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Journal of Chemistry
Volume 2015, Article ID 293054, 12 pages
http://dx.doi.org/10.1155/2015/293054
Research Article

Phragmites karka as a Biosorbent for the Removal of Mercury Metal Ions from Aqueous Solution: Effect of Modification

1Institute of Chemistry, University of the Punjab, Lahore 54590, Pakistan
2College of Earth and Environmental Sciences, University of the Punjab, Lahore 54590, Pakistan

Received 3 December 2014; Revised 3 February 2015; Accepted 3 February 2015

Academic Editor: Wenshan Guo

Copyright © 2015 Muhammad Hamid Raza et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Batch scale studies for the adsorption potential of novel biosorbent Phragmites karka (Trin), in its natural and treated forms, were performed for removal of mercury ions from aqueous solution. The study was carried out at different parameters to obtain optimum conditions of pH, biosorbent dose, agitation speed, time of contact, temperature, and initial metal ion concentration. To analyze the suitability of the process and maximum amount of metal uptake, Dubinin-Radushkevich (D-R) model, Freundlich isotherm, and Langmuir isotherm were applied. The values of for natural and treated biosorbents were found at 1.79 and 2.27 mg/g, respectively. The optimum values of contact time and agitation speed were found at 50 min and 150 rpm for natural biosorbent whereas 40 min and 100 rpm for treated biosorbent, respectively. The optimum biosorption capacities were observed at pH 4 and temperature 313 K for both natural P. karka and treated P. karka. values indicate that comparatively treated P. karka was more feasible for mercury adsorption compared to natural P. karka. Both pseudo-first-order and pseudo-second-order kinetic models were applied and it was found that data fit best to the pseudo-second-order kinetic model. Thermodynamic studies indicate that adsorption process was spontaneous, feasible, and endothermic.

1. Introduction

The generic name Phragmites is attributed to Trinius as the earlier Phragmites Adans is considered a redundant name for Saccharum Linn. Phragmites represents a part of Arundo Linn. It is a cosmopolitan genus of four species: P. karka (Retz.), Phragmites australis (Cav.), Phragmites japonicus Steud, and Phragmites mauritianus Kunth. Phragmites australis Trin and P. karka Trin are allopatric and are found to be in temperate and tropic species, respectively [1, 2]. P. karka (Retz.) is also known as Arundo karka (Retz.), Arundo roxburghii Kunth, Phragmites maxima, Phragmites nepalensis and Phragmites roxburghii (Kunth) Steud. It is a perennial reed having creeping rhizomes. Culms are erect up to 1.5–3 m high. Length and width of leaf blades range as 20–60 cm or more 8–32 mm, respectively, which are pungent and sometimes stiff. Panicles are 6–10 cm wide and 20–30 cm long, and the lowest node is usually few branched. Spikelets are 12–18 mm long, and the rhachilla hairs are 6–10 mm long, abundant, and silky [3]. The fertile lemmas are narrowly lanceolate and 9–13 mm long. The middle and upper stem internodes of American reed are smooth, shiny, and red-brown to dark red-brown during the growing season, while in common reed (Phragmites australis) middle to upper stem internodes are dull, ridged, and tan colored during the growing season. It is a perennial grass, abundantly found across railway lines and roadsides, near streams, and in marshes, ponds, wetlands, barrages, swamps, and water head works in New Guinea, Africa, Northern Australia, and Asia and is also widely distributed in Pakistan. In Pakistan, it is distributed in Punjab and Kashmir regions and temperate regions of both hemispheres. It is said to be poisonous to cattle (Duthie) and is not used as fodder. P. karka is mostly used as weaving material and decoration and for making musical instruments. It is also an excellent stabilizer of eroding river banks [13].

