Lead (Pb) Sorption Kinetics by Clay Minerals: Bentonite and Zeolite

Water pollution emanating from heavy metal contamination has been considered to be a global concern in recent years (Beidokhti et al., 2019; Tathe and Kolape, 2021). An increase in metallic substances in the water bodies particularly groundwater has been observed due to increased discharge of metals from industrial processes and urbanisation activities (Olukanni et al., 2014; Silvy and Teenamol, 2019). The levels of heavy metals exceed permissible limits in the system pose risk and health hazards to human beings through their cyclic transfer (Lingamdinne et al., 2018). The soil and waste water polluted with inorganic metal pollutants are difficult to treat as they are non-biodegradable and remain for longer period (Khan et al., 2017). Out of various heavy metal pollutants, lead (Pb) ranks second, after As, on priority lists of Hazardous Substances of Agency for Toxic Substances and Disease Registry (ATSDR, 2015) and US Environmental Protection Agency (USEPA) owing to its widespread occurrence and potentially high ecotoxicity (Kushwaha et al., 2018). The existence of >15 µg Pb²⁺ per litre in drinking water was found to be toxic to human beings (Khan et al., 2017). The soluble Pb bioaccumulates in living organisms, causing toxic effects on terrestrial and aquatic biota through unintended ingestion of Pb by consuming contaminated water and crop produces (Hamid et al., 2020). Ingestion of lead can cause anaemia, kidney malfunction, brain tissue damage and even death in human beingsunder extreme poisoning conditions (Mutter et al., 2017).

Lead ions in aqueous solutions could be removed by various processes  like ion exchange, adsorption, biosorption, precipitation, coagulation or electro-coagulation, cementation, osmosis and electro-dialysis (Arbabi, 2015) using various natural and synthetic materials. Most of the Pb removal techniques and materials are expensive and are not feasible for application in real life conditions on a larger scale. In this context, adsorption is a preferable methodwhich could be adopted to remediate contaminated waters using low-cost materials as adsorbents (Salman et al., 2017). Adsorption is a very efficient technique for metal contaminant removal and performance of the adsorbent is highly reliant on physicochemical properties of adsorbent, nature of contaminant and conditions of adsorption processes. To investigate the chemical adsorption rate, the rate-determining step in adsorption process, adsorbent material, effect of pH and time, chemical kinetic models were employed to describe the reactions (Ray et al., 2020) which permits to optimize the mechanism of adsorption pathways, to express the dependence of adsorbent surface properties, determine adsorbent capacities and design the adsorption systems effectively (Largitte and Pasquier, 2016). Adsorption efficiency is generally influenced by various factors like adsorbent dosage, initial metal concentration, contact time, particle size and pH of solution (Karri et al., 2017). Thus, there is a need to recognise and investigate efficient, low-priced and readily available adsorbents (Karri et al., 2017) to know the mechanisms of metal removal for choosing suitable sorbents under specific situations.

On the other hand, clay minerals have gained attention as effective and cheap sorbing and immobilizing materials for Pb because of their high net negative charge, cation exchange capacity, porosity, surface area, affinity towards Pb, lower cost, strong adsorption capacity, wider acceptability and easier availability (Melichova and Handzusova, 2013). Bentonite, a 2:1 clay mineral with two Si tetrahedral sheets sandwiching an Al octahedral sheet (Guerra et al., 2013) and zeolites (CaAl₂Si₄O₁₂·nH₂O), the three dimensional alkaline porous negatively charged alumino-silicates, neutralized by introducing exchangeable cations in structural sites. The negative charge arises due to the partialsubstitution of silicon (Si⁴⁺) with aluminium (Al³⁺, Hamidpour et al., 2010; Sangeetha and Baskar, 2016; Akbar and Zahedi, 2016). These clay minerals are capable of removing Pb from aqueous solution without emitting any toxic substances into the environment (Guerra et al., 2013). Thus, the study aims to investigate and compare the adsorption rate of bentonite and zeoliteat varying contact time, initial Pb concentrations and sorbent doses. The data is also fitted in differentkinetic models and kinetic parameters were computed to choose the best model to describe Pb removal.

Pb standards and sorbents
 
The Pb standards were prepared using analytical grade lead nitrate (Pb(NO₃)₂) salt and using the 2000 mg l^-1 stock solution, different Pb concentrations viz., 100, 250, 500, 1000 and 1500 mg l^-1 were prepared. Bentonite and zeolite clay minerals were used as sorbents which were of analytical grade purchased from M/s. Sigma Aldrich, Bangalore.
 
Batch sorption experiments
 
Batch sorption experiments were performed at room temperature with various sorbents applied at three different doses (1, 2.5 and 5 g) to the solutions having varied initial Pb²⁺ concentrations (100, 250, 500, 1000, 1500 and 2000 mg l^-1) at successive time intervals (24, 48, 72, 96 and 120 hrs). The experiment was conducted in duplicates with a blank simultaneously to quantify the amount of Pb adsorbed and desorbedby the sorbents.
 
