Higher adsorption capacity of Spirulina platensis alga for Cr(VI) ions removal: parameter optimisation, equilibrium, kinetic and thermodynamic predictions

Abstract

This study discusses about the biosorption of Cr(VI) ion from aqueous solution using ultrasonic assisted Spirulina platensis (UASP). The prepared UASP biosorbent was characterised by Fourier transform infrared spectroscopy, X‐ray diffraction, Brunauer–Emmet–Teller, scanning electron spectroscopy and energy dispersive X‐ray and thermogravimetric analyses. The optimum condition for the maximum removal of Cr(VI) ions for an initial concentration of 50 mg/l by UASP was measured as: adsorbent dose of 1 g/l, pH of 3.0, contact time of 30 min and temperature of 303 K. Adsorption isotherm, kinetics and thermodynamic parameters were calculated. Freundlich model provided the best results for the removal of Cr(VI) ions by UASP. The adsorption kinetics of Cr(VI) ions onto UASP showed that the pseudo‐first‐order model was well in line with the experimental data. In the thermodynamic study, the parameters like Gibb's free energy, enthalpy and entropy changes were evaluated. This result explains that the adsorption of Cr(VI) ions onto the UASP was exothermic and spontaneous in nature. Desorption of the biosorbent was done using different desorbing agents in which NaOH gave the best result. The prepared material showed higher affinity for the removal of Cr(VI) ions and this may be an alternative material to the existing commercial adsorbents.

1 Introduction

For many years, heavy metals removal from water is going to be a main concern. In edible agricultural crops and drinking water sources, heavy metals that are present would be dangerous to human. These heavy metals are toxic in nature, e.g. these heavy metals spoil nerves, liver and bones, and also hinder functional group of essential enzymes [1, 2]. Heavy metals are available in waste water including copper, chromium, zinc, lead, cadmium, mercury and so on, from various industries like dyes and pigments, metal cleaning and plating baths, paper and pulp, refineries, ceramic paints, tanning, wood preserving, catalysis chemical manufacturing, glass and so on, and in that from 5 to 220 mg/dm³ of chromium(VI) is present which run out into environment. This released compound will eventually pollute water and soil [3, 4, 5, 6, 7]. Two forms of chromium are available, such as hexavalent and trivalent. In which, hexavalent is generally available and dangerous to biological activities [8]. Cr(VI) allowable limits for portable water and the effluent which is released from industries are 0.05 and 0.1 mg/l. Above the permissible limit, Cr(VI) is going to provide the harmful effect to the environment. It is essential to provide the treatment technologies for the removal of heavy metals from the water and wastewater. There are extensive ranges of chemical and physical processes that are available such as ultrafiltration, chemical precipitation, electro‐chemical precipitation, ion exchange, reverse osmosis, electro‐dialysis and adsorption [9, 10, 11, 12, 13]. Compared with other methods, the adsorption method is very cheap and effective method for the removal of toxic pollutants from wastewater [14, 15, 16]. The major benefits of the adsorption processes are high selectivity, recovery of the heavy metal ions, simplicity of design, less sludge volume produced and the meeting of strict discharge specification. An effective adsorptive material should have some appropriate active functional groups that should interact with heavy metal ions and also have an insoluble porous matrix [17, 18]. Conventionally, the most widely used adsorbent was of activated carbon. But the initial cost of activated carbon was high and also in problem with the tedious regeneration process which restricts its application in the wastewater treatment. Hence, the usage of low‐cost adsorbent interest is increased nowadays for the removal of heavy metal ions from aqueous solution [19, 20]. There are various type of adsorbents used for the removal of heavy metals. In the beginning, the researchers employed the living organisms for heavy metal removal, upon noticing that dead biomass has the potential to adsorb the heavy metals. Most of the studies focused to remove the heavy metals by biological method using dead biomass. The maximum amount of heavy metal removal using non‐living biomass of algae [21], bacteria [22], fungi [23] and yeasts [24] was reported. Among the biomasses and metal ions, the affinity is highly important which causes the latter occupation and which bound to the biomass. Compared with the above studied biomasses, algae are commonly offered, because of its cheapness, easy processing, quick, large and extensive availability [25]. Due to the presence of proteins, lipid or polysaccharides in the cell wall structure, the metal binding capacities of the biological materials, i.e. marine alga was found to be high. In recent years, algae are preferred tremendously for heavy metals removal. Algae are found to have metal binding capacities which can accumulate wide range of metal species [26, 27].

This study investigated the preparation of ultrasonic assisted Spirulina platensis (UASP) for the removal of Cr(VI) ions from aqueous solution. Spirulina is a blue green algae. It is an alternative source of protein for human food. Spirulina dry biomass was used by several scientists to remove the heavy metals from the aqueous solutions. Spirulina was used not only to remove the heavy metals, but also to remove the food dyes from aqueous solutions [28]. Biosorbent separates the heavy metals based on the metal binding capacity of material. The optimisation of adsorption parameters like pH, initial metal ion concentration, adsorbent dose, contact time and temperature was carried out. The adsorbent characterisation was done to determine the functional group in adsorbent by using Fourier transform infrared spectroscopy (FTIR), to understand the crystallinity of adsorbent by X‐ray diffraction (XRD), to study morphology and compound present in the material using scanning electron spectroscopy and energy dispersive X‐ray (SEM‐EDX) and thermal stability of the material is tested by thermogravimetric analysis (TGA). Adsorption isotherm and kinetics were studied to understand the mechanism of Cr(VI) ion onto the UASP. Thermodynamic study was carried out to estimate the Gibb's free energy (ΔG °), enthalpy (ΔS °) and entropy (ΔS °) changes. Desorption was also carried out to recover and reuse the UASP biosorbent.