In this study, biosorption potential of P. karka (Trin Retz.) Steud, locally known as Khagra reed, was studied for heavy-metals uptake from aqueous solution, in single metal ion system as novel application. Heavy-metals pollution is a worldwide problem as they are nonbiodegradable and hazardous to human health and environment due to high bioaccumulation and magnification potential [27]. Lead, mercury, and cadmium are known as the three famous pollutants due to their high impact on the environment [8]. Natural sources of mercury include volcanic action, weathering of mercuriferrous rocks, degassing from surface water, biogenic emissions, and natural forest fires [9], while anthropogenic sources include chemical manufacturing, metallurgical and metal finishing industries, pulp and paper industry, pharmaceuticals, seed dressings, fungicides, dental preparations, thermometers, paints, and fluorescent and ultraviolet lamps [9, 10]. In humans, symptoms of mercury poisoning are malfunctioning of central nervous system, visual constriction, loss of memory hearing, and renal disturbance [11, 12]. Due to its high toxicity, European Union recommended maximum permissible limit of Hg in drinking and wastewater as 0.001 and 0.005 mgL−1, respectively [13, 14]. According to World Health Organization and China guidelines, permissible level of inorganic mercury in drinking water is 1.0 µg L−1 [15].

The term “Biosorption” describes the removal of the heavy metals from a solution by nonmetabolically mediated passive binding to living or dead biomass [1, 16]. This process is based on the fact that certain natural materials of biological origin have property of metal sequestering [17]. Depending on origin and physicochemical conditions, biosorption involves different processes which include metal ion exchange, coordination complexes, covalent linkage to biomass components, and physiologically mediated intracellular uptake [18, 19]. From last two decades, it is being considered as one of the most suitable economic and green technologies. The material reusability, no toxic sludge production, low operating cost, improved selectivity, and compressed operation time for certain metals are some of its advantages over conventional techniques [11, 20]. Microorganisms, fungi, algae, lichens, aquatic macrophytes and animals, waste materials and terrestrial plants in natural, and treated and modified forms are now being used to check their sorption potential by many researchers of the world. Furthermore, chemically treated biosorbents including orange peels, bagasse pith, coconut husks, and polyacrylamide-grafted banana stalks have been used for the biosorption of mercury ions [21].

The effects of agitation speed, biosorbent dose, pH, initial metal ion concentration, and temperature and contact time on adsorption potential were studied in this research. Characteristics of biosorbent were also studied by Fourier transform infrared spectroscopy. In addition, equilibrium studies were performed by applying Langmuir isotherm, Freundlich isotherms, and D-R model to calculate values and suitability of the sorption process. Kinetic studies were performed by applying pseudo-first-order and pseudo-second-order kinetics. Thermodynamics studies were also performed to analyze spontaneity of the biosorption process.

2. Material and Methods

2.1. Raw Biomass

Plant biomass selected for biosorption studies (P. karka, Trin) was collected from railway headquarters, Mughalpura, Lahore (25°58′25′′N, 68°38′15′′E). Leaves and stems of the plants were used for sorption studies because they are waste materials while its rhizome is used in herbal medicines.

2.2. Preparation of Biosorbent

P. karka (Trin) is a long plant with length about two meters. So, for ease of washing and handling it was first manually broken down into four to five pieces per plant. Those pieces were then washed with distilled water twice in the laboratory to remove all the litter. Washed plant biomass was first air dried for one day and then manually chopped with Chinese axe to convert into small size pieces for ease of oven drying. Oven drying was done for 14 hours at 90°C till constant mass was obtained. Oven dried plant biomass was ground in a shredder and then passed through mesh number 50 of US standard sieves to get biosorbent of same particle size, that is, 297 µm.

2.3. Chemical Treatment of Biosorbent

Powdered biosorbent was divided into two parts; one was kept in air tight glass container as such in natural form and used for further experimentation (named as natural P. karka), while second part was treated according to the method described elsewhere [1]. For treatment purpose, 200 g of powdered biosorbent was soaked in solution containing 1000 ml ethanol, 500 mL NaOH (0.5 mol/L), and 500 mL CaCl2 (1.5 mol/L) for 24 hours [1, 1114]. After repeated filtration and decantation of biosorbent, it was first dried in open air and then oven dried to get its constant mass. The material was stored in airtight plastic container and named as treated P. karka.

