Sequestration of Pb(II) and Ni(II) ions from aqueous solution using microalga Rhizoclonium hookeri: adsorption thermodynamics, kinetics, and equilibrium studies

In the present study, the microalga Rhizoclonium hookeri (RH) was effectively applied to remove the metal ions [Pb(II) and Ni(II)] from aqueous solution in batch adsorption mode experiments. The adsorption process was influenced by several operating parameters such as initial metal ion concentration, contact time, pH, particle size, adsorbent dose, and temperature. The maximum monolayer adsorption capacity of the RH was found to be 81.7 mg g −1 and 65.81 mg g −1 for Pb(II) and Ni(II) ions, respectively, at optimum conditions. The calculated thermodynamic parameters illustrated that the adsorption process was found to be spontaneous and endothermic in nature. Experimental data were analyzed in terms of pseudo-first order, pseudo-second order, and Elovich kinetic models. The results showed that the removal of Pb(II) and Ni(II) ions followed the pseudo-second order kinetics. The adsorption isotherm data were described using two and three parameter models. The results indicate that the adsorption data were best fitted with the Sips isotherm model. Consequently, the microalga RH with good adsorbability and reusability could be used as an effective adsorbent for the adsorption of Pb(II) and Ni(II) ions from wastewater.


INTRODUCTION
Industrial and domestic wastewater has become a serious threat to water resources around the world. This wastewater mostly contains hazardous chemicals, such as heavy metals and organic toxicants which are persistent and gradually accumulate in food chains and can cause irreparable damage to people and the environment (Xu et al. ).
become a major concern for researchers in recent years. Several conventional methods have been employed for the removal of Pb(II) and Ni(II) from aqueous solution, such as chemical precipitation (Purkayastha et al. ), membrane separation (Alzahrani & Mohammad ), ion exchange (Fu & Wang ), reverse osmosis (Bhattacharya et al. ), electrodialysis (Slesarenko ), and electrochemical treatment (Tran et al. ). However, these methods are restricted due to their own demerits such as high cost, complexity, low efficiency, or waste disposal. Hence, researchers turned their attention towards adsorption technology, which is recognized to be an efficient method for the removal of these heavy metals from aqueous solution.
This adsorption can be done with various adsorbents, such as polymers (Gao et al. ), clay minerals (Lee & Tiwari ), oxides (Minju et al. ), carbon materials (Yang & Jiang ), and biosorbents (Dong et al. ). Among these, biosorbents seem to be very efficient for removal of heavy metals with low concentration and inexpensive material, fast metal recovery, and regeneration. Mostly, waste biomass has been used as a biosorbent, and mainly consists of various functional groups such as alcohol, aldehydes, ketones, carboxylic, ether, and phenol. These functional groups play a vital role in binding the heavy metals towards the surface of the biosorbents. To date, several studies on the potential of biosorbents such as non-living biomass (e.g., bark, lignin, shrimp, krill, squid, crab shell, etc.) Algae are a renewable natural biomass found abundantly all over the world because they have the capability to survive in dual environment conditions such as in fresh and marine brackish water. They also have several attractive features, such as low cost, excellent reusability, high metal sorption capacity, and CO 2 sequestration. In these respects, algae have been used as an effective biosorbent material for the sorption of heavy metals. In this study, algal biomass, Rhizoclonium hookeri (RH), was used as an effective biosorbent for the removal of Pb(II) and Ni(II) from wastewater. This species has the aforementioned biosorption properties and, in addition, it also has high mechanical stability to adapt to extreme conditions during the regeneration process.

Chemicals and equipment
The chemicals lead ( where C 0 and C e are the initial and final concentrations (mg L À1 ) of metal ions, respectively.

