Development and characterization of N-substituted derivative of 2-sulfanylacetamide on Phyllanthus emblica seed coat as novel adsorbent for remediation of As(III) from water

Increasing use of various forms of arsenic in different fields is intimidation to the environment as well as human health. Trivalent arsenic is mainly used in pesticides, insecticides, herbicides, and treated wood products. Arsenic contamination in drinking water is a global problem, affecting millions of people. The US Environmental Protection Agency and World Health Organization consider that all forms of arsenic cause human health implications. Arsenic is number one for hazardous ions as per US-ATSDR (Agency for Toxic Substances and Disease Registry) and group ‘A’ carcinogen material in its priority list issued in 2001. Many studies have confirmed the toxicity of arsenite (Mandal & Suzuki 2002; Osuna-Martínez et al. 2021). According to EPA and WHO guidelines the concentration of arsenic in public water supply should be less than 0.01 mg/L (USEPA 2001; WHO 2011). India, Bangladesh, Canada, Chile, China, Cambodia, Germany, Japan, USA, Thailand, and Mexico are severely affected with arsenic contamination (Bagchi 2007).

Arsenic contamination has affected number of places throughout the world. India as well as other international countries such as Bangladesh, Canada, Chile, China, Cambodia, Germany, Japan, USA, Thailand, and Mexico are rigorously affected with arsenic contamination (Bagchi 2007; Shaji et al. 2020). More than 238 million people in 21 states in India were affected by a high level of arsenic in usable water as per Times of India report (Jadhav 2017). More than 70 million people in 96 districts in 12 States in India were affected by high level of arsenic in water as per India Today report (ITWD 2018). As per a recent article in Science News published on May 21st, 2020 regarding the risk of arsenic contaminated water, more than 220 million people may be at high risk around the world (Gramling 2020). Arsenic concentration above the recommended limit can causes severe health effects such as abdominal pain, diabetes, pulmonary disease, diarrhea, cardiovascular disease, and cancer (Palma-Lara et al. 2020).

Many methods have been proposed for the removal of dissolved arsenic ions from water (Nicomel et al. 2016; Alka et al. 2021). An adsorption technique is widely used because of effective cost but it becomes necessary to explore cost effective and efficient materials for adsorption or removal of metals especially heavy metals and apart from good adsorption, efficient regeneration of adsorbent should be feasible up to certain cycles (Saini et al. 2018, 2019).

A wide range of bulk and nano-materials have been synthesized and modified for removal of arsenic trivalent ions from water (Balsamo et al. 2010; Natale et al. 2013; Hu et al. 2017). Different iron oxide coated sand (0.041–0.136 mg/g adsorption capacity) (Thirunavukkarasu et al. 2005), MnO₂ loaded resin (53) (Lenoble et al. 2004), Oxisol (2.6) (Ladeira & Ciminelli 2004), activated alumina grains (3.48) (Lin & Wu 2001), nanoscale zero-valent iron (2.47) (Kanel et al. 2005), hydrous titanium dioxide (33.4), nanocrystalline TiO₂ (37.5) (Pirilä et al. (2011), α-Fe₂O₃ (1.9) (Prasad et al. (2011)), CuO (26.9) (Martinson & Reddy (2009), polymer nanocomposite (Fe₃O₄) (2.83) (Vunain et al. (2013)) and many other metallic oxide as well as activated carbon have been widely applied as adsorbent for removal of trivalent arsenic from water (Mohan & Pittman 2007). The main disadvantages of these synthesized adsorbents are high cost, low removal efficiency, and more hazardous towards the environment. To overcome these problems natural and modified natural materials have been proposed and applied for removal of arsenic from water. Rice husk biochar (19.3), Hydrilla verticilata (11.65), tea waste (7.36), pomegranate peel (5.57), corn cob (4.33), bagasse (1.35), mango bark (1.25), orange peel (OP) (1.08), biochar of rice straw (0.44), and banana peel (0.84) are used as natural adsorbents for removal of arsenic from water (Mohan & Pittman 2007; Shakoor et al. 2016, 2019). However, iron-coated rice husk biochar (30.7), thiol functionalized sugarcan bagasse (28.57), citric acid modified water melon rind (3.89), modified cassia fistula biochar (0.78), thioglycolated sugarcane carbon (0.08) are used as modified natural adsorbents for removal of arsenic from water (Mohan & Pittman 2007; Shakoor et al. 2016, 2019).