2.4. Analytical Instruments and Reagents

All chemicals used were of pure analytical grade purchased from Sigma Aldrich Inc. Glassware was soaked in 20% (v/v) nitric acid for 24 hours and then rinsed two to three times using double distilled water. Mercury stock solution (1000 mg/L) was prepared by dissolving specific quantity of mercuric chloride HgCl2 salt in doubly distilled water. Working and standard solutions of mercury were prepared just before use by required dilutions of stock solution. NaOH (0.1 M) and HCl (0.1 M) solutions were used for pH adjustment of solutions by digital pH meter (Iono Lab pH 720). Oven used was of Memmert (Germany) and solutions were agitated after addition of biosorbent on orbital shaker (IKA Germany Yellow line OS 10 basic). Filtrates were analyzed by AAS (Perkin Elmer Analyst 700 equipped with MHS 15 CVAAS system). The bandwidth of spectrum was 0.7 nm for mercury ions isolation, while 6 mA operating hollow cathode lamp was used. Sodium borohydride (2% w/v) in sodium hydroxide was used as reducing agent.

2.5. Batch Biosorption Procedure

Biosorption of mercury, Hg(II) metal ions, on natural and treated plant biomass (P. karka) was studied in batch fashion. The effects of temperature, pH, initial metal ion concentration, contact time, biosorbent dose, and agitation speed on adsorption capacity of biomass were investigated.

Batch pH experiments were performed for 200 mL solution of Hg(II) ions (10 mg/L) in Erlenmeyer flask (250 mL) with 2 g biosorbent, while the pH was varied from 2 to 8. The pH of solutions was adjusted initially and not thereafter. The flasks were agitated on orbital shaker at 150 rpm speed for 30 minutes at 298 K temperature. After each experiment, solutions were immediately filtered by using Whatman filter papers of grade 40 and filtrates were analyzed using CVAAS. Same process was done for other conditions by changing one parameter and keeping the rest constant.

The effect of biosorbent dose on sorption capacity was determined at six different doses in range of 1.0–6.0 g by keeping rest of parameters constant. The effect of temperature on the adsorption capacity of the biosorbent was determined at different temperatures (i.e., 20, 30, 40, and 50°C). Effect of agitation speed on sorption was studied at 100, 150, 200, 250, and 300 rpm. For determination of optimum contact time samples were drawn at regular intervals at 20, 30, 40, 50, and 60 min. Effect of initial metal ion concentration on biosorption was also studied at six different concentrations in range of 10–60 ppm. All experiments were performed in triplicate fashion and their average values were considered in analysis. Percent biosorptive removal and metal uptake capacity were calculated by the following equations: where (mg/L) and (mg/L) are the initial and final metal ion concentrations of solution, respectively, is the volume of metal ion solution (L), and is the mass of the biosorbent (g).

3. Results and Discussion

3.1. FTIR

FTIR spectroscopy is used to analyze the chemical functional groups on the biosorbent. FTIR spectra of the natural as well as treated biosorbent were recorded in 600–4000 cm−1 range and are shown in Figure 1.

Figure 1: FTIR of natural P. karka and treated P. karka (biosorbents).