Lead adsorption studies
 
Bentonite and zeolite clay minerals as sorbents was weighed and transferred into 50 ml centrifuge tubes to which 25 ml of Pb²⁺ solution was added at different concentrations. The centrifuge tubes were shaken on a mechanical shaker for the postulated time intervals of 24, 48, 72, 96 and 120 hours. After the stipulated time, tubes were centrifuged for 3 minutes at 5000 rpm and the supernatant was filtered using Whatman No.42 filter paper and stored in refrigerated conditions for further analysis. The Pb²⁺ concentration was determined in the filtrate at different time period using an atomic absorption spectrophotometer (Model: GBC AvantaPM). The amount of Pb²⁺ adsorbed (qe, mg g^-1) was calculated (Ray et al., 2020) according to equation (1):
 

 
Where:
C0 and Cf= Concentrations of Pb²⁺ (mg l^-1) in the initial and final solutions, respectively.
V= Volume of Pb²⁺ solutions added (25 ml).
‘m’= Mass of sorbents (g).
The adsorption rate (AR, %) was calculated as below.
 

 
Kinetic models
 
Lead adsorption data acquired from the investigation was fitted into zero, first, second order kinetics, intraparticle diffusion model, power function and pseudo-second order kinetic models. The details of the models used and their constants are furnished in Table 1.


 
Statistical analysis
 
All the statistical analysis was carried out using SPSS software. A simple variance analysis was done (ANOVA) for factorial completely randomized block design with threefactorial arrangement (Pb²⁺ concentration, sorbent dose and incubation interval) and two replications. The least significance test was used to detect the differences between the means at p<0.005. Simple correlation between various factors were performed using the procedure described by Snedecore and Cochran (1967) with the help of SPSS software.

Lead (Pb²⁺) adsorption
 
Effect of Pb concentrations
 
The effect of initial Pb²⁺ concentrations of the solution on Pbadsorption rate was investigated and depicted in Fig 1a. The removal of Pb2+ relied on the initial Pb concentration which varied from 88.1 to 46.7% for bentonite and 87.7 to 61.8% for zeolite. At lesser Pb²⁺ concentrations, no much variation was noted and remains the same for bentonite (88.1%) and zeolite (87.7%) but with increasing Pb2+ concentrations, zeolite (61.8%) exhibited superior adsorption rates. The adsorption rate was higher in zeolite at 100 to 1000 mg l^-1 (87.7 to 79.7%) but decreased steeply from 79.7 to 61.8% when the initial Pb concentration in the solution increased from 1000 to 2000 mg l^-1 however in bentonite the rate of Pb removal dropped linearly. The adsorption rates declined for both the sorbents with increasing Pb²⁺ concentrations since the sorbent surfaces were loaded with Pb ions viz., loading effect (Keles et al., 2010) where the fraction of available surfaces for Pb²⁺ adsorption was higher at lower concentrations than that of higher concentrations (Elboughdiri, 2020). At lower concentrations, more active sites would be available on the adsorbent to adsorb the Pb ions resulting in higher adsorption rates (Melichova and Hromada, 2013; Glatstein and Francisca, 2015). However, at higher concentrations, the active sites would be saturated with adsorbed Pb ions resulting in more Pb ions left un-adsorbed in the solution (Pawar et al., 2016; Hussain and Ali, 2021).
 
Effect of incubation intervals
 
The Pb²⁺ adsorption rate by the sorbents were studied at different incubation time intervals (Fig 1b) and a linear increase in the adsorption rate was discovered with the advancement of time inboth the sorbents. The adsorption rate increased from 0 to 63.2% for bentonite and 72.9% for zeolite within 24 hours of incubation. This higher adsorption rate might be ascribed to the accessibility of enormous active vacant sorption sites existing on the clay minerals initially and an enhanced concentration gradient between the solute in bulk solution and the solute near sorbent surfaces (Hamidpour et al., 2010; Pawar et al., 2016) resulting in faster ion exchange. It was then saturated with the passage of time (Zhang et al., 2016; Hussain and Ali, 2021) resulting in a slow and steady growth in the adsorption rate (63.2 to 77.3% for bentonite and 72.9 to 82.5% for zeolite from 24 to 120 hrs).
 
Effect of sorbent dose
 
The effect of sorbent dosage on the removal of lead ions at various contact time intervals was investigated with different adsorbent dosage (1.0-5.0 g 25 ml^-1) and the outcomes were depicted in Fig 1c. Higher adsorption rates were noticed with increasing sorbent dosage irrespective of type of sorbent studied. The Pb²⁺ adsorption rate varied from 63.5 to 78.2% in bentonite and 70.4 to 86.5% in zeolite clay minerals with the increasing sorbent doses from 1 to 5g owing to the increase in number of adsorption sites (Elboughdiri, 2020; Keles et al., 2010). The absorption of Pb ions increases with the quantity of benton­ite and zeolite added because of the accessibility of greater surface area and greater number of ex­changeable sites in clay structure at a time when the initial metal concentration is constant, resulting in higher Pb2+ removal efficiency (Melichova and Hromada, 2013; Zhang et al., 2016 and Elboughdiri, 2020). Similar findings were reported by Hamidpour et al., (2010); Pawar et al., (2016) and Hussain and Ali (2021).


