Binary adsorption of [Pb(II) þ Co(II)] from aqueous solution using thiolated saw dust

Technology advancement contributed to an increase in industrial activities, resulting in the introduction of metal ions into water resources at concentrations well above the WHO limits. Heavy metals are highly toxic and carcinogenic; usually occur as multicomponent mixtures in aquatic environment. In present study, batch experiments have been conducted to study the dependence of varying concentration, time, pH and temperature on the uptake of Pb(II) as pure component under equilibrium conditions using thiolated saw dust. Saw dust has been chemically modi ﬁ ed with thioglycolic acid and characterised using proximate and FTIR analyses, degree of thiolation has also been determined. To determine the effect of presence of Co(II) ions on the uptake of Pb(II) ions, batch experiments for [Pb(II) þ Co(II)] mixture have been carried out for concentration ratios of 1:0, 1:1, 1:2, 1:3, 1:4 of Pb:Co at pH 5 and data has been interpreted using Langmuir competitive isotherm shows that adsorption of Pb(II) has been suppressed by the presence of Co(II) ions in the binary solution, hence the adsorption process is antagonistic in nature. Study also indicates the possibility of simultaneous removal of both metal ions using low cost bioadsorbent, which is economical specially for application in small scale industries. Pb(II) ions thiolated saw in of Pb(II) in maximum pH further in a in amount adsorbed, pHpzc The pHpzc of thiolated saw dust is 4.5. At a higher concentration of H þ ions with positively Pb(II) in competitive effect maximum of further in shows a in of Pb(OH) The relative ease of hydrogen of the thiol saw with


INTRODUCTION
In the last few decades, there has been a tremendous increase in the deterioration of quality of freshwater due to rapid growth of human activities and the accelerated pace of industrialization (Czikkely et al. 2018). Lead and cobalt are highly toxic and are released into natural waters from various industrial activities including mining, refining ores, batteries and metal plating (Rashid & Yaqub 2017). The toxicity of these metals is due to their persistence and bioaccumulation in the body which causes severe damage to the nervous system, reproductive system and kidneys, gastrointestinal irritation, lung cancer etc (Ketsela et al. 2020;Okolo et al. 2020). A large number of methods have been used for the removal of these heavy metal ions from wastewater, these are chemical precipitation, ion exchangers, chemical oxidation/reduction, reverse osmosis, electro dialysis, and ultrafiltration. However, most of these methods have their limitations related to disposal problems, low efficiency, sensitive operation and production of secondary sludge (Rashid & Yaqub 2017;Czikkely et al. 2018) that increases its cost for the application in small scale industries. Consequently, there is a growing demand for efficient, novel and cost-effective techniques for the remediation of wastewater containing heavy metal ions present in the form of single or multicomponent mixture ( Jain et al. 2016;Cifci & Meric 2017). The adsorption of heavy metal ions using agricultural wastes or biosorption materials has attracted much attention over other conventional treatment methods, due to low cost, regeneration, high efficiency, lesser sludge production and possibility of adsorbate recovery (Gupta et al. 2015;Alalwan et al. 2020). Agricultural waste materials have shown promising removal efficiency for metal ions from effluents either in unrestricted type or by further chemical treatment (Mahmood et al. 2017;Singh et al. 2017). Literature studies reveal that chemical modification of bioadsorbents enhances their surface capability that is attributed to the presence of hemicellulose, lignin or other functional groups (Mahmood et al. 2017). The present study aims to check the surface assimilation behaviour of Pb(II) ions from aqueous solution in pure as well as in the presence of Co(II) ions from binary mixture of [Pb(II) þ Co(II)] using thiolated saw dust (Robert & Girish 2018;Chikri et al. 2020) as adsorbent. Agricultural wastes as adsorbents are cost effective for wastewater treatment especially for simultaneous removal of contaminants in the form of mixture as compared to the use of conventional absorbents. Also there is no need for regeneration, which makes the sorption process of treating wastewaters cost effective and more economical for small scale industries.

Preparation of adsorbent
Saw dust of Eucalyptus tereticornis, obtained from local mill in Pinjore, district Panchkula, Haryana. It was washed with distilled water then air dried and ground. Sieve analysis was performed using mechanical sieves and activation of adsorbent was done by soaking saw dust in excess of 0.3 M HNO 3 for 24 hr followed by washing and drying. For chemically modification, 100 g sample was treated with 0.1 M thioglycolic acid, continuously stirred for 24 hr at 25°C-30°C and then centrifuged, washed and dried at 80°C and supernatants were discarded (Robert & Girish 2018;Chikri et al. 2020).