The two peaks A and B represent FTIR spectra of natural P. karka and treated P. karka, respectively. Peak (I) of natural biosorbent sample at 3370.56 cm−1 indicates presence of (O-H) hydroxyl group in cellulose, hemicellulose, pectin, and lignin; it is shifted after treatment to 3290.56 cm−1 peak (I). Peak (II) at 2900.94 cm−1 in natural biosorbent is representative of stretching vibration of alkanes (C-H), that is, methyl, methylene, or methoxy group. This peak is shifted to 2947.23 cm−1 in treated biosorbent with greater intensity. Peak (III) represents presence of aldehydes group in natural P. karka at 2885.51 cm−1, whereas in treated biosorbent peak (III) is observed at 2885 cm−1 representing nitriles and alkyne groups. Peak (IV) at 1730.15 cm−1 represents carbonyl group (C=O) of aldehydes, whereas peak (IV) at treated biosorbent is at 2133 cm−1 representing carbonyl group of aldehydes and ketones. Peak (V) at 1633.71 cm−1 indicates presence of carboxylate group (COO-) and it is shifted to 1595.13 cm−1 in treated biosorbent. Peak (VI) for both biosorbents is at 1506 cm−1 representing N-O (nitro groups). Peak VII is at 1425.4 cm−1 which is shifted to 1435.04 cm−1 in treated biosorbent and represents presence of (C-H) group of aliphatic or aromatic hydrocarbons. Peaks (IX) in range of 1300–1000 cm−1 (1240.3 and 1163 cm−1 for natural and treated biosorbents, resp.) are representative of (C-O) stretching of alcohols (R-OH) and carboxylic acids (RCOOH). The appearance of peak (IX) at 1163 cm−1 in treated biosorbent shows presence of aldehydes and aliphatic amines specifically. The addition of two more peaks (X, XI) in treated biosorbent at 1064 cm−1 and 896 cm−1 again shows the presence of nitrile and unsaturation in the carbon chain. Results confirmed the changes in some functional groups of treated biosorbent.

3.2. Effect of pH on Biosorption Determination of pHpzc

Heavy metal ions sorption on the surface of biosorbent is a pH dependent process, which affects the functional groups of biosorbent and the metal ions chemistry in solution [2227]. As a result, pH affects the availability of metal ions in solution and metal binding sites on biosorbent surfaces. As shown in Figure 3, Hg(II) adsorption capacity at pH 3 was 0.3055 mg g−1 and 0.338 mg g−1 for natural P. karka and treated P. karka, respectively. At low pH values below 4, competition between protons and metal ions exists for same binding sites; hence, metal binding capacity decreases [28, 29]. Different functional groups on plant surface are important for the sorption of mercury. A previous work on parsley stated that carboxylate groups of plants play a major role in mercury ions bindings [29]. At lower value of pH (pH < 4), H+ ions may bind with negatively charged hydroxyl groups on plant surfaces or other lone pair carriers groups such as carbonyl. Hence, there arises a competition between H+ ions and metal ions in the solution. The researchers found a similar situation that occurs in case of caster leaves, where the carboxylate groups are considered as attracting sites on leaves [9]. Adsorption capacity increased with further increase in pH and reached optimum values of 0.510 mg g−1 (80.1%) and 0.579 mg g−1 (95.8%) at pH 6 and pH 7 for natural P. karka and treated P. karka, respectively. This value is in agreement with many previous researchers who have reported optimum biosorption of mercury by different biomasses at pH 6 or near values [17, 3032]. An increase in biosorption levels with increasing pH can also be explained by relating the biosorption to the number of available negative charges on the surfaces. Beyond optimum pH values, decrease in adsorption capacities was observed. This is because at high pH, solution has excess number of OH- ions, which enhance the probability of salt or hydroxide formation with metal ions in the solution. In case of alkali and alkaline earth metal ions, this probability can be ignored as their hydroxides are soluble but for heavy metal ions like mercury and so forth, it contributes significantly. Hence, there is a decrease in metal removal efficiency (Figure 2).

Figure 2: Point zero charge (pHpzc) values for the biosorption of mercury ions on natural P. karka and treated P. karka (concentration = 10 mg/L, time = 30 minutes,  K, agitation speed = 150 rpm, dose of biosorbent = 2 g/100 mL, and pH = 2–8).
Figure 3: Effect of pH change on biosorption capacity of natural P. karka and treated P. karka (concentration = 10 mg/L, time = 30 minutes,  K, agitation speed = 150 rpm, dose of biosorbent = 2 g/100 mL, and pH = 2–8).