In the present work Phyllanthus emblica, an excellent fruit popularly known as Indian gooseberry (Annapurn 2012; Zhao et al. 2015; Mao et al. 2016; Chaphalkar et al. 2017) has been utilized first time as a novel adsorbent for removal of As(III) from water in pristine and modified form. P. emblica seed coat is derivatized in the form of N-substituted 2-sulfanylacetamide derivative using thioglycolic acid and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimidehydrochloride (EDC). Activated ester leaving group of thioglycolic acid with EDC is displaced by nucleophilic attack of primary amino acids on seed coat. Pristine P. emblica (PPE) and derivatized P. emblica (DPE) products are characterized using FTIR, SEM-EDX, and XRD techniques. Surface area of the adsorbents can be calculated using BET method. PPE and DPE are applied as adsorbents for removal of trivalent arsenic from water by batch study. The effect of pH of solution, concentration of arsenic ions, adsorbent dose and time of contact are observed for the adsorption efficiency of the adsorbents. This study will be significant for implementation of technology for arsenic remediation in real scenario conditions.

All chemicals and reagents used in the present work were of analytical grade purity and purchased from Central Drug House (CDH) (P) Ltd New Delhi India. Sodium arsenite (NaAsO₂) was used for preparing trivalent arsenic ions stock solution. H₂SO₄, EDC, thioglycolic acid (HSCH₂CO₂H) were used without further purification for modification of the pristine adsorbent.

Mini Flex II XRD manufactured by Regaku (Japan) was employed to measure the X-ray diffraction patterns for the pristine and derivatized P. emblica seed coat samples utilized in this research. IR Affinity-1S Shimadzu (Mozambique) instrument was used to measure the spectra of both materials in the middle IR range 4,000–667 cm⁻¹ using KBr method. The BET instrument, Micromeritics ASAP 2020 (Germany), porosity and surface analyser, was employed to know the surface area and size of both the sample used in this research through N₂ adsorption. The SEM imaging of samples was carried out using JSM 6510Lv SEM instrument manufactured by Jeol (Japan), operating at an accelerating voltage of 15 kV. 1 cm to 5 μ area may be imaged with magnification 20x-30,000x and spatial resolution of 50–100 nm by using conventional SEM. EDX of adsorbents was carried out by the Oxford INAX-ACT model instrument manufactured by Oxford (London), to know the chemical composition of PPE and DPE. Microwave Plasma-Atomic Emission Spectrophotometer (MP-AES 4200) manufactured by Agilent Technologies Inc. (USA) was used for determination of concentration of arsenic in the solution.

A stock solution of 100 ppm trivalent arsenic ions was prepared by utilizing sodium arsenite. The solutions of different concentration required to conduct batch study was obtained from stock solution after dilution. Samples are subjected to atomic emission spectroscopy (AES) to determine the amount of arsenic in the filtrate.

The presence of primary amine was confirmed by a weak peak at 1,575 cm⁻¹, which is due to the N–H (bending vibration) group of primary amines. It is deformed in the derivatized (DPE) product at 1,608 cm⁻¹ due to modification of the primary amine into amide with the interaction of thioglycolic acid (Zhang et al. 2015). More intense peak at 1,222 cm⁻¹ represents C–N stretch of aliphatic amines. Also, a weak broad band at 889 cm⁻¹ is due to N–H wagging of primary amines. However, it becomes strong broad band for secondary amide at 891 cm⁻¹. The peak at 1,703 cm⁻¹ becomes more intense at 1,724 cm⁻¹ due to formation of amide stretching vibration of C = O group present in the compound. The weak peak at 2,358 cm⁻¹, is due to the presence of –SH group (Shen et al. 2018). A new peak at 1,087 cm⁻¹ indicates C–S axial stretching of thioglycolic acid (Papaleo et al. 1996).

The BET t-plot method, an extension of the Langmuir isotherm, was used to find out the specific surface area of PPE and DPE. The properties of PPE and DPE were characterized by utilizing nitrogen gas at an ambient temperature of 22 °C with Micromeritics ASAP 2020, USA. The results showed that the BET surface area (0.3152±0.0081 m²/g) and Langmuir surface area (0.3571±0.0054 m²/g) of PPE are changed after modification. DPE showed high surface area compared to un-derivatized adsorbent due to the presence of more modified groups on the surface (BET surface area 0.5812 m²/g, Langmuir surface area 0.6372 m²/g).

A batch study was conducted with PPE and DPE product to evaluate the adsorption characteristics in different conditions of concentration of arsenic (20 to 100 mg/L), pH (1 to 9), time of contact (20 to 120 minutes), and adsorbent dose (0.03 to 0.50 g/L). Solutions with the desired concentration of arsenite were prepared from the stock solution of trivalent arsenic using double distilled water. Conical flasks with solutions of different concentrations and specific adsorbent dose were kept over the rotatory shaker for shaking followed by filtration. Filtrate is subjected to AES to determine the amount of arsenic in the filtrate.

