High biosorption of cationic dye onto a novel material based on paper mill sludge

The valorization of paper mill sludge (PMS) is the main goal of this study. The emissions of PMS continue to increase at global scale, especially from packaging paper and board sectors. The raw sludge was used to prepare an adsorbent to remove toxic pollutants from wastewater, the methylene blue (MB), an organic dye. Firstly, the physico-chemical characterization of PMS was done determining the crystalline phases of PMS fibers, the content of main elements, and the pH zero point charge, which was determined at around pH 7. The adsorption of MB on PMS powder was studied at 18 °C with an agitation of 200 rpm, being the best operating conditions 30 min of contact time, 250 mg L−1 of initial MB concentration and 0.05 g in 25 mL of adsorbent dose. Experimental data of MB adsorption was fitted to Langmuir and Freundlich isotherm equations. The Langmuir model was more accurate for the equilibrium data of MB adsorption at pH 5.1. The PFOM and PSOM were adjusted to experimental adsorption kinetics data, being PSOM, which describes better the MB adsorption by PMS powder. This was confirmed by calculating the maximum adsorption capacity with PSOM, which was 42.7 mg g−1, being nearly similar of the experimental value of 43.5 mg g−1. The analysis of adsorption thermodynamics showed that the MB was adsorbed exothermically with a ΔH0 = − 20.78 kJ mol−1, and spontaneously with ΔG0 from − 0.99 to − 6.38 kJ mol−1 in the range of temperature from 291 to 363 K, respectively. These results confirm that the sludge from paper industry can be used as biosorbent with remarkable adsorption capacity and low cost for the treatment of wastewater. PMS can be applied in the future for the depollution of the effluents from the textile industry, which are highly charged with dyes.

Industries generate millions of tons of wastes each year.Their management can be difficult and expensive, and new solutions other than disposal must be developed to valorize them and reintroduce new added value products into the production chain.This option contributes with the decrease of waste emissions and the protection of human and environmental health.
Paper manufacturing generates a large quantity of wastes, and landfill is the most common option because it is simple and low expensive 1 .The global scale production in pulp and paper sector is 184.4Mt/year of pulp and 402.790Mt/year of paper 2,3 , which generates about 400 Mt/year of paper mill sludge 4 .
Several studies have been performed to valorize paper mill sludge (PMS), for example in the sectors of agricultural and forestry spreading 5 , composting, incineration, gasification, cement production and concrete preparation 6 , manufacture of animal feed 7 , water decontamination 8,9 , and transformation into biofuels (biogas 10 , biodiesel 11 , and bioethanol) and chemicals (isoprene) 12 .
In this context, the PMS from the paper industry has been used as an adsorbent to remove pollutants from wastewater.This residue is emitted in huge volumes and in several countries; and in some of them it is uncontrolled discharged to the environment 13,14 .When PMS is disposed in landfills, this represents particularly challenges and poses severe environmental hazards because of the high content of toxic, organic and heavy metal pollutants due to the treatment of paper at the industry like the use of dichromates for wood treatment 15 .organic residues, and capital investments to preserve the environnement.In order to reach this goal, we had the following specific objectives: (1) to show the valorization potential of residual PMS which constantly increases, and (2) to control emissions of MB as representative molecule of cationic dye high toxic pollutant, the residual MB, which is present in the effluents of textile industries.The selection of this pollutant is based on the capacity of MB to liberate aromatic amines such benzidine methylene, which is carcinogenic 53,54 , its presence in aquatic ecosystems can decrease the photosynthetic activity of plants because it obstructs the sunlight penetration 55 .

Adsorbent preparation
The PMS was from the FADERCO industrial unit at Setif, which is the leader of personal hygiene products in Algeria.The PMS was dried at 105 °C (Memmert,Germany) during 24 h, and then crushed using a grinder.The powder was sieved to obtain fine particles smaller than 100 μm.The PMS powder was stored in a plastic bottle in a desiccator until further use.

Physico-chemical characterization
The PMS was characterized determining the pH zero point charge (pHZPC), identification and quantification of elements (X-Ray fluoresence spectrometry, XRF), the functional groups on the surface (Infrared spectroscopy, FTIR) and the crystallography (X-ray Diffraction, DRX).This characterization gives us information on both structure and composition of the paper sludge, in order to determine its potential as an adsorbent of organic pollutants in water pollution control.

