as in an expansion of pores and activation of the adsorbent [44]. The maximum capacity yield of the process was achieved for a concentration of 10 mg/dm³ at 40 °C, which was approx. 90% at pH 7.