2 Materials and methods

2.1 Synthesis of Cr(VI) ion solution

Potassium dichromate (molecular formula: K₂ Cr₂ O₇, molecular weight: 294.19 g/mol) was purchased from Merck specialities private limited. A stock solution of 1000 mg/l of Cr(VI) ions was prepared by dissolving 2.83 g of potassium dichromate in 1000 ml of distilled water. The desired concentration of Cr(VI) ion solutions (50–750 mg/l) was prepared by diluting the stock solution using distilled water.

2.2 Synthesis of UASP

Spirulina platensis (micro algae) was purchased from Arwind enterprise, Vellore, Tamil nadu, India. This biomass was washed with distilled water and then dried in sunlight for 3 days. Then this dried biomass was grounded into powder form and then stored in the plastic container. The prepared materials were abbreviated as raw Spirulina platensis (RSP). Surface modified Spirulina platensis (SMSP) powders were prepared by using the ratio 1:2 by weight percentage of treated RSP with concentrated sulphuric acid. Then this mixture was left 24 h and then rinsed with distilled water to remove the excess acid for several times until pH≃7 is reached. At 80°C, the washed material was dried for 3 h. To increase surface area, this dried material was finely grounded. This surface modified alga was used for the treatment to get the UASP. The UASP adsorbent was synthesised by agitating 3.0 g of SMSP with 40 ml of distilled water. Sonication was continued with a speed of 500 rpm for 1 h and the working frequency of ultrasonicator was 24 kHz. This treated Spirulina platensis mixture was filtered, dried at 40°C for 24 h in oven [29] and then stored in desiccators to avoid moisture adsorption. UASP was used as an adsorbent for Cr(VI) ions removal from aqueous solution.

2.3 Experiments

Batch adsorption experiments were carried out to study the effect of various parameters such as dose, initial concentration, pH, contact time and temperature for Cr(VI) ions removal from aqueous solution. For all adsorption experiments, 100 ml of desired concentration of aqueous Cr(VI) ion solution with the optimised amount of UASP were taken in the 100 ml stoppered conical flask. These solutions were kept in temperature controlled incubation shaker at 80 rpm agitation with different temperatures and time intervals. After the agitation, these mixtures were centrifuged. The concentrations of residual Cr(VI) ions in the solution were then determined using atomic adsorption spectroscopy (AAS, SL 176, Elico Company, India). The percentage removal of Cr(VI) ion was calculated by using the following equation:

$$\text{\%}\mathrm{Removal}\mathrm{of}\mathrm{Cr}\left(\mathrm{VI}\right)\mathrm{ion}=\frac{\left(C_0-C_{\mathrm{e}}\right)}{C_0}\times 100$$ (1)

where C ₀ and C _e are the initial and equilibrium concentrations of Cr(VI) ions (mg/l) of solution.

2.4 Equilibrium studies

The adsorption isotherm study was conducted by mixing 1 g/l of UASP biosorbent into 100 ml of Cr(VI) ion solution with the various concentrations of Cr(VI) ion solutions from 50 to 750 mg/l at an optimum condition. The mixtures were agitated in the temperature controlled incubator at equilibrium time interval. Then the solutions were centrifuged and collected as a supernatant, which was used to analyse the concentrations of residual Cr(VI) ion. In this study, the four adsorption isotherm models such as Langmuir, Freundlich, Temkin and Redlich–Peterson models were investigated to study the Cr(VI) ion removal from aqueous solution by UASP. The amount of Cr(VI) ion adsorbed per unit mass of UASP as an adsorbent from aqueous solution was calculated by using the following equation:

$$q_{\mathrm{e}}=\frac{\left(C_0-C_{\mathrm{e}}\right)V}{m}$$ (2)

where q _e is the equilibrium adsorption capacity, V is the volume of Cr(VI) ion in the solution (L) and m is the mass of adsorbent used (g).

2.5 Kinetic studies

The kinetic studies were carried out using 1 g/l of UASP in the Cr(VI) ion solution of different concentration (50–750 mg/l) in a series of 100 ml stoppered flasks. They were stirred in temperature controlled shaker with different contact time (5–45 min) at a room temperature. After reaching equilibrium, the mixture was centrifuged and the Cr(VI) ion concentration in the supernatant was measured. The adsorption kinetic models such as pseudo‐first‐order, pseudo‐second‐order, Elovich and intraparticle diffusion models were applied to the kinetic data.

2.6 Thermodynamic studies

In the thermodynamic study, adsorption processes were conducted by adding 1 g/l of UASP adsorbent in a series of 100 ml of initial Cr(VI) ion concentration (50–750 mg/l) in the stoppered conical flask at different temperature (303–333 K) kept in the temperature controlled incubator for stirring. After adsorption, the adsorbents were separated from the mixture and the filtered solutions were tested to determine the Cr(VI) ion removal. From the study, the thermodynamic parameters like enthalpy (ΔH °), entropy (ΔS °) and Gibb's free energy (ΔG °) were calculated.