In point of zero charge, pHpzc represents the pH where adsorbent surface becomes electrically neutral. Above the pHpzc value, surface of biosorbent acquired negative charge, whereas below the pHpzc point surface acquired positive charge. Hence, cationic biosorption will be favorable above the pHpzc point and anionic biosorption will be optimum below the pHpzc point. pHpzc was determined for the biosorption of mercury ions on the natural P. karka and treated P. karka using the method described in [28, 33], Figure 2. The values of pHpzc were found at 4.5 and 5 for natural P. karka and treated P. karka, respectively (Figure 3). The results are in agreement with the pH values obtained for the same experiment showing maximum biosorption at pH 6 and 7 for natural P. karka and treated P. karka, respectively (above pHpzc values, adsorption of mercury ions is cationic adsorption, Figure 3).

3.3. Effect of Agitation Speed on Biosorption

Shaking speed is important in biosorption process because it increases the chances of interaction between metal ions and biomass. Adsorption increases with increase in shaking speed. At slow speed, biosorbent settles down and buries many active sites under its top layers. So, only top layer takes part in adsorption process because the under buried layers do not have contact with metal ions. Figure 4 indicates that maximum adsorption of Hg(II) ions for natural P. karka was 0.329 mg g−1 (65.8%) at 150 rpm, while for treated P. karka maximum adsorption of Hg(II) attained was 0.446 mg g−1 (89.1%) at 100 rpm. Treated P. karka has 23.3% more biosorptive removal capacity than natural P. karka. A decrease in adsorption was observed by further increase in shaking speed because at high agitation speed, random collisions between particles (adsorbate-adsorbate, adsorbent-adsorbate, and adsorbent-adsorbent) do not provide enough time to heavy metal ions to bind with surface of the biosorbent [33]. Moreover, increase in biosorption capacity in treated biosorbent is due to the addition of some functional groups, which cause increase in the binding capacity of the material.

Figure 4: Effect of agitation speed on adsorption capacity of natural P. karka and treated P. karka (concentration = 10 mg/L, time = 30 minutes,  K, agitation speed = 100–300 rpm, dose of biosorbent = 2 g/100 mL, and pH = 6 and 7 for natural P. karka and treated p. karka, resp.).
3.4. Effect of Dose on Biosorption

The effect of biosorbent dose on biosorption efficiency for Hg(II) ions was investigated. As shown in Figure 5, the maximum metal uptake was found to be 0.107 mg/g (89.3%) for natural biomass at concentration of 1 g/100 mL and for treated biomass was 0.398 mg/g (93.2%) at 1 g/100 mL. The uptake capacities decreased at higher dosages. The above facts can be illustrated as a result of partial aggregation that occurs at high doses of biomass, resulting in decreasing the number of active sites on the surfaces of biomass [22, 23]. A further increase in biomass concentration over optimum dose does not lead to appreciable improvement in biosorptive metal removal due to the saturation of the adsorbent surface with metal ions and the establishment of equilibrium between the ions remaining nonadsorbed in the solution and which bound to the adsorbent.

Figure 5: Effect of biosorbent dose on adsorption capacity of natural P. karka and treated P. karka (concentration = 10 mg/L, time = 30 minutes,  K, agitation speed = 150 and 100 rpm natural P. karka and treated p. karka resp., dose of biosorbent = 1–6 g/100 mL, and pH = 6 and 7 for natural P. karka and treated p. karka, resp.).
3.5. Effect of Contact Time on Biosorption

Time of contact is also one of the most important parameters that helps to use biosorbent successfully in practical and the rapid biosorption application [2732, 34, 35]. Time needed for metal sorption helps to determine the amount of solutions that can be treated and impacts on the process selection and its design [26]. Figure 6 indicates the biosorption capacity of natural P. karka and treated P. karka versus time for Hg(II) ions removal from aqueous solution. The maximum sorption capacity of Hg(II) was attained in first 20 min for both natural P. karka and treated P. karka. After that, there was slight increase in sorption with time and sorption equilibrium was reached in 50 min (0.4365 mg g−1, 87.3%) and 40 min (0.4545 mg g−1, 90.9%) for natural P. karka and treated P. karka, respectively. Most of the mercury ions sorption was attained in first 20 min that may be due to availability of more sorption sites in the beginning of experiment. Initially, with increase in time, metal ions find more adsorption/binding sites; hence, metal adsorption amount of metal ions increased. The maximum amount of metal ions adsorbed at 50 min and system reached equilibrium. At equilibrium state, maximum adsorption of metal ions illustrated the covering of maximum binding sites with metal ions. Hence, rate of adsorption may decrease to a constant value with further increase in time as in case of natural P. karka which is due to complete coverage of active sites to the surface of biosorbent and increase in rate of desorption with respect to time as well, which is in agreement with some previous researches on different biosorbents [35, 2732, 34].