Adsorption equilibrium experiments
The adsorption equilibrium experiments were performed in 250 mL Erlenmeyer flasks with 100 mL metal ion solutions with an initial metal ions concentration of 250-1,000 mg L À1 .
The pH was maintained at 3.0-4.5 for Pb(II) and Ni (II) ion solution at 25 W C. The solution was allowed to shake in a shaker at optimum conditions. On reaching the equilibrium time, the aqueous solutions were filtered using a 0.45 μm Whatman filter paper and analyzed with an AAS.
The following formula was used to find the amount of metal ions adsorbed by the adsorbent at equilibrium: where q e is the adsorption capacity at equilibrium (mg g À1 ), V is the volume of metal ion solution (g), C e is the concentration of metal ions in the solution at equilibrium (mg L À1 ), and m is the mass of adsorbent (g). The data obtained from the adsorption equilibrium studies were used to fit with the different adsorption isotherm models to check the types of adsorption process. The two parameter models, namely, Langmuir () and Freundlich (), and the three parameter models, namely, Redlich & Peterson () and Sips () were used to fit the adsorption equilibrium data.
Two parameter adsorption isotherm model The Langmuir adsorption isotherm model of non-linear form is given as (Langmuir ): where q e is the adsorption capacity at equilibrium (mg g À1 ), K L is the Langmuir equilibrium constant related to the affinity of metal ions to the biosorbent (L mg À1 ), q m is the maximum monolayer adsorption capacity (mg g À1 ), and C e is the concentration of the metal ions in the solution at equilibrium (mg L À1 ).
The Freundlich adsorption isotherm model is given as (Freundlich ): related to the bonding energy and n is a measure of the deviation from the linearity of adsorption (g/L). The significance of 'n' is as follows: n ¼ 1 (linear); n < 1 (chemical process); n > 1 (physical process).

Three parameter adsorption isotherm model
The Redlich-Peterson adsorption isotherm model, a combination of Langmuir-Freundlich, is given as (Redlich & Peterson ): where K RP is the Redlich-Peterson isotherm constant (L g À1 ), β RP is the exponent which lies between 0 and 1, α RP is the Redlich-Peterson isotherm constant (L mg À1 ) 1/βRP .
The significance of 'β' is as follows: β ¼ 1 (Langmuir model); The Sips adsorption isotherm equation is given as (Sips ): where α s is the Sips model constant (L g À1 ) 1/βs , K s is the Sips model isotherm constant (L g À1 ) βs , and β S is the Sips model exponent. The constant β S is often regarded as the heterogeneity factor, with values close to 1 indicating a homogeneous binding site and values greater than 1 indicating a heterogeneous adsorption system.

Adsorption kinetic experiments
The adsorption kinetic studies were carried out by varying the contact time for the present adsorption system. The contact time was varied from 5 to 70 min and the temperature was given by the following equation: where q t is the quantity of metal ions absorbed onto the adsorbent at any time interval t (mg g À1 ), C o is the initial concentration of metal ions (mg L À1 ), C t is the concentration of metal ion solution at any time t (mg L À1 ), m is the mass of the adsorbent (g), and V is the volume of the metal ion solution (L). The kinetics models such as pseudo-first order, pseudo-second order, and Elovich kinetic models were fitted with the observed kinetic data.
The pseudo-first order kinetic model is given by the following equation (Lagergren ): where q t is the quantity of metal ions adsorbed at any time t (mg g À1 ), q e is the quantity of metal ions adsorbed at equilibrium (mg g À1 ), t is the time (min), and k 1 is the pseudo-first order kinetic rate constant (min À1 ).
The pseudo-second order kinetic model is given by the following equation (Ho & McKay ): where q t is the quantity of metal ions adsorbed at any time t (mg g À1 ), q e is the quantity of metal ions adsorbed at equilibrium (mg g À1 ), t is the time (min), and k 2 is the pseudosecond order kinetic rate constant (g mg À1 min À1 ).
The Elovich kinetic model is given by the following equation (Low ): where α E is the initial adsorption rate (mg g À1 min À1 ), β E is the desorption constant related to the activation energy of chemisorption (g mg À1 ).