3 Result and discussion

3.1 Fourier transform infrared spectroscopy

The adsorptive nature of adsorbent (RSP, SMSP and UASP) based on the functional groups were determined by using the FTIR analysis in the range of 4000–400 cm⁻¹. Fig. 1 shows the FTIR spectrum of (a) RSP, (b) SMSP and (c) UASP before and (d) after adsorption. The peak at 3276 cm⁻¹ represents the OH stretching vibration and NH₂. Peaks observed at 2960, 2920 and 2851 cm⁻¹ corresponds to asymmetric and symmetric stretching of CH₂ groups, respectively. The adsorption bands at 1626, 1535 and 1451 cm⁻¹ could be assigned to the scissor bending of NH₂ group. C–N stretch of amide (or) amine groups were observed at 1236, 1154 and 1079 cm⁻¹ [30, 31]. Due to the sulphuric acid treatment, high carbonisation was observed in SMSP by decreasing intensity of the peak due to CH₂ stretching vibrations at 2927 cm⁻¹. The peak for UASP was observed at 3277 cm⁻¹ representing the OH stretching vibration mixed with NH₂. Peaks were present at 2955, 2918 and 2850 cm⁻¹ due to stretching of CH₂ groups. The adsorption bands at 1626, 1531 and 1452 cm⁻¹ were obtained due to the scissor bending of NH₂ group. At 1227, 1053 cm⁻¹, C–N stretch of amide (or) amine groups were observed. After adsorption, the intensities of the UASP peaks were observed to be less and there was also disappearance of the peaks at 2955 and 2850 cm⁻¹. It may be because of the attraction of Cr(VI) ions from aqueous solution. The functional groups OH stretching vibration mixed with NH₂ and asymmetric and symmetric stretching of CH₂ groups are responsible for attraction of Cr(VI) ion from solution onto the UASP adsorbent. This UASP material was found to be more carbonaceous. The FTIR spectrum confirms that the UASP material has potential to remove the Cr(VI) ion from aqueous solution by the adsorption process.

Fig. 1.

FTIR analysis of adsorbents

3.2 SEM and EDX

SEM is a very effective analysis to illustrate the surface morphology of the sample. Fig. 2 shows that rougher and irregular pores developed on the surface of the UASP material due to the ultrasonication of the SMSP. These active pore sites were responsible for the adsorption of Cr(VI) ion onto the UASP. The chemical composition of UASP by EDX analysis was observed from Fig. 2. The peaks of C (59.68 wt%), O (24.67 wt%), N (12.21 wt%) and Mo (3.44 wt%) were observed. The major compounds present in UASP material are C, O and N. The percentage of carbon present in the UASP was significantly more, which confirmed that the adsorbent was more carbonaceous and had better active sites to occupy Cr(VI) ion. Finally, it can be concluded that UASP has more adequate morphology for the Cr(VI) ion removal from aqueous solution.

Fig. 2.

SEM image and EDX analysis for UASP

3.3 XRD and TGA

The size, diameter and crystalline nature of the sample were studied by XRD. Fig. 3 a illustrates the XRD pattern of UASP prepared from Spirulina platensis which shows crystallinity of UASP and the broad peaks indicated less particle size. Three peaks were shown by the UASP at 2θ  = 9.549°, 19.455° and 46.033° corresponding to (111), (222) and (554) planes. Table 1 (a) clearly explained the crystal lattice and the average lattice parameter. UASP was in the form of cubic in structure which is confirmed by cubic structure calculation shown in Table 1 (a). The diameters of the particles were 3.362, 2.421 and 39.624 nm, respectively, as shown in Table 1 (b), which was determined by Debye–Scherrer formula using full width half maximum (FWHM) values shown in Table 1 (b). The size and the shape of the particles were calculated by using Debye–Scherrer formula [32]

$$D=\frac{0.9\lambda }{\beta \cos \theta }$$ (3)

Fig. 3.

XRD image and TGA for UASP

Table 1.

(a) Simple peak indexing for UASP

2 Theta (2θ)	Sinθ	Sin2θ	$1\times {\displaystyle \frac{\sin 2\theta }{{\sin }_{min}2\theta }}$	$2\times {\displaystyle \frac{\sin 2\theta }{{\sin }_{min}2\theta }}$	$3\times {\displaystyle \frac{\sin 2\theta }{{\sin }_{min}2\theta }}$	h 2  + k 2  + l 2	Cubic lattice (h k l)	a, Å
(a)
9.549	0.0831	0.0069	1	2	3	3	(111)	16.0417
19.455	0.1688	0.0285	4.1207	8.2415	12.3623	12	(222)	16.0417
46.033	0.3908	0.1527	22.068	44.136	66.2039	66	(554)	16.0417

where D is the particle diameter, λ is the wavelength of X‐ray (0.1541 nm), β is the FWHM and θ is the diffraction angle. This XRD analysis confirmed that the structure of the UASP particle was the face centred cubic crystal structure. The average lattice parameter is 16.0417 Å. This adsorbent UASP has more active sites for the adsorption of Cr(VI) ion.

Table 1.

(b) particle size of UASP adsorbent

2 Theta	Theta	Cubic lattice	a, Å	FHWM in degree	FHWM in radians	Size of particle (D), nm
(b)
9.549	4.7745	(1 1 1)	16.0417	2.373	0.0414	3.362
19.455	9.7275	(2 2 2)	16.0417	3.331	0.0581	2.421
46.033	23.0165	(5 5 4)	16.0417	0.218	0.0038	39.624

The thermal stability of UASP was analysed by the TGA shown in Fig. 3 b. The two stages of degradation were obtained for UASP. In the first stage, the weight loss was observed at 96.7°C caused by decomposition of water molecules and moisture present in the environment. At 291.4°C, the second stage of degradation was existed due to the decomposition of amine or amide group. After complete decomposition, 10.8% of char yield was present in the UASP which apparently explained that the material had excellent thermal properties and it may be as a result of surface modification of Spirulina platensis.