Figure 6: Effect of contact time on adsorption capacity of natural P. karka and treated P. karka (concentration = 10 mg/L, time = 20–60 minutes,  K, agitation speed = 150 and 100 rpm natural P. karka and treated p. karka, resp., dose of biosorbent = 1 g/100 mL, and pH = 6 and 7 for natural P. karka and treated p. karka, resp.).
3.6. Kinetic Studies of Adsorption

Rate of metal uptake by a particular adsorbent is calculated from kinetic studies and it is important for design of biosorption process [36]. Comparison of calculated and experimental uptake () data and shape of graph helps to find which model fits best to the biosorption process. Moreover, value of coefficient of determination () above 0.98 is an indication of model suitability [37]. The pseudo-first-order (Lagergren kinetic model) describes sorption of solid or liquid systems based on solid capacity: where and (mg/g) are adsorption capacities at time and at equilibrium, respectively, and is the first-order rate constant (min−1). Linear form of pseudo-first-order kinetics model is written by the following equations:

Consider pseudo-second-order rate kinetics by where is constant for second-order rate (mg g−1 min−1). Its linear form is written by the following equation:

The rate-controlling mechanism may be varying during the adsorption process. There are three possible mechanisms that may be cropping up. Diffusion process or mass transfer through external surfaces controls the early stage of this process. Then there is a reaction or a stage of constant rate and then it finally reached diffusion stage after that biosorption process slows down significantly [12]. Data shows values for pseudo-first-order as 0.502 and 0.972, while for pseudo-second-order, values are 0.994 and 0.999 for natural P. karka and treated P. karka, respectively (Table 1). So, for pseudo-second-order kinetic values of for both natural p. karka and treated P. karka are in agreement as . The experimental metal uptake values are also much closer to the values calculated from pseudo-second-order model. Hence, the pseudo-first-order kinetics is more deviated from experimental values as compared to pseudo-second-order kinetics. These results revealed that the pseudo-second-order mechanism is overpowering and chemisorption might be the rate-limiting step that controls the biosorption process [38, 39]. The plots of pseudo-first-order and pseudo-second-order kinetic models are shown in Figure 7.

Table 1: Parameters of pseudo-first-order and pseudo-second-order kinetic models for the biosorption of mercury ions on the surface of natural P. karka and treated P. karka.
Figure 7: (a) Pseudo-first-order rate kinetics of Hg(II) for natural P. karka and treated P. karka. (b) Pseudo-second-order rate kinetics of Hg(II) for natural P. karka and treated P. karka (concentration = 10 mg/L, time = 20–60 minutes,  K, agitation speed = 150 and 100 rpm natural P. karka and treated p. karka, resp., dose of biosorbent = 1 g/100 mL, and pH = 6 and 7 for natural P. karka and treated p. karka, resp.).
3.7. Effect of Initial Concentration of Hg(II) on Biosorption

Factor of initial metal ion concentration is important for determining the types of effluents that can be treated with biosorbent. Figure 8 shows that metal uptake capacity of the biosorbent increased with increase in initial metal ion concentration while it decreased in percentage removal of metal ions for both natural P. karka and treated P. karka. Adsorption capacity () ranged from 0.439 and 0.449 mg of Hg(II)/g of biosorbent for initial concentration 10 ppm to 1.7685 and 2.169 mg of Hg(II)/g of biosorbent for initial concentration 60 ppm for natural and treated biosorbents, respectively. The results gave evidence of the great ability of material to remove mercury ions from solutions [40, 41].