Estimation of adsorbate concentration
Metal ion concentration was estimated titrimetrically (Singh et al. 2017) before and after adsorption using the standard methods with EDTA solution in single component and binary mixture. 5 mL solution of PbCl 2 /CoCl 2 solution was taken in 250 mL titration flask and then 10 mL of distilled water was added. To this 0.7 gm of hexamine was added as buffer and 2-3 drops of xylenol orange was added as indicator. The titrations were carried out with 0.01M and 0.001M EDTA solutions for the estimation of Pb(II) and Co(II) ions respectively.

Batch adsorption studies
Single metal ion adsorption 250 mg of adsorbent was weighed and placed in contact with 10 mL of metal ion solutions for the concentration range of 30-300 mg/L at varying pH ranges 2-8 (adjusted using 0.1M HCl/NaOH). Test tubes were agitated for regular time intervals until equilibrium is attained. After agitation and centrifugation, supernatant was analysed titrimetrically (Singh et al. 2017) using the standard EDTA solution.

Binary metal ion adsorption
Binary adsorption was carried out for [Pb(II) þ Co(II)] mixture for concentration ratios 1:0, 1:1, 1:2, 1:3, 1:4 for fixed Pb(II) ion concentration of 60, 80 and 100 mg/L. The pH of the solution was adjusted at 5, where maximum adsorption was obtained for Pb(II) in single adsorption study. After sample preparation, 250 mg of adsorbent was agitated with 10 mL of solution until equilibrium is attained. Samples were analysed after centrifugation, titrimetrically for Pb(II) and Co(II) ion concentration (Singh et al. 2017).

RESULTS AND DISCUSSION
Characterization of thiolated saw dust Adsorbent has been characterised using sieve and proximate analyses, degree of thiolation, point of zero charge (pH pzc ) and FTIR spectroscopy (Perkin Elmer spectrophotometer) (Robert & Girish 2018).

Sieve analysis
Sieve analysis was performed using mechanical sieves to obtain adsorbent particles of size between 125 and 250 μm for adsorption experiments (Singh et al. 2017).

Proximate analysis
The loss of ignition was determined by heating 5 g of unmodified saw dust sample at 100°C for 1 hr to get moisture content and further heated upto 700°C in muffle furnace to get the ash content. The moisture content was found to be 8.9% and ash content at 700°C was 40.3% (Singh et al. 2017).

Surface area analysis
Since surface area is an important parameter to characterise the adsorbent surface area, analysis has been carried out for untreated saw dust using iodine adsorption method (Robert & Girish 2018). Amount of iodine absorbed was estimated by titrating a blank and against iodine containing saw dust with standard thiosulphate solution using starch as indicator and the surface area of unmodified saw dust was found to be 179.7 m 2 /g.
Degree of thiolation 0.5 g of thiolated saw dust was placed in contact with 20 mL of 0.5N iodine solution and agitated on for 10 minutes, then allowed to settle. The unreacted iodine was back titrated with 0.1N sodium thiosulphate using starch as indicator. The titre values were recorded and the degree of thiolation was calculated (Robert & Girish 2018) using Equation (1) and found to be 2.4 for thiolated saw dust.
where T is degree of thiolation, V and V 0 is the volume (mL) of 0.1M Na 2 S 2 O 3 solution used in blank titration and 0.5 g thiolated saw dust respectively, W is weight of adsorbent sample and M is molarity of Na 2 S 2 O 3 .

Point of zero charge
25 mL of 0.01M NaCl solutions were placed in different test tubes and pH of each solution was adjusted in the range 2-12 using 0.1M HCl/0.1M NaOH solution, as the case may be. Then 0.25 g of chemically modified saw dust was added in each test tube and agitated mechanically for 1 hr and allowed to stand for 24 hr to attain equilibrium at 25°C-30°C. The zeta potential (mV) value of each solution was determined and plotted against initial pH to determine the point of zero charge (Singh et al. 2017;Robert & Girish 2018) and pH of point zero charge for thiolated saw dust was found to be 4.5.