3.4 Brunauer–Emmet–Teller (BET) analysis

The surface area and pore volume of the adsorbent was measured by using BET analyser (Micromeritics ASAP 2020 Surface Area and Porosity Analyzer V3.00 H, USA). The adsorbent samples were degassed via helium purging at 150°C for about 12 h. The surface properties of the adsorbent were evaluated by using the adsorption isotherm of nitrogen gas and measured at 77 K. The estimated values of BET surface area and pore volume of the newly prepared UASP were 765.1 m² /g and 0.513 cm³ /g, respectively.

3.5 Optimisation of operating parameters on Cr(VI) ion biosorption

The effect of UASP dose (Fig. 4 a) for an initial concentration of 50 mg/l to remove Cr(VI) ion was performed with different adsorbent doses (0.2–1.4 g/l). The removal of Cr(VI) ions was increased with increase in the adsorbent dose in the range of 0.2–1 g/l. This may be due to the fact that the active sites of the adsorbent was increased with the increase in adsorbent dose [33]. Also it can be explained that the exposed surface area for the adsorption was increased with the increase in the adsorbent dose. After the addition of 1 g/l dose, the removal was remained constant. This may be due to the reduction in the concentration gradient between the Cr(VI) ions in the adsorbent surface and Cr(VI) ions in the liquid solution. The maximum removal of Cr(VI) ions was obtained to be 99.814% for UASP at an optimum dose of 1 g/l.

Fig. 4.

Optimisation of parameters for Cr(VI) ion removal by UASP

The effect of pH for the removal of Cr(VI) from aqueous solution by UASP adsorbent was studied (in the range of 2–7). Heavy metal removal by adsorption depends on the pH of aqueous solution that influences the species of biosorbent, biosorbent's surface charges and degree of ionisation [34]. Based on the pH, chromium ions are available in five different forms, which are H₂ CrO₄ (pH < 1), HCrO₄ ⁻ and $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}$ (2<pH>6) and ${\mathrm{CrO}}_4^{2-}$ (pH>6). Fig. 4 b shows that the percentage of removal of Cr(VI) ion by UASP decreased when pH was increased from 2 to 7. At low pH, the amino groups in the UASP adsorbent surface were the highly protonated cations ($-{\mathrm{NH}}_3^{+}$), which were easily attracted by the negatively charged Cr(VI) ions (in the form of ${\mathrm{HCrO}}_4^{-}$ and $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}$ at 2<PH>6) from the solution. When pH was increased, the adsorption is weakened due to the large quantity of OH⁻ ions, which hindered the diffusion of chromate ions (${\mathrm{CrO}}_4^{2-}$) [35, 36, 37]. The following reactions clearly explained the protonation of amino groups of UASP and electrostatic attraction of the positively charged amino groups (${\mathrm{NH}}_3^{+}$) in the UASP adsorbent and the negatively charged chromium ions (${\mathrm{HCrO}}_4^{-}$ and $\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}$) in the solution.

The amino group's protonation reaction

$$\mathrm{R}-\mathrm{N}{\mathrm{H}}_2+{\mathrm{H}}^{+}\leftrightarrow \mathrm{R}-{\mathrm{NH}}_3^{+}$$ (4)

The adsorption of chromium ion from aqueous solution to the UASP

$${\mathrm{HCrO}}_4^{-}+\mathrm{R}-\mathrm{N}{\mathrm{H}}_2\leftrightarrow {\mathrm{HCrO}}_4^{-}\dots .{}^{+}\mathrm{N}{\mathrm{H}}_3-\mathrm{R}$$ (5)

$$\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}+\mathrm{R}-\mathrm{N}{\mathrm{H}}_2\leftrightarrow \mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}\dots .{}^{+}\mathrm{N}{\mathrm{H}}_3-\mathrm{R}$$ (6)

The maximum (99.814%) Cr(VI) removal was observed around pH = 3 using UASP adsorbent which may be due to higher H⁺ ions on the adsorption surface at acidic pH. Hence, there was a strong electrostatic attraction between positively charged UASP surface and negatively charged Cr(VI) ions.

The initial concentration of metal ions could change the efficiency of the adsorption process and hence it is an important factor in its removal using adsorption. The mass transfer resistance between the solid metal and aqueous phases is overcome by the initial concentration of solution and it gives important driving force to them [38]. In this study, various concentrations of aqueous solution were prepared in the range from 50 to 750 mg/l with optimum 1 g/l of UASP biosorbent. Fig. 4 c illustrates the percentage of removal of Cr(VI) which decreased from 99.975 to 75.523% for UASP with increasing initial concentration of Cr(VI), due to the adequate active sites at lower concentration that adsorbate could easily be occupied. However, there were no enough active sites available at higher concentration. Due to the saturation of binding sites, Cr(VI) ions were not possible to adsorb completely onto UASP.