Solids
To determine the profile of solids in PMS, 3 g of PMS was dried at 105 °C as indicated before to determine water and total solids content.Then dried solids were calcined at 450 °C in a muffle oven (SHIMADZU) for 2 h to determine the content of nonvolatile solids (ashes) and volatile solids (organic matter).

pH zero point charge (pHZPC)
The pHZPC of an adsorbent is the pH at which the adsorbent surface becomes electrically neutral 56 .It was determined as follows: in ten Erlenmeyer flasks of 50 mL were put 50 mg of adsorbent.50 ml of distilled water were added and solutions of HCl (37%, Sigma-Aldrich, Germany) or NaOH (98%, Sigma-Aldrich, Germany) were used to adjust the pH of PMS suspension from 2 to 12.The suspensions were agitated for 24 h at 200 rpm.
As soon as the stirring is over, the final pH values are taken, using a pH metre sension™ (pH 31).The ΔpH (pHf-pHi) as a function of pHi is plotted and the intersection of the ΔpH curve with the x-axis at its zero value corresponds to the pHZPC, which is the isoelectric point of PMS powder.

Identification and quantification of elements
The qualitative and quantitative determination of elements was performed by XRF (ZSX Primus IV spectrophotometer).Seven g of PMS was crushed and pressed to form a lozenge before proceeding with the analysis.The sample is irradiated with a beam of X-rays, each element of the sample emits its own fluorescence radiation.

Surface area of material
The specific surface area calculated by the Brunauer, Emmett and Teller theory (S BET ) and the pore volume of dried PMS powder were determined by nitrogen gas adsorption at 77.3 K and an equilibration interval of 15 s (Micromeritics ASAP 2000, Germany).

Morphology of PMS powder
The scanning electron microscopy (SEM) technique was employed to observe morphology of PMS dried powder.The SEM micrograph (Quanta 250FEG) was used to observe PMS morphology at different microscopic scales by electron beam bombardment.The PMS dried powder is fixed on a support for insertion into the scanning electron microscope.The electron beam is scanned horizontally and vertically over the sample surface.When the electrons in the beam interact with the sample, different signals are emitted.To explore the structure of materials at microscopic levels, a zoom of 50 and 100 um was achieved.

Functional groups present on the surface of PMS
The determination of functional groups was performed by Fourier transform infrared analysis (FTIR) (SHI-MADZU FTIR-8400 S spectrometer) before and after adsorption.The spectra were measured between 4000 and 400 cm −1 .

Crystalline phases
The crystalline phases of PMS were identified using X-ray diffraction (XRD) (Bruker D2).The material was bombarded with a monochromatic and parallel X-ray beam with a wavelength of 1.54060 A°, produced by a copper anticathode.
The peak position and FWHM from XRD data are determined and introduced in Scherrer equation to calculate the crystal size of the PMS powder".

Preparation of adsorbate (the pollutant)
The MB (Fig. 1) is a cationic dye was used as adsorbate.
A solution of 1 g L −1 of MB (LabKem, 89% of purity, index CI 52015, molar mass of 319.85 mol g −1 ) was prepared.The concentration of MB before and after the adsorption tests was determined using UV-visible spectrometer (Shimadzu UV-VIS, Germany) at wavelength of 664 nm that corresponds to the maximum absorbance of the MB.

Adsorption tests
The adsorption is influenced by a variety of physicochemical parameters.Their effect on adsorption was analyzed according to the experimental plan shown on Table 1.
The adsorption assays were performed varying the initial dye concentration from 90 to 400 ppm, contact time from 1 to 60 min, initial pH from 2 to 9, mass of PMS dried powder from 0.05 to 0.9 g, and temperature from 18 to 90 °C.All the assays were performed under stirring at 200 rpm.100 mg of adsorbent was added in 25 ml of a solution of MB with fixed initial concentration and initial pH.At the end of each assay, the suspension was filtered with cellulose filters 0.45 µm and the filtrate was analyzed using UV-visible spectrometer to determine final MB concentration.
The amount of adsorbed MB was calculated using the following equation where C 0 : represents the initial dye concentration (mg L −1 ).C t : represents the dye concentration at any time (mg L −1 ).V: represents the volume of dye solution (L).m: represents the mass of adsorbent (g).