Fourier transform infrared spectrum (FTIR)
FTIR of saw dust (Figure 1(a)) shows bands at 3,386 cm À1 indicates the existence of free hydroxyl group (O-H), 2,924 cm À1 may be assigned to symmetric or asymmetric C-H and symmetric stretching vibration of CH 2 , 1,738 cm À1 to stretching vibration of C-O bonds (-COOH, -COOCH 3 ) and 1,242 cm À1 , 1,054 cm À1 to SO 3 stretching (Singh et al. 2017). The FTIR spectrum of thiolated saw dust (Figure 1(a)) shows bands at 3,356.3, 2,918.6 and 1,722.2 cm À1 are attributed to surface hydroxyl groups, stretching vibration of CH 2 and stretching vibration of C-O bonds (-COOH) respectively. Band at 1,624.7-1,510.0 cm À1 is indicative of carboxylic acid and 1,350.0-1,000.0 cm À1 of O-H of alcohol and aliphatic ethers. After thiolation, band present below 800 cm À1 are indicative of sulphur functional (-SH) groups (Sharma et al. 2009(Sharma et al. , 2012. After adsorption of Pb(II) ions, most of the shifted peaks in FTIR spectrum of thiolated saw dust (Figure 1(b)) were observed at 3,406.6-3,356.3 cm À1 attributed to complexation of Pb(II) ions with ionised -OH groups and bonded -OH bands of carboxylic acids (Robert & Girish 2018). The changes in peaks observed between 1,067.7 and 1,057.9 cm À1 , caused by stretching vibration of SO 3 , indicate the importance of SO 3 group in the adsorption of lead onto thiolated saw dust. The band present below 800 cm À1 are finger print zone of sulphur functional (-SH) groups (Sharma et al. 2009).

Effect of pH and point of zero charge (pHpzc)
Batch experiments for the removal of Pb(II) ions using thiolated saw dust have been conducted for at varying pH in the range 2-8 (beyond which precipitation of Pb(II) ions occurs). It has been observed that adsorption of Pb(II) ions increases with increase in pH and maximum removal was obtained at pH 5, further increase in pH shows a decrease in amount adsorbed, which is associated with pHpzc (Singh et al. 2017;Robert & Girish 2018). The pHpzc of thiolated saw dust is 4.5. At low pH, a higher concentration of H þ ions compete with positively charged Pb(II) ions. With an increase in pH, the competitive effect of H þ ions decreases and maximum adsorption of positively charged Pb(II) ions on negatively charged adsorbent surface occurs at pH 5 (i.e pH . pHpzc). A further increase in pH shows a decrease in the amount of Pb(II) adsorbed due to the formation of soluble or insoluble Pb(OH) 2 (Singh et al. 2017). The relative ease of exchanging hydrogen atoms of the thiol groups of thiolated saw dust with Pb(II) ions also results in improved level of adsorption.

Effect of initial concentration and contact time
Maximum removal was obtained at lower concentration of Pb(II) ions, which decreases with increase in concentration from 30 to 300 mg/L. However, the adsorption capacity increases with increase in concentration. Adsorption takes place rapidly in the initial 15 minutes and equilibrium adsorption of 7.5 mg/g was attained within 150 minutes. This may be attributed to the availability of larger number of adsorption sites for a smaller number of metal ion species at higher dilution, which decreases with time (Dhiman & Sharma 2016;Singh et al. 2017).

Adsorption isotherm modelling
The equilibrium data obtained at pH 5 and contact time 150 min for adsorption of Pb(II) ions were analysed using Freundlich, Langmuir and Temkin adsorption isotherm models to determine the equilibrium metal ion concentrations and adsorption capacity of thiolated saw dust (Dhiman & Sharma 2016;Singh et al. 2017).

Freundlich isotherm
Figure 1 | FTIR spectrum of (a) unmodified and thiolated saw dust before adsorption (b) thiolated saw dust after adsorption of Pb(II) ions.

Uncorrected Proof
Langmuir isotherm where C e is the equilibrium concentration (mgL À1 ) and q e is the equilibrium amount adsorbed (mgg À1 ), n and K f are Freundlich isotherm constants, K L related to affinity of the binding sites (Lmg À1 ), a L the Langmuir isotherm constant. The values of Freundlich and Langmuir constant are presented in Table 1. Results from Freundlich and Langmuir isotherm models suggest, favourable and monolayer adsorption occurs with high correlation coefficient value which indicate the effectiveness of thiolated saw dust. Langmuir model can be expressed in terms of separation factor or equilibrium parameter R L (Dhiman & Sharma 2016), The values of R L is found to be 0.260 for Pb(II) ions indicating favourable adsorption (Dhiman & Sharma 2016).
Temkin isotherm where A T is the equilibrium binding constant (Lg À1 ) corresponding to maximum binding energy, B T ¼ (RT)/b T , is constant related to heat of adsorption (Jmol À1 ), b T is Temkin isotherm constant, T is absolute temperature (K) and R is universal gas constant, 8.314 J mol À1 K À1 . The values of isotherm constants have been calculated using slope and intercept from the respective straight line plot of linear isotherm model (Table 1), which indicates the applicability of the Temkin isotherm model (Dhiman & Sharma 2016, 2019a).