It is also necessary to study the effect of contact time on the adsorption of Cr(VI) ions from aqueous solution using UASP adsorbent. This adsorption study was carried out using optimum 1 g/l of UASP adsorbent with different initial concentration such as from 50 to 750 mg/l under the contact time varied from 5 to 45 min. It is clear from Fig. 4 d that the adsorption of Cr(VI) ions increased sharply on increase in the contact time which may be due to the more availability of unfilled active sites of UASP adsorbent. Equilibrium was reached at 30 min after which the adsorption remained constant. Maximum percentage of removal of Cr(VI) ion occurred at the optimum contact time of 30 min with varied concentrations.

Temperature is the important parameter for metal removal from solution. This study was carried out at various temperatures such as 303, 313, 323 and 333 K for the removal of Cr(VI) ion using 1 g/l of UASP adsorbent from aqueous solutions concentration ranging from 50 to 750 mg/l at 30 min contact time as shown in Fig. 4 e. Fig. 4 e certainly revealed that the percentage removal of Cr(VI) ion decreased with increasing temperature. At 303 K, maximum percentage of removal was obtained for different Cr(VI) ion solution which confirms that maximum adsorption occurred at low temperature.

3.6 Adsorption equilibrium studies

In this isotherm experiment, the amount of adsorption capacity and the nature of adsorbent (UASP) were studied for different initial concentration of Cr(VI) ion solutions. Adsorption equilibrium was achieved using initial concentration in the range of 50–750 mg/l with 1 g/l of adsorbent UASP. The existed equilibrium data were analysed using Langmuir, Freundlich, Temkin and Redlich–Peterson isotherm model. Langmuir isotherm model is the monolayer adsorption model and the equation of the Langmuir isotherm [39, 40] is as follows:

$$q_{\mathrm{e}}=\frac{q_{\mathrm{m}}{K}_{\mathrm{L}}{C}_{\mathrm{e}}}{1+K_{\mathrm{L}}{C}_{\mathrm{e}}}$$ (7)

where q _m is the maximum monolayer adsorption capacity (mg/g), K _L is the Langmuir constant (l/mg) related to affinity of the metal ions to the adsorbent.

Freundlich model is the multilayer adsorption model [41] which can be explained as follows:

$$q_{\mathrm{e}}=K_{\mathrm{F}}{C}_{\mathrm{e}}^{1/n}$$ (8)

where q _e is the amount of adsorption at equilibrium (mg/g), K _F is the Freundlich constant [(mg/g)(l/mg)^1/n ] and n is the Freundlich exponent constant.

Temkin model is also the empirical equation originally projected by Temkin [42] which is used to understand the indirect adsorbate/adsorbate attraction and its equation is given by

$$q_{\mathrm{e}}=B\log A_{\mathrm{T}}+\mathrm{B}\log C_{\mathrm{e}}$$ (9)

where A _T is the Temkin isotherm equilibrium binding constant (l/g), B  = RT /b, b is the constant related to the heat of sorption (J/mol), T is the absolute temperature (K) and R is the gas constant (8.314 J/mol K).

Redlich–Peterson model is a combination of both Langmuir and Freundlich models [43]. At lower concentration, it follows Langmuir model, whereas at higher concentration limit it behave as Freundlich. This equation can be explained as follows:

$$q_{\mathrm{e}}=\frac{K_{\mathrm{RP}}{C}_{\mathrm{e}}}{1+{\alpha }_{\mathrm{RP}}{C}_{\mathrm{e}}^{{\beta }_{\mathrm{RP}}}}$$ (10)

where K _RP is the Redlich–Peterson constant (l/g), α _RP and β _RP are the Redlich–Peterson parameters (l/mg). The significance of this model lies on the value of β. Freundlich and Langmuir isotherms are preferred when β  = 0 and β  = 1, respectively.

Plot of C _e versus q _e (Fig. 5) was used to determine the parameters such as the equilibrium adsorption capacity (q _e), Langmuir constant (q _m and K _L), Freundlich constants (K _F and n), Temkin constants (A and B) and Redlich–Peterson constants (K _RP, α _RP and β _RP) for these four isotherm models. Table 2 shows the non‐linear regression coefficients such as R ² (coefficient of determination) and error values, i.e. sum of squared errors (SSE) and root mean squared errors (RMSE). Based on the high values of R ² and low error values, the best fitted isotherm model was found to fall in the following order: Freundlich>Redlich–Peterson >Langmuir>Temkin. The Freundlich parameter ‘n’ lies between 0 and 10 which clears that the adsorption of Cr(VI) ions onto UASP was physical adsorption. The results showed that the Freundlich model was best obeyed with the adsorption equilibrium data than the other isotherm models. This indicates the heterogeneous and multilayer adsorption of Cr(VI) ions onto the UASP. The Langmuir maximum monolayer adsorption capacity of the adsorbent was obtained as 577.9 mg/g. From the Temkin model, it can be seen that the heat of adsorption (b) values for the present adsorption system was found to be <8 kJ/mol and which clears the weak interaction between the Cr(VI) ions and the adsorbent. This indicates that the present adsorption system is of physical adsorption. From the Redlich–Peterson isotherm, it can be seen that the value of β _RP was found to be 0.7398. β _RP is an exponent which lies between 0 and 1. Significance of β _RP is as follows: β _RP  = 0, this Redlich–Peterson equation was transformed into Henry's law equation; β _RP  = 1, this Redlich–Peterson equation was transformed into Langmuir equation. In other way, it can be discussed that at low Cr(VI) ions concentration the Redlich–Peterson equation was transferred to Henry's law and at high Cr(VI) ions concentration this can be transferred to Freundlich isotherm model. From the isotherm results, it can be concluded that the present adsorption system was best discussed by the Freundlich isotherm model based on the higher correlation coefficient and low error values.