Figure 8: (a) Effect of initial metal ion concentration on adsorption capacity of natural P. karka and treated P. karka. (b) Langmuir adsorption isotherm for natural P. karka and treated P. karka (concentration = 10–60 mg/L, time = 50 and 40 minutes for natural P. karka and treated p. karka resp.,  K, agitation speed = 150 and 100 rpm natural P. karka and treated p. karka, resp., dose of biosorbent = 1 g/100 mL, and pH = 6 and 7 for natural P. karka and treated p. karka, resp.).
3.8. Adsorption Study of Isotherms

Adsorption isotherms are important for developing an equation which is true representative of the adsorption data and help to design the adsorption process. Mostly Langmuir isotherm, Freundlich isotherm, and D-R model were applied to the data. According to Langmuir isotherm, monolayer adsorption occurs on a homogeneous surface of adsorbent and no interaction is found between adsorbed molecules on surface of sorbent [42]: where (mg/g) is metal uptake capacity at equilibrium, (mg/g) is maximum uptake capacity, (L/mg) is Langmuir constant, and (mg/L) is concentration of adsorbate at equilibrium. The linear form of the Langmuir isotherm is Feasibility and shape of Langmuir isotherm are calculated by following equation of equilibrium parameter [43]: Values of show that Langmuir isotherm is not favorable if , linear if , favorable if , and irreversible if [44]. All values of for natural P. karka and treated P. karka lie between 0 and 1, which means Langmuir isotherm is favourable for both types of biomass.

According to the Freundlich isotherm, adsorption is a multilayer process and occurs on heterogeneous surfaces [42] (10): where and are Freundlich constants. Linear form of Freundlich equation is

Dubinin-Radushkevich model is used for estimation of porosity of the biomass and adsorption mechanism based on the potential theory assuming heterogeneous surface. The linear form of (D-R) isotherm model is where is a constant representing the mean free energy of biosorption per mole of the biosorbate (mol2/ kJ2), is the theoretical saturation capacity of biosorbent (mg g−1), and is the Polanyi potential, which can be calculated by where (K) is the absolute temperature and (J mol−1 K−1) is the gas constant. Hence, by plotting versus , it is possible to calculate the value of from the intercept and the value of by the slope. The plots of Langmuir, Freundlich, and D-R models are shown in Figures 8(b), 9(a), and 9(b), respectively. The biosorption mean free energy (kJ/mol) is calculated as follows [45]:

Figure 9: (a) Freundlich adsorption isotherm for natural P. karka and treated P. karka. (b) Dubinin-Radushkevish (D-R) model fornatural P. karka and treated P. karka (concentration = 10–60 mg/L, time = 50 and 40 minutes for natural P. karka and treated p. karka, resp.,  K, agitation speed = 150 and 100 rpm natural P. karka and treated p. karka, resp., dose of biosorbent = 1 g/100 mL, and pH = 6 and 7 for natural P. karka and treated p. karka, resp.).

Table 2 shows the different parameters calculated from Langmuir, Freundlich, and D-R equations for both natural P. karka and treated P. karka biomass. The values of are calculated as 1.787 and 2.268 mg/g for natural P. karka and treated P. karka, respectively. The values of correlation factor () values for Langmuir isotherm are 0.978 and 0.991, which shows good fit of the model to both types of biosorbents. Hence, it shows homogeneous distribution of active sites on the surface of biosorbent [2227]. The value is greater than a number of reported materials (Table 4).

Table 2: Langmuir isotherm, Freundlich isotherm, and D-R model parameters for the biosorption of mercury ions on the surface of natural P. karka and treated P. karka.

The slope and intercept of Freundlich model were used to calculate the constant indicative of relative adsorption capacity, and factor , and that indicative of adsorption intensity. The value of tells about favorability of adsorption process. Adsorption is said to be better if is in the range of 2–10, good if in range of 1-2, and average if below 1. Here natural and treated biosorbents have value of , 2.219 and 1.729, respectively. This is an indication of better adsorption for natural biosorbent and good for treated biosorbent. The values of coefficient of approximation () were 0.991 and 0.996 for natural and treated biosorbents, respectively, that pointed out a good fit of Freundlich model [24, 2729].

The slope and intercept of D-R model (Equation (12)) are used to calculate maximum adsorption capacity, and (mg/g) was 1.36 mg/g and 1.58 mg/g, respectively, for natural P. karka and treated P. karka. These values are not comparable with experimental values. Moreover, the values of coefficient of approximation () are 0.8373 and 0.7853, respectively, for natural and treated biosorbents which indicate data is not fit to the D-R model for both types of biosorbents as compared to Langmuir and Freundlich isotherms. The values of adsorption energy are calculated to be 1.0 kJ/mol and 1.18 kJ/mol for natural and treated biosorbents, respectively. These values are below 8, which indicate physical nature of adsorption. Biosorption process may be chemisorption if and physisorption for  kJ/mol [2732].

3.9. Effect of Temperature on Adsorption of Hg(II)

Study of temperature is an important parameter for all energy dependent mechanisms as in metal biosorption, which is mostly physicochemical in nature [31]. Economic suitability of any process is always based on energy efficient process. Figure 10(a) shows effect of temperature on Hg(II) sorption capacity of natural P. karka and treated P. karka at a temperature range 293–323 K. Optimum removal capacity was obtained at 313 K for both types of biosorbents, that is, 0.4545 mg g−1 and 0.46 mg g−1, respectively. Increase of temperature beyond optimum value, that is, 40°C, leads to a decreased in metal uptake.

Figure 10: (a) Effect of temperature on adsorption capacity of natural P. karka and treated P. karka. (b) Thermodynamics studies for natural P. karka and treated P. karka (concentration = 10 mg/L, time = 50 and 40 minutes for natural P. karka and treated p. karka, resp., –328 K, agitation speed = 150 and 100 rpm natural P. karka and treated p. karka, resp., dose of biosorbent = 1 g/100 mL, and pH = 6 and 7 for natural P. karka and treated p. karka, resp.).
3.10. Thermodynamics

Thermodynamics parameters of biosorption process are calculated by studying effect of temperature on the sorption process. Values of Gibbs free energy change (), entropy change (), and enthalpy change () change with temperature [46]. Gibbs free energy change is calculated to determine feasibility and spontaneity of the biosorption process by (12) where (kJ/mol) is Gibbs free energy change, is universal gas constant (8.3145 J/mol K), and is equilibrium constant . The value of (kJ/mol) gives information about the endothermic or exothermic nature of the process, while (J/molK) indicates whether sorption process is ordered or random. Both parameters were calculated from the following equation:

Energy and entropy factors are considered to determine which processes will occur spontaneously. Negative value of indicates that the reaction is spontaneous. The linear plot between temperature and Gibb’s free energy change () gives the enthalpy change () from intercept and entropy change () from slope of (13). The values of enthalpy change were found to be +26.688 and +24.034 kJ/mol and entropy change as +101.6 and +94.62 J/mol K, for natural P. karka and treated P. karka, respectively.

Negative values of indicate that sorption process of mercury is spontaneous and feasible for both natural biosorbent and treated biosorbent [47]. Increasing trend in feasibility of biosorption process is observed up to 40°C afterwards spontaneity and feasibility of process decreased by increasing temperature to 50°C. Positive values of indicate that reaction is endothermic while negative values of means there is very less randomness in sorption process at solid/liquid interface (Table 3).

Table 3: Values of thermodynamic parameters for the biosorption of mercury ions on the surface of natural P. karka and treated P. karka.
Table 4: A comparison of mercury metal ions adsorbed by different biosorbents.

4. Conclusion

Current study on sorption and desorption behavior of natural P. karka and treated P. karka (Trin) indicates that its Hg(II) uptake capacity is 1.79 and 2.27 mg/g, respectively. Sorption process fits Langmuir and Freundlich models and follows pseudo-second-order rate kinetics. Thermodynamic studies show that the sorption process is spontaneous and endothermic. Studies indicate that P. karaoke can be used effectively for treatment of Hg(II) loaded effluents and treated absorbent effectively increases the metal sorption capacity.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

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