To determine the effect of concentration of arsenic on adsorption, different concentrations of arsenic (20 to 100 mg/L) were used at fixed conditions of adsorbent dose, pH dose, contact time, and temperature. Adsorption of arsenic ions on the surface of adsorbent was quite significant at lower concentration due to more availability of binding sites on the adsorbent (Figure S1(a)) (Bessaies et al. 2020).

To determine the effect of PPE and DPE adsorbent dose on the adsorption, different doses of the adsorbents were used with other factors held constant. 0.03–0.50 g/L of adsorbent was taken into different conical flasks and to each flask 60 mg/L solution of arsenite at other constant conditions was added. Conical flasks with solutions were kept over the rotatory shaker for shaking then the flasks were removed from the rotatory shaker and filtered. The filtrate is subjected to AES to determine the amount of arsenic in the filtrate. The results showed that with increasing the amount of PPE and DPE adsorbents up to 0.1 g/L, removal of arsenic increases and after that no appreciable increase was observed. The initial rise in adsorption of arsenic is due to more adsorptive sites and a high amount of adsorbent available for adsorption of the ions (Figure S1(b)) (Bessaies et al. 2020).

Batch study was conducted at different pH conditions, i.e. 1 to 9, to determine the adsorption of arsenic by the PPE and DPE adsorbents in different media. The pH of solution was maintained by using H₂SO₄ and NaOH. The study was carried out by using 60 mg/L solution of arsenite and 0.06 g/L of adsorbent dose at pH of 1 to 9. Results indicate that adsorption of trivalent arsenic ions was dependent on pH (Figure S1(c)). Removal of trivalent arsenic was found to be increased by a decrease in the acidity of mixture. Maximum removal was achieved in neutral solution (pH 7). Further increase in the basicity of solution arsenic ions caused precipitation in the solution (Yin et al. 2022).

To determine the effect of time on adsorption of ions from the prepared solution, different time intervals were taken at other constant factors. Adsorption of arsenic ions increased with increase in the time up to 120 minutes and after that no significant increase was observed for PPE and DPE materials as more sites were available in the beginning of adsorption, which results more adsorption (Figure S1(d)) (Duarte et al. 2021). After 120 minutes sites are occupied by the arsenic ions and adsorption becomes almost constant. In case of derivatized material availability of more adsorption groups were leads to increase in the adsorption capacity of the material. So, it can be concluded that pH 7 and contact time of 120 min are favorable conditions for maximum removal from contaminated water with different concentrations of arsenic ions keeping the adsorbent dose from 0.06–0.3 g/L.

Experiments conducted in batch studies helped to optimize the conditions for adsorption isotherm studies. As the maximum removal was observed in a neutral aqueous sample after 120 minutes. These conditions were selected for further studies taking 0.06 g/L of adsorbent after studying the effect of pH, contact time, and adsorbent dose on removal of arsenic ions from the aqueous sample. Isotherm parameters were obtained by using the well-known Langmuir, Freundlich, and Temkin models. With the help of adsorption isotherms, it can be identified that whether the process of adsorption occurred on a homogenous surface with uni-layered binding or on a heterogeneous surface with multi-layered binding (Kumar & Chawla 2013; Kumar et al. 2015; Saini et al. 2018, 2019). These isotherms were applied to get the adsorption capacity of P. emblica for trivalent arsenic ions.

The value of b and A (L/min) can be calculated using intercept and slope of the linear plot of q_e versus lnC_e (Figure 6(c)). The results are summarized in Table 1. The value of B was found to be 14.26 J/mol for pristine material and 16.73 J/mol for derivatized material indicating physical adsorption on the surface of adsorbents. However, the low values of b represent a feeble interaction between trivalent arsenic and both adsorbents, which sometimes favors ion exchange mechanism.

The rate of adsorption of arsenic on P. emblica adsorbents were studied by using time-dependent data. Time-dependent data were obtained from 20 to 120 minutes' time duration at optimum dose of adsorbent and optimum concentration of arsenic ions at normal temperature. In the initial stage of time (15 to 20 minutes) fast removal of arsenic ions was achieved due to availability of active sites. The progress of removal after 20 minutes was quite slow due to occupation of the active sites by the arsenic ions. Time dependent data were also analyzed by using pseudo-first- and second-order kinetics models to known the mechanism of reaction, i.e. physiosorption or chemisorptions.

The value of q_e, k_1,q_e, and k₂ are tabulated in Table 2. The value of