Fig. 5.

Adsorption isotherm fitting curve for Cr(VI) ion removal by UASP

Table 2.

Isotherm results for UASP‐Cr(VI) ions system

Isotherm model	Parameter	R 2	SSE	RMSE
Langmuir	q m  = 577.9 (mg/g)	0.9374	2.683	4.543
	K L  = 0.0991 (l/mg)
Freundlich	K F  = 158.3 ((mg/g)(l/mg)(1/n))	0.9846	0.598	1.345
	n  = 3.819
Temkin	A T  = 26.91 (l/g)	0.8827	5.027	6.218
	B  = 26.83
	b  = 93.89 J/mol
Redlich–Peterson	α RP  = 1.568 (l/mg)	0.9840	6.618	2.256
	β RP  = 0.7398
	k RP  = 2.54 (l/g)

In Table 3, the maximum monolayer adsorption capacity of the UASP adsorbent was compared with the different adsorbents and clears that the UASP adsorbent has very large and impressive adsorption capacity.

Table 3.

Comparison of maximum monolayer adsorption capacity of different adsorbents for the Cr(VI) ion removal

Adsorbents	q m, mg/g	References
UASP	577.9	Present work
acid and thermal activated bentonite	250	[44]
PANI‐Fe/OMC	172.33	[45]
acid activated bentonite	133.33	[44]
thermal activated bentonite	89.28	[44]
raw bentonite	66.66	[44]
neem sawdust	58.82	[46]
mango sawdust	37.73	[46]
wheat husk	28.08	[46]
C. virgatum (red algae)	26.5	[47]
sugarcane bagasse	23.8	[46]
orange peel	19.8	[46]
oxidised carbon (peanut shell)	16.26	[48]
unoxidised carbon (peanut shell)	14.31	[48]
custard apple (Annona Squamosa) peel powder	7.874	[49]

3.7 Adsorption kinetics and mechanism

In the kinetic experiment, initial Cr(VI) ion concentrations (50–750 mg/l) on 1 g/l of UASP adsorbent were carried out with varying contact time (5–45 min). The kinetics for the adsorption Cr(VI) ions by UASP was examined with pseudo‐first‐order, pseudo‐second‐order and Elovich models.

First, pseudo‐first‐order kinetic model [50] equation is explained as

$$q_t=q_{\mathrm{e}}\left(1-\exp \left(-k_{1}t\right)\right)$$ (11)

where k ₁ is pseudo‐first‐order kinetics rate constant (min⁻¹) and t is the time (min).

Second, pseudo‐second‐order kinetic model [51] is explained as

$$q_t=\frac{q_{\mathrm{e}}^2{k}_{2}t}{1+q_{\mathrm{e}}{k}_{2}t}$$ (12)

where k ₂ is the pseudo‐second‐order kinetic rate constant (min⁻¹).

Finally, Elovich kinetic model [52] is explained as

$$q_t=\left(1+{\beta }_E\right)\ln \left(1+{\alpha }_E{\beta }_{E}t\right)$$ (13)

where α_E and β_E are the initial adsorption rate mg/(g min) and desorption rate constant.

Parameters and correlation coefficient (R ²) for these kinetic models are shown in Tables 4 (a) and (b). From Fig. 6, higher R ² (0.968–0.994) values were obtained for all the initial concentrations which clears that pseudo‐first‐order was applicable when compared with all other kinetic models. Also comparing the calculated adsorption capacity (q _e, cal) and experimental adsorption capacity (q _e, exp), the adsorption capacities evaluated from pseudo‐first‐order for various initial concentrations were very closer to the experimental adsorption capacity than the other models. R ² value obtained from Elovich explains only the heterogeneous adsorbent in adsorption process rather than the adsorption mechanism. From this result, on increasing the concentration of Cr(VI) ion solution, increase in the adsorption capacity and decrease in the rate constant was observed. As at the lower concentration, there was lesser competition for adsorbate to occupy the active sites of adsorbent, but at the higher concentration, the competition for the active sites of adsorbent was significantly very high [53].

Table 4.

(a) Adsorption kinetics parameters and coefficient of determination values for the adsorption of Cr(VI) ions (50–400 mg/l) onto UASP adsorbent and (b) adsorption kinetics parameters and coefficient of determination values for the adsorption of Cr(VI) ions (450–750 mg/l) onto UASP adsorbent