Adsorption thermodynamic study
In this thermodynamics study the adsorption process was carried out at different temperatures for a constant metal ion concentration of 100 mg L À1 and the adsorbent dosage was added as 1 g L À1 . After the adsorption process, the supernatant solution was analyzed using AAS and the observed results were used to find the thermodynamic parameters: where K c is the equilibrium constant, C Ae is the amount of metal ions adsorbed on the adsorbent per liter of solution (mg L À1 ), C e is the equilibrium metal ion concentration in solution (mg L À1 ), T is the temperature (K), and R is the gas constant (8.314 J mol À1 K À1 ).

RESULTS AND DISCUSSION
Effect of temperature and thermodynamic study The effect of temperature on the adsorption of Pb(II) and Ni(II) ions using the microalga RH was examined at different temperatures (25 to 40 W C) with a fixed initial metal concentration (1,000 mg L À1 ) at the adsorbent dose of 1 g L À1 and at a pH of 4.5. As Figure 1(a) and 1(b) illustrate, the percentage removal and metal uptake capacity of Pb(II) and Ni(II) ions onto the adsorbent was slightly dependent on the solution temperature. It can be seen that the sorption of Pb(II) and Ni ( (11) and (12). The values of ΔH o and ΔS o were calculated from the slope and the intercept of the linear plot of log K c versus 1/T (Figure 2). The estimated thermodynamic parameters for the adsorption of Pb(II) and Ni(II) ions using RH are shown in Table 1. The negative value of Gibbs free energy increased when the temperature was increased from 25 to 30 W C, which suggests that a greater number of active sites was available at higher temperature, and it was concluded that the adsorption process was a spontaneous process. The positive value of standard enthalpy change (ΔH o ) and the standard entropy change (ΔS o ) indicated that the adsorption process was an endothermic process and enthalpy driven, respectively.

Effect of particle size
The particle size of RH was an influence in the removal of Pb(II) and Ni(II) ions from the aqueous solution. The  show the considerable deviations in the percentage removal and metal uptake capacity of Pb(II) and Ni(II) ions using the different particle sizes of adsorbent material. Consequently, the smallest particle size of 0.5 mm has better removal efficiency and metal uptake capacity when compared with 0.6, 0.7, and 1.0 mm particle sizes. Generally, the larger particle size has a tortuous pathway, longer diffusion path length to its interior surface and diffusional resistance due to the restriction posed by the pore size. Conversely, the particle size of 0.5 mm adsorbent material has a larger surface area which enhances the adsorption capacity. Therefore, the particle size of 0.5 mm was selected as the optimum particle size for the effective removal of Pb(II) and Ni(II) ions.   show that the removal of metal ions was increased with an increase in adsorbent dose from 1.0 to 5.0 g L À1 . This may be due to the fact that the active sites of the adsorbent were increased with the increase in the adsorbent dose.
The equilibrium adsorption capacity value was decreased with the increase in adsorbent dose. This result might be due to the availability of more active sites at lower dosage as compared to higher dosage, which has less active sites     Tables 2 and 3 show that the experimental values of adsorption capacity (q e , exp) were very close to the theoretical adsorption capacity (q e , cal) of the pseudo-second order kinetic model. The results suggest that the pseudo-second order kinetics is more applicable for the adsorption of Pb(II) and Ni(II) ions onto the RH. Consequently, the chemical adsorption was accepted as the rate controlling step in the present adsorption system.

Adsorption isotherm
Adsorption isotherm plays a vital role in optimizing the design of an adsorption system. The adsorption isotherm study gives the specific relationship between the  The maximum monolayer adsorption capacity (q max ) of the present adsorbent was compared with the other adsorbents for the removal of metal ions ( Table 6). The results show that the RH was found to be higher for metal ions as compared to the other low-cost adsorbents.

CONCLUSION
Based on the observations in the present study, it can be concluded that the microalga RH is an effective adsorbent for the removal of Pb(II) and Ni(II) ions from aqueous solution due to high efficiency, low-cost, reusability, and fast  for Pb(II) and Ni(II) ions, respectively. The thermodynamic studies showed that the adsorption process was spontaneous, enthalpy driven, and endothermic in nature.