 
Adsorption kinetics
 
The kinetics of Pb sorption in solid-metal ions system reflects the rate of Pb sorption on sorbents and the time required to reach equilibrium, which is crucial for the understanding of sorption process and sorbent performance. The adsorption data thus obtained was fitted into various kinetic models and the results have been reported in Table 2 and 3.





The higher coefficient of determination (R²= 0.999**), represented the validity of pseudo-second order model for lead sorption by both the sorbents followed by intraparticle diffusion and power function model. According to pseudo second order kinetics, rate-determining step in Pb²⁺ sorption was chemical sorption (Dragan et al., 2010). A linear variation in amount of Pb sorbed per unit time (t/qt) at time (t) for both bentonite and zeolite at different initial Pb²⁺ concentrations was explained in Fig 2. The initial sorption rate ‘h’, pseudo-second-order rate constant ‘k’, amount of Pb²⁺ adsorbed at equilibrium ‘qe’ acquired from the pseudo second-order model was listed in Table 2 and 3. Increasing Pb²⁺ concentration decreased the initial Pb²⁺ sorption rate (h) from 10.1 to 0.32 mg g^-1 min^-1 for bentonite and 12.9 to 0.35 mg g^-1 min^-1 for zeolite as Pb²⁺ concentration increased from 100 to 2000 mg l^-1. The magnitude of Pb²⁺ adsorbed at equilibrium (qe) augmented from 1.28 to 15.5 mg g^-1 for bentonite and 1.26 to 19.2 mg g^-1 for zeolite with increasing Pb²⁺ concentration. Comparing the two sorbents, the initial rate of Pb²⁺ sorption (h), rate constant (k) and equilibrium concentration (qe) were relatively higher for zeolite than bentonite. There was a decline in pseudo-second order rate constant (k) for the adsorption of Pb²⁺ on both bentonite and zeolite clays, as initial Pb²⁺ concentration was increased (Table 2 and 3). This shows that, time required for adsorption of Pb2+ to attain equilibrium is extended with increased initial Pb concentrations. The linearity of plots for the pseudo-second order model in Fig 2 indicates that, chemical reaction through exchange or sharing of electrons between sorbent and sorbate is the main rate-controlling step throughout the adsorption process (Vadivelan et al., 2005) and the mechanism follows a pseudo-second order reaction scheme. These outcomes are in harmony with the interpretations of Kragovic et al., (2012); Guerra et al., (2013) and Pawar et al. (2016).



The intraparticle diffusion model also showed good correlation with higher R² values for both the sorbents ranging from 0.964** to 0.983** for bentonite and 0.947** to 0.999** for zeolite indicating that this model could also describe the adsorption process in a better way. Here in addition to chemisorption, physisorption also contributed for the Pb adsorption onto the sorbents. However, the R² values for bentonite was slightly higher than zeolite. The diffusion rate constant escalated from 0.03 to 0.73 mg^-0.5 kg 0.5 for bentonite and 0.73 to 0.72 mg^-0.5 kg 0.5 for zeolite with increasing initial Pb concentrations depicting higher diffusion of Pb ions into the surface of sorbents when the concentration gradient is enhanced. However, the plot of ‘qt’versus ‘t1/2’(Fig 3) showed a deviation from originfor theexclusion of Pb²⁺. This result might be ascribed to the difference in mass transfer at the beginning and completion of sorption process (Pawar et al., 2016). These results are in line with the inferences reported by Katsou et al., (2011) and Javanbakht et al., (2019).

This study demonstrates that both bentonite and zeolite clay minerals as sorbents showed higher Pb²⁺ sorption from aqueous solutions. However, zeolite possess higher Pb removal efficiency than bentonite clay with lesser desorption of adsorbed Pb²⁺. The adsorption rate increased with increasing sorbent dose and at successive time intervals for both the sorbents. The different models tested for sorbent-metal ion interactions, showed that pseudo second order model explained the Pb²⁺ adsorption mechanism viz., chemisorption followed by intraparticle diffusion model. Chemisorption is the main rate limiting step in Pb²⁺ sorption and desorption was highly influenced by the net negative charges of the sorbents used. Hence it could be concluded that, zeolite is the effective sorbent used for removing Pb²⁺ from the aqueous systems and pseudo second order model best describes the Pb²⁺ sorption than other models.

The authors greatly acknowledge the support rendered by the Department of Soil Science and Agricultural Chemistry, TNAU, Coimbatore and Department of Science and Technology (DST)- INSPIRE (IF 210247), Government of India for funding the research.

This research work was supported by the Department of Science and Technology (DST), Government of India under INSPIRE (IF 210247) Fellowship program.

The authors do not have any conflict of interest to declare.