C 0, mg/l	q e (exp)	Pseudo‐first‐order			Pseudo‐second‐order			Elovich			Intraparticle diffusion
		k 1, min−1	q e, mg/g	R 2	k 2, g/mg min	q e, mg/g	R 2	αE , mg/(g min)	βE , g/mg	R 2	k 4, mg/(g min1/2)	R 2
(a)
50	50.01	0.0995	51.16	0.994	0.0020	60.5	0.992	0.155	5.33	0.986	8.66	0.924
100	99.97	0.0849	104.1	0.994	0.00068	130	0.992	0.037	13.5	0.986	17.01	0.952
150	149.36	0.05	293	0.983	0.00015	362.1	0.987	0.005	45.8	0.983	40.11	0.972
200	196.99	0.0683	212.9	0.990	0.00022	280.7	0.988	0.008	33.4	0.984	32.73	0.971
250	243.81	0.05	293	0.983	0.00015	362.1	0.987	0.005	45.8	0.983	40.11	0.972
300	288.467	0.0583	323.9	0.99	0.00011	444.1	0.986	0.001	87.5	0.965	47.11	0.971
350	332.98	0.05	393.2	0.986	7.5 × 10−5	545.3	0.981	0.001	95.4	0.977	53.78	0.966
400	375.52	0.0491	444.1	0.985	5.9 × 10−5	635.7	0.981	0.001	102.	0.976	60.26	0.963
(b)
450	416.73	0.045	505.6	0.984	4.6 × 10−5	737.5	0.980	0.001	109.5	0.976	66.35	0.959
500	457.52	0.041	572.2	0.982	3.5 × 10−5	852.7	0.978	0.0007	128.9	0.975	72.18	0.952
550	491.75	0.038	635.7	0.980	2.8 × 10−5	966.8	0.976	0.0005	151	0.973	77.16	0.944
600	517.97	0.036	686.8	0.977	2.3 × 10−5	1061	0.973	0.0004	169.5	0.969	80.97	0.936
650	547.49	0.032	765.5	0.972	1.8 × 10−5	1200	0.969	0.0002	201.8	0.966	84.69	0.924
700	568.65	0.030	822	0.969	1.4 × 10−5	1327	0.966	0.0002	225.6	0.963	87.54	0.915
750	568.94	0.030	832.2	0.968	1.4 × 10−5	1354	0.964	0.0002	233	0.961	87.51	0.909

Fig. 6.

Adsorption kinetic fitting curve for Cr(VI) ion removal by UASP (initial concentration = 50–750 mg/l, dose = 1 g and volume of the sample = 100 ml)

To determine the rate limiting steps for the adsorption of Cr(VI) ions on UASP, the adsorption mechanism was essential, which was not described by pseudo‐first‐order, pseudo‐second‐order and Elovich kinetic models. Hence, intraparticle diffusion model was used to explain the diffusion mechanism [54]. The intraparticle diffusion model is explained as follows:

$$q_t=k_p{t}^{1/2}$$ (14)

where k _p is the intraparticle diffusion rate constant (mg/(g min^0.5)), which can be determined by the slope from linear plot q _t versus time shown in Fig. 6. Tables 4 (a) and (b) clear that when the concentration of adsorbate in the solution was increased from 50–750 mg/l, the rate constant value also increased but the correlation coefficient decreased. The adsorption of Cr(VI) ions on UASP was explained by three consecutive stages. The external surface adsorption or rapid adsorption was occurred at the first stage. This diffusion normally was neglected by stirring and so it was not a rate limiting step. In the second stage, gradual diffusion was observed into the pores of adsorbent. The rate of diffusion was controlled by intraparticle diffusion in this step. In the final stage, intraparticle diffusion was slow down because of the lower adsorbate concentration in the solution and the diffusion take place on micropores of the adsorbent [55]. Intraparticle diffusion is only the rate controlling step, if the plot is linear and passed through the origin. From Fig. 6 it was observed that the plot is not linear and it is not passed through the origin but it has different regions. This indicates that the intraparticle diffusion is not only the rate controlling step in the removal of Cr(VI) ions by UASP.

3.8 Thermodynamic studies

From thermodynamic study, the spontaneity of the process and type of process were studied based on the thermodynamic parameters such as entropy (ΔS °), enthalpy (ΔH °) and Gibbs free energy (ΔG °). These parameters were explained as follows:

$$K_c=\frac{C_{\mathrm{Ae}}}{C_{\mathrm{e}}}$$ (15)

$$\mathrm{\Delta }{G}^{\circ }=-RT\ln \left(K_c\right)$$ (16)

$$\mathrm{\Delta }{G}^{\circ }=\mathrm{\Delta }{H}^{\circ }-T\mathrm{\Delta }{S}^{\circ }$$ (17)

$$\log K_c=\frac{\mathrm{\Delta }{S}^{\circ }}{2.303R}-\frac{\mathrm{\Delta }{H}^{\circ }}{2.303RT}$$ (18)

where K_c is the equilibrium constant, C _e is the equilibrium concentration of Cr(VI) ions of solution (mg/l), C _Ae is the amount of metal ions adsorbed on the adsorbent per litre of solution (mg/l), R is the gas constant (8.314 J/(mol K)) and T is the temperature (K).

Slope and intercept of the plot of 1/T and Log K _c (Fig. 7) were used to determine ΔH ° and ΔS °. ΔG ° was calculated using the above equation. The result from Table 5 shows that the negative values of ΔG ° indicated that the adsorption of Cr(VI) ion on the adsorbent process was feasible and spontaneous. The negative value of ΔH ° explains that Cr(VI) ion removal by adsorption on adsorbent was exothermic process. The value of ΔS ° was found to be negative which shows that the adsorption of Cr(VI) ion onto the adsorbent was enthalpy driven.

Fig. 7.

Thermodynamic studies for Cr(VI) ion removal by UASP

Table 5.