Error analysis for isotherm studies
Error functions of non-linear regression methods were used for the assessment of optimum adsorption isotherm model for the removal of Pb(II) ions using thiolated saw dust (Dhiman & Sharma 2019a).

Kinetic studies
The equilibrium data obtained at pH 5 for Pb(II) ions have been analysed for kinetic studies using pseudo first order (Equation (11)) and pseudo second order (Equation (12)) rate equations, log(q e -q) ¼ log q e À k ad X t 2:303 (11) where q e and q (mg g À1 ) are the amounts of metal ion adsorbed at equilibrium and at any time taken for study respectively, t (min) is the time of contact and k ad is the adsorption rate constant (min À1 ), K 2 is equilibrium rate constant of pseudo second order adsorption (gmg À1 min À1 ). The values of kinetic constants are presented in Table 1. A straight line plot obtained from plot of t/q t vs. t indicates the applicability of pseudo second order kinetics (Dhiman & Sharma 2019a). The possibility of intraparticle diffusion was studied, using Morris Weber model, where q is the amount of metal ion adsorbed at different time intervals (mgg À1 ), K p is the intraparticle diffusion constant (mgg À1 min À1 ) and t is contact time (min.). K p as calculated from the slope of the linear plot of q vs t 1/2 (Table 1). A straight line plot has been obtained, which does not pass through origin, indicates that intraparticle diffusion occurs but is not the rate determining step (Singh et al. 2017;Dhiman & Sharma 2019a).

Effect of temperature
In order to study the effect of temperature on the adsorption of Pb(II) ions using thiolated saw dust, adsorption capacity was determined at 25°, 30°, 35°, 40°and 45°C. Results show that the adsorption capacity increases from 7.05 mg/g to 7.28 mg/g to 7.94 mg/g to 8.05 mg/g to 8.54 mg/g with increase in temperature from 298 to 303 K to 308 to 313 to 318 K respectively. The increase in adsorption capacity with increase in temperature was observed due to increase in number of sorption sites generated with breakage of some internal bonds near the active surface sites of adsorbent for each metal ion (Dhiman & Sharma 2016;Singh et al. 2017).

Thermodynamic studies
The thermodynamic parameters for the adsorption of Pb(II) ions have been determined using following equations K c is the equilibrium constant, T is the temperature (K) and R is gas constant. The positive values of enthalpy change (ΔH 0 ) and entropy change (ΔS 0 ) ( Table 1) suggests the endothermic nature of adsorption and increase in randomness at solid solution interface during adsorption. The negative value of ΔG 0 indicate feasible and spontaneous nature of adsorption (Singh et al. 2017;Dhiman & Sharma 2019a).