Isotherm models
The adsorption models of Langmuir 58 and Freundlich 59 were fitted to experimental results.According to the Langmuir model, the equilibrium is reached when a monolayer of adsorbate molecules saturates the adsorbent.The Langmuir model is expressed as follows: Linearizing Eq. ( 3): (2)  where C e : is the concentration at equilibrium (mg L −1 ).q e : is the quantity of product adsorbed per unit mass of adsorbent (mg g −1 ).q m : is the maximum theoretical adsorption capacity (mg g −1 ).k l : is the constant of thermodynamic absorption equilibrium (L mg −1 ).
The Freundlich's model is based on adsorption on heterogeneous surfaces and its mathematical expression is as follows 61 : where K f : is a constant that indicates the adsorption capacity of the adsorbent.n: is a constant that represents the intensity of adsorption and indicates the kind of adsorption that occurs in the system.If n < 1 the chemical adsorption is present and if n > 1 the physical adsorption is present.
The parameters K f and n are determined by linearizing Eq. ( 5) using logarithms and plotting the resulting linear equation where log K f is the y-intercept for a linear equation and 1/n is the slope.

Modeling of adsorption kinetics
The order of the reaction is an essential feature to define and understand the reaction process.In the present study, two kinetics models were used, the pseudo-first-order (PFO) and pseudo-second-order (PSO) models.
a.The Lagergren equation expresses the adsorption of pseudo-first-order 62 : where K 1 is the rate constant of PFO model, and q e and q t as defined before.
The integration of Eq. ( 7) for t, in the range from 0 to t, and for q, from 0 to q t , the integrated equation becomes:

So:
The equation becomes: The log plot (q e -q t ) in function of (t) gives a line with a slope of-K 1 /2.303 and the y-intercept equal to log (q e ).b.The adsorption of pseudo-second-order is expressed by the equation 63 : where K 2 is the rate constant of PSO model, and qe and qt as defined before.The integration of Eq. ( 9) for t, in the range from 0 to t, and for q, from 0 to qt, the integrated equation becomes: The equation becomes: The constants can be determined by plotting t/q t as a function of t, which gives a line with 1/K 2 q e 2 as y-intercept and 1/q e as slope.

Thermodynamic parameters
The thermodynamics of adsorption is expressed by the following equation 64 : (5) ) log q e − q t = log (q e ) − k 1 2.303 t (10) e log (q e −q t ) = e log (qe) + e ( where ∆H°: is the enthalpy (kJ mol −1 ).∆S°: is the entropy in (J mol −1 K −1 ).T: is the temperature (K).R: is the ideal gas constant (R = 8.314 J mol −1 K −1 ).q e : is the adsorption capacity at equilibrium (mg g −1 ).C e : is the equilibrium concentration (mg L −1 ).This equation let to know the ΔH and ΔS of adsorption and the ΔG can be calculated using the general equation of thermodynamics 64 :

Solids
The content of nonvolatile solids (ashes) for dried PMS powder was determined by gravimetry, being around 1.2% (w/w) and the volatile solids (VS) content was 98.8%.The VS content of PMS powder is in the range of some VS observed in literature shown in Table ST1 (all tables ST are in supplementary information file 1).

pH zero-point charge (pHZPC)
Figure 2 shows the graph of ΔpH as a function of pHi.The point of zero charge for the dried PMS powder was observed at pHi of 7, 14.This point corresponds to ΔpH = 0, which means that pHf = pHi.Where the surface of the PMS powder is electrically neutral.
The range [0-7] is characterized by ΔpH > 0 (pHf > pHi).In this range, the pH of the solution was smaller than the pHZPC.The medium is acidic causing the protonation of the solution, which positively charges the surface of the PMS powder and gives it an anionic exchange capacity.
The range [7-12] is characterized by ΔpH ˂ 0 (pHf <pHi).In this range, the pH of the solution was bigger than the pHZPC.The medium is basic causing the deprotonation of the solution, which negatively charges the surface of the PMS powder and gives it a cationic exchange capacity.

Specific surface area of PMS powder
The S BET and the pore volume of a solid material are main factors that influence the adsorption capacity.The PMS had low values of these parameters; the S BET was 4.3 m 2 g −1 and the pore volume 0.004 cm 3 g −1 .For example, the S BET reported for date pits was 8.1 m 2 g −170 , and for barbecue charcoal 15.5 m 2 g −1 , whereas the pore volume for the latter was 0.022 cm 3 g −170 .

Morphology of PMS powder
According to the SEM micrograph of PMS (Fig. 3), the particles presented a morphology of long and non-oriented micro-fibers 71 with irregular structure.This increases the surface turbulence and random contact between biosorbent and MB molecules, counterbalancing the small specific surface area discussed below.This structure also increases the exposure of functional groups, which are the responsible of MB removal.www.nature.com/scientificreports/elements, which were identified, was lower than 1% (w/w).In the case of Mn, Ni, Cu, Br, Zr and Pb, their content was lower than 0.01% (w/w), for this reason, they are not included in the Table ST2.The content of carbon and oxygen in the PMS powder of some mills reported in literature is shown in Table ST3.According to these data, the PMS used in the present study had an averaged content of these elements, being representative powder of PMS.

Functional groups present on the surface of PMS powder
The Infra-Red (IR) spectra for dried PMS powder is shown in Fig. 4 before the adsorption and after the adsorption of MB.The main elongations observed in Fig. 4 are identified in Table ST4.For the PMS before the adsorption, the IR spectra presents a wide and moderate elongation band around 3446 cm −1 which corresponds to vibration of OH functional group, a fine and weak elongation band around 2921 cm −1 for the vibration of CH methyl alkane functional group, a fine and moderate elongation band around 1780 cm -1 for the vibration of C=O functional group, a fine and weak elongation band around 1435 cm −1 for the vibration of C-H alkane functional group, and a fine and weak elongation band around 1058 cm −1 for the vibration of C-O-C functional group.
For the PMS after the adsorption, the IR spectra presents a wide and intense elongation band around 3425 cm −1 which corresponds to vibration of OH functional group, a fine and moderate elongation band around 2925 cm −1 for the vibration of CH methyl alkane functional group, a fine and moderate elongation band around 1771 cm −1 for the vibration of C=O functional group, fine and moderate elongation band around 1430 cm −1 for the vibration of C-H alkane functional group, and a fine and intense elongation band around 1064 cm −1 for vibration of C-O-C functional group.
These changes observed between spectra in Fig. 4 (intensity of some bands) indicated the possible involvement of those functional groups on the surface of the dried paper mill sludge in sorption process.

Crystalline phases
Figure 5 shows the X-ray diffraction analysis of dried PMS powder.The diffraction spectrum shows a series of well-defined diffraction peaks, which correspond to predominantly well-crystallized solid phases.The phases that can be identified are the kaolinite corresponding to 2Ɵ of 12.5° (peak 1), the cellulose 2Ɵ of 20.0° (peak 2), quartz 2Ɵ of 21.9° (peak 3) and calcite 2Ɵ of 32.1° (peak 4).These phases are like those found in other pulp mill sludge 73,74 .Cellulose and lignin are the originally compounds present in PMS.The other compounds (kaolinite, quartz, and calcite) are introduced into paper sludge from many sources, such from water contaminants or residues from the paper production process.In this way, PMS is a complex mixture of different materials resulting from the papermaking process (Raw materials, chemicals additives used) 73,74 .
The lignin is the main responsible of MB adsorption due to its amorphous structure and functional groups (active sites), which can interact with the MB molecules through chemical and physical bonds, for example: phenolic, hydroxyl and methoxy groups 75 .The crystal size of the crystalline structure of cellulose was calculated around 8.4 nm using the equation of Scherrer 76 .

Effect of the initial dye concentration
Figure 6 shows the adsorption capacity of adsorbent as a function of the initial concentration of MB.The adsorption capacity increased with initial concentration of MB in the range from 90 to 250 mg L −1 .Then, the adsorption capacity was nearly constant for initial concentration of MB higher than 250 mg L −1 .This means that in the range from 250 to 400 mg L −1 the activated sites were saturated without adsorbing more molecules of MB.Thus, the maximum adsorption capacity that can be obtained with PMS powder is 45.2 mg g −1 .Based on infrared spectra, the elongation band of C-O-C functional group increased after the adsorption of MB.This suggests that main binding site for MB is the sulfur (S) of MB skeleton shown in Fig. 1.According to the infrared spectra, a bond between the S of MB and an oxygen present in the surface of PMS is formed.www.nature.com/scientificreports/capacity increased slightly after 30 min.This means that activated sites were nearly saturated at 30 min and the activated sites available will be occupied slowly until reach the equilibrium at time higher than 120 min.According to Fig. 7, the adsorption capacity that can be obtained in short time (30 min) is 42.3 mg g −1 , at 120 min it increased slightly to 43.5 mg g −1 .This phenomenon can be explained by a quick adsorption step of MB on easily accessible sites, followed by a molecular diffusion towards less accessible adsorption sites, before reaching an adsorption equilibrium where all sites become occupied.Quick adsorption is caused by the high affinity (adsorbent-adsorbate).

Effect of initial pH
The surface properties of the adsorbent and the ionization dissociation of functional groups on the adsorbate molecule are affected by the pH of aqueous solution 77 .Figure 8 shows the adsorption capacity of adsorbent as a function of the initial pH of MB solution.The adsorption capacity increased with intial pH.Since the MB molecule is in the form of cation in the aqueous solution, it will compete with other cations for the active sites in the PMS, such as protons.The increase of pH corresponds to a decrease of protons in the solution.The concentration of 250 ppm of MB corresponds to 7.8*10 -4 mol L −1 , and for the same concentration of protons the solution must have a pH of 4.7.Under this pH the concentration of protons will be higher that concentration of MB and for pH above this value the concentration will be lower.As shown in Fig. 8, the MB adsorption increased slightly from pH 2.1-4.2 because the number of protons was much higher than MB molecules.The dynamic of competition for active sites will change in the range from 4.2 to 5.1, became the MB molecules preferably adsorbed at pH near of 4.5 and higher.In the range from 5.1 to 9.0 the activated sites available for MB adsorption were occupied by its molecules.Thus, the maximum adsorption capacity that can be obtained with PMS powder is 44.9 mg g −1 , being the pH of 5.1 the lowest pH, which let to adsorb the highest amount of MB.

Effect of adsorbent dosage on MB removal
Figure 9 shows the adsorption capacity of PMS and the MB removal percentage as a function of the PMS adsorbent dosage.The adsorption capacity decreased in the range from 72.3 to 6.8 mg g −1 with increasing adsorbent dosage from 0.05 to 0.90 g in a constant volume of 25 mL.The addition of more adsorbent in a solution containing a fixed concentration of MB will decrease the ratio of MB molecules/active sites.This will cause that the adsorption capacity will decrease even if more MB has been adsorbed.The latter was confirmed by the color removal, which increased with PMS adsorbent dosage from 58 to 98%.The maximum adsorption capacity for PMS powder was 72.3 mg g −1 for a dosage of 0.05 g in 25 mL of MB solution.However, this adsorption capacity corresponds to low color removal of 58%.The best dosage of PMS in terms of MB removal, according to a practical point of view, was 0.3 g.

Effect of temperature
Figure 10 shows the adsorption capacity of adsorbent as a function of temperature decreasing in the range from 291 to 343 K.Then, the adsorption capacity was nearly constant for temperature higher than 343 K.The best adsorption capacity of PMS for MB was 50 mg g −1 at 291 K, which was the room temperature.The adsorption occurred spontaneously without needing to heat, being an exothermic reaction.This means that the rise in temperature decreases the dye removal because the strength of the bonds between the adsorbent active sites and the MB molecules weakens 78 .

Isotherm models
Table 2 shows the results for the two models of adsorption, Langmuir and Freundlich.The Langmuir model, which describes a monolayer adsorption, was the model, which best fitted to experimental data (Fig. 11), showing a correlation factor R 2 of 0.99.The theoretical adsorption capacity according to Langmuir was 46.1 mg g −1 , which was close to the experimental value observed of 45.2 mg g −1 .This suggests the formation of a monomolecular layer of adsorbate, which is due to a rapid decrease of intermolecular forces.The Langmuir model confirms that   the adsorption reaction was instantaneous and reversible, the absence of interaction between the adsorbed species and the presence of a homogeneous distribution of the adsorption energies 79 .

Modeling of adsorption kinetics
The Pseudo first order model (PFOM) and the Pseudo second order model (PSOM) were adjusted to experimental data of kinetics adsorption (Fig. 12).The parameters for PFOM and PSOM obtained by linear regression are shown in Table 3. Figure 12 shows that the PSOM fitted better than PFOM to adsorption capacity of PMS, with a determination coefficient R 2 of 0.99.The adsorption capacity calculated using the PSOM was 42.7 mg g −1 , which was closer to the experimental value of 43.5 mg g −1 than that of PFOM, which was only 1.1 mg g −1 .

Thermodynamic parameters
Table 4 shows the thermodynamic parameters of adsorption capacity of PMS powder for MB.The ΔG calculated with Eq. ( 16) was always negative in the range of temperature from 291 to 363 K.This indicates that the MB adsorption process on dried PMS particles was a physisorption 80 and a spontaneous phenomenon 81,82 .ΔH and ΔS were calculated using the y-intercept and the slope of the Eq. ( 15) adjusted to experimental data (Fig. 13) with a R 2 of 0.92.The ΔH°s was − 20.778 kJ mol −1 , and the ΔS° − 74.821 J K −1 mol −1 .The negative values of ΔH and ΔS indicates that the adsorption of MB was exothermic and a random decrease of molecular interactions between adsorbate-adsorbent on the solid-solution interface 83 .

Representativeness of this study with other studies on MB adsorption by biosorbents
The biosorption of MB has been studied by other biosorbents, such as tea wastes, olive pomace, cereal husk (rice, wheat, etc.), fruit peel (orange, banana), sawdust (cherry, spruce, etc.) and seaweed.The adsorption capacity reported for these residues was observed in the range from 85.2 mg g −1 for tea waste to 5.2 mg g −1 for seaweed.
The PMS has high potential to be used as biosorbent thanks to its adsorption capacity of 72.3 mg g −1 under the optimal conditions determined in this study.As shown in Table 5, the adsorption capacity of PMS was slightly lower than the best of biosorbents reported in literature (tea waste) and it presents an adsorption capacity for MB in the range of activated carbon from 47.2-269.3mg g −1 .

Conclusions
The adsorption of MB onto PMS powder particles was studied.The PMS issued from paper industry presented high potential to be used as biosorbent to remove dyes from aqueous solutions.The optimal range of initial concentration of MB, contact time, initial pH, temperature and PMS dose on adsorption capacity was determined.The maximum adsorption capacity was 72.3 mg g −1 , under a MB concentration of 250 mg L −1 , 30 min, pH of 5.1, 18 °C and 0.05 mg of biosorbent in 25 mL of solution.This removal capacity is competitive with other biosorbents and even with activated carbon.According to this study, the use of PMS let to remove up to 98% of the color given by MB to wastewater.The MB adsorption on PMS powder followed a Langmuir behavior and a pseudo second order kinetics, suggesting that adsorption is a physicochemical process, and the adsorbent particles could be regenerated for being reused in new adsorption cycles.The MB adsorption is a spontaneous and exothermic process; this means that for regenerating is necessary to introduce an energy source.
The high removal capacity of PMS suggests that this material is a good option to depollute effluents charged with dyes; for example, textile sector which is the main consumer of MB.In addition, the use of a byproduct as adsorbent contributes to sustainable development of regions.This circular economy concept focuses on preserving the environment.The promising use of PMS as biosorbent opens the door to deepen the scientific research in the use of emergent materials to remove other pollutants.

Figure 2 .
Figure 2. Surface charge of adsorbent as a function of pH.

Figure 7 Figure 5 .
Figure7shows the adsorption capacity of adsorbent as a function of the contact time.The adsorption capacity increased with the contact time, being a strong increase in the range from 1 to 30 min.Then, the adsorption

Figure 6 .
Figure 6.Adsorption capacity of PMS for MB as a function of the initial concentration.

Figure 7 .
Figure 7. Adsorption capacity of PMS for MB as a function of the contact time.

Figure 8 .
Figure 8. Adsorption capacity of PMS for MB as a function of the pH.

Figure 9 .
Figure 9. Adsorption capacity of PMS for MB and removal percentage as a function of adsorbent dosage.

Figure 10 .
Figure 10.Adsorption capacity of PMS for MB as a function of the temperature.

Table 3 .
Kinetics parameters of Pseudo first order model (PFOM) and Pseudo second order model (PSOM) for MB blue adsorption.*q e was calculated by adjusting PFOM or PSOM to experimental data.

Table 4 .
Thermodynamic parameters of MB adsorption on PMS.

Table 5 .
Adsorption capacity of biosorbents for MB.