Adsorption thermodynamic studies for the removal of Cr(VI) ions from aqueous solution by UASP adsorbent

C 0, mg/l	ΔH °, kJ/mol	ΔS °, J/mol/K	ΔG °, kJ/mol
			303 K	313 K	323 K	333 K
50	−127.213	−352.307	−20.897	−15.653	−12.520	−9.241
100	−81.317	−216.554	−15.836	−12.875	−10.526	−8.493
150	−54.317	−142.953	−11.964	−9.610	−8.000	−7.021
200	−43.061	−108.469	−10.418	−8.550	−7.416	−6.502
250	−31.803	−75.133	−9.081	−8.033	−6.986	−6.250
300	−27.514	−64.028	−8.036	−7.353	−6.450	−5.576
350	−24.201	−55.086	−7.440	−6.837	−6.111	−5.251
400	−22.823	−52.903	−6.826	−6.082	−5.240	−4.827
450	−18.180	−39.289	−6.302	−5.658	−5.123	−4.663
500	−15.589	−31.88	−5.883	−5.477	−4.995	−4.480
550	−13.092	−25.465	−5.359	−4.995	−4.521	−4.207
600	−10.448	−19.013	−4.627	−4.398	−4.151	−3.657
650	−9.451	−17.309	−4.180	−3.916	−3.661	−3.316
700	−8.124	−14.762	−3.661	−3.355	−3.173	−2.909
750	−7.203	−14.322	−2.838	−2.636	−2.482	−2.166

3.9 Effect of salts and desorption study

The industrial waste water contains various other salts along with the chromium ions. These salts were observed to influence the removal of Cr(VI) ions by UASP biosorbent. In this study, different salts such as NH₄ Cl, NH₄ NO₃, KNO₃, MgSO₄ and Cr(VI) pure were used to study the change in removal of Cr(VI) ions using UASP biosorbent. In this analysis, 100 ml of 100 mg/l of initial concentration of Cr(VI) ion solution with optimum quantity of UASP adsorbent were taken with different salts in the stoppered conical flask. The mixture was kept for 60 min with 80 rpm stirring speed in the temperature controlled incubator. Fig. 8 a shows the effect of various salts during the adsorption of Cr(VI) ion on UASP biosorbent. The result shows that 99.81% of removal was observed for Cr(VI) alone and 55.252% of removal of Cr(VI) in the presence of ${\mathrm{SO}}_4^{2-}$, which was responsible for the maximum reduction in the removal of Cr(VI) ions. The effects of NH₄ Cl, NH₄ NO₃, KNO₃ salts in Cr(VI) ion removal were less compared with ${\mathrm{SO}}_4^{2-}$ ion dissolved in solution. These results clear that the presence of salts inhibit the Cr(VI) ion removal from aqueous solution by UASP biosorbent.

Fig. 8.

Effect of salts in the Cr(VI) ion removal and desorption of Cr(VI) ion from UASP

Desorption and regeneration of adsorbent is important to improve the efficiency of adsorbent and also reduce the cost of the adsorption process. The ability of desorption and regeneration of UASP was investigated using desorbing agents such as NaOH, KOH, HNO₃, HCl, H₂ SO₄ and HCHO, respectively. In this study, 0.1 g/l of Cr(VI) ion adsorbed on UASP powder with 100 ml of NaOH, KOH, HNO₃, HCl, H₂ SO₄ and HCHO as desorbing agents in 100 ml Erlenmeyer flask was stirred for 1 h at room temperature. The percentage of Cr(VI) ion released after desorption with different desorbing agents was calculated using AAS analyser. Fig. 8 b shows that NaOH was more efficient than other desorbing agents like HNO₃, KOH, HCl, H₂ SO₄ and HCHO. Desorption of Cr(VI) ions from Cr(VI) loaded UASP with NaOH was 94.48% of metal recovery. These results reveal that UASP can be an efficient adsorbent for the Cr(VI) ions removal from aqueous solution. The scheme of the adsorption/desorption mechanism is given in Fig. 9.

Fig. 9.

Scheme of the adsorption/desorption mechanism

4 Conclusion

UASP was studied for the removal of Cr(VI) ions from aqueous solution. The functional groups, size, crystalline structure, thermal stability and surface properties of UASP adsorbent were determined by using FTIR, XRD, TGA and SEM‐EDX analysis. For Cr(VI) ion‐UASP system, the optimum conditions in the adsorption were 50 mg/l of initial concentration of Cr(VI) ion, 1 g/l of adsorbent dose, 2.0 of pH, 30 min of contact time and temperature of 303 K. The percentage of Cr(VI) ions were increased with increasing the amount of adsorbent. The maximum adsorption was observed at the optimum adsorption dose. On further addition of dose, there was no much change in the removal of Cr(VI) ion by UASP because excess active sites were available even after the adsorption process. Hence, the minimum quantity of adsorbent was needed for this adsorption. The contact time for this process was increased to increase the removal percentage of Cr(VI) ions. The removal of Cr(VI) ions was increased when the temperature and pH of solution were decreased. At low pH, maximum removal was occurred. The adsorption isotherm was used to understand that Freundlich isotherm model was fitted well compared with other models based on the maximum correlation coefficient with less error values. This adsorption kinetics for Cr(VI) ion removal clearly explained that the pseudo‐first‐order model was the best model for this present study compared with other models. Thermodynamic parameters such as Gibb's free energy (ΔG °), enthalpy (ΔH °) and entropy (ΔS °) were calculated from thermodynamic study, which reveals that the UASP‐Cr(VI) system was spontaneous, exothermic. The desorption of the Cr(VI) ion from UASP using desorbing agents like NaOH, KOH, HNO₃, HCl, H₂ SO₄ and HCHO was used to improve efficiency of adsorbent. In the desorption process, 94.487% of Cr(VI) ions were removed from UASP surface using NaOH as a desorbing agent.