Effect of mutual interference in binary solutions
In order to study the effect of presence of Co(II) ions on the amount adsorbed of Pb(II) ions using thiolated saw dust, batch studies have been undertaken for [Pb(II) þ Co(II)] mixture by varying concentration ratios in the range 1:0, 1:1, 1:2, 1:3 and 1:4 for fixed Pb(II) ions concentration of 30, 60 and 90 mg/L and the concentration of secondary metal ion was varied accordingly (Singh et al. 2017;Thakur et al. 2020). The pH of the solution was adjusted at 5, where maximum adsorption was obtained for both metal ions in single adsorption studies. A fixed amount of 250 mg of adsorbent was agitated with 10 mL each of the solution until equilibrium is attained at 120 minutes (Singh et al. 2017). Mahmood et al. (2017) also studied the removal of Cd(II) and Zn(II) using natural and modified biomass and maximum removal of approximately 90.1% was obtained using 0.5 g adsorbent at 5 mg/L concentration. Whereas in present study maximum removal of approximately 85-87% was obtained in single metal ions study using 0.25 g thiolated saw dust for 30 mg/L concentration of Pb(II) and Co(II) ions, which shows better efficiency of thiolated saw dust for very small adsorbent dosage.
For [Pb(II) þ Co(II)] mixture, it has been observed that with increase in Co(II) ions concentration, amount adsorbed of Pb(II) ions decreases from 2.90 to 1.47, 3.70 to 1.81 and 4.15 to 2.94 respectively for fixed Pb(II) ions concentration of 30, 60 and 90 mg/L for contact time of 120 minutes. Results suggests significant decrease in the uptake of Pb(II) ions in the presence of Co(II) ions. Similar results were obtained by Singh et al. (2017) for the competitive adsorption of manganese and iron using esterified saw dust as adsorbent.
The adsorption behaviour of the binary mixture of [Pb(II) þ Co(II)] is shown in Figure 2. Maximum percentage removal of Pb(II) ions in mixture at pH 5 (i.e pH of maximum removal of Pb(II) ions in single component adsorption) for the fixed primary adsorbate concentration of 30 mg/L at 120 minutes. Equilibrium adsorption capacity for single component adsorption was found to be 2.90 mg/g for Pb(II) ions at fixed primary adsorbate concentration of 30 mg/L for the contact time of 120 minutes. With concentration ratios varying from 1:1, 1:2, 1:3 and 1:4 of [Pb(II):Co(II)], the amount of Pb(II) ions adsorbed decreases from 1.80 to 1.65 to 1.57 to 1.47 mg/g for fixed Pb(II) ions concentration of 30 mg/L, 2.45 to 2.12 to 2.05 to 1.81 mg/g for fixed Pb(II) ions concentration of 60 mg/L and 3.35 to 2.55 to 2.21 to 1.94 mg/g for fixed Pb(II) ions concentration of 90 mg/L at 120 minutes. Therefore, the binary adsorption data suggests that adsorption of primary adsorbate was strongly depressed in the presence of secondary component when they are present together in the form of binary set of metal ions (Singh et al. 2017;Dhiman & Sharma 2019b;Thakur et al. 2020).
For the analysis of adsorption equilibrium data obtained from binary set of [Pb(II) þ Co(II)], q 0 e /q e ratios have been determined, where q e is amount adsorbed for single component system at equilibrium and the prime denotes the equilibrium amount adsorbed in the presence of other component (Singh et al. 2017;Dhiman & Sharma 2019b). The value of q e 0 /q e ratio was found to be 0.88, for [Pb(II) þ Co(II)] mixture, for fixed primary metal ion concentration of 30 mg/L for contact time of 120 minutes, where maximum removal was obtained. The q e 0 /q e ,1, indicates the antagonistic behaviour of binary adsorption (i.e the effect of mixture is less than that of the individual adsorbate in the mixture) (Dhiman & Sharma 2019b;Thakur et al. 2020).

The Langmuir competitive model
Competitive adsorption of [Pb(II) þ Co(II)] mixture, the Langmuir multicomponent isotherm (Singh et al. 2017;Dhiman & Sharma 2019b) has been applied to the binary sorption equilibrium data where K L1 and K L2 indicates the heat of adsorption, C e1 and C e2 are equilibrium concentrations and q e1 and q e2 are equilibrium adsorption capacity of Pb(II) and Co(II) ions in [Pb(II) þ Co(II)] mixture (Singh et al. 2017), q m is the maximum adsorption capacity corresponding to primary adsorbate in mixture (Figure 3). The values of Langmuir multicomponent isotherm parameters are K L ¼ 0.040 and q m1 ¼ 2.87 mg/g.

CONCLUSIONS
The data demonstrate the ability of thiolated sawdust as an effective, sustainable, and low-cost adsorbent for removal of lead ions due to high porosity and availability of functional sites. The equilibrium adsorption capacities evaluated for pure component solution and binary mixtures shows that chemical modification of the sawdust using thioglycolic acid enhances its  Uncorrected Proof adsorption capacity and were found to depend on the porous surface of the adsorbent and nature of the specific electrostatic interactions that are controlled by the solution pH. The application of Langmuir multicomponent isotherm shows that the affinity of Pb(II) ions was depressed by the presence of Co(II) ions in the binary solution, hence the effect of the mixtures seemed to be antagonistic. The study also suggests that thiolated saw dust seems to be good adsorbent for the simultaneous removal of both metal ions. Therefore, the present study highlights the future expectations of thiolated sawdust as a viable, cost effective and efficient bioadsorbent for the removal of heavy metal ion pollutants from wastewater in the pure form and multicomponent mixtures, specifically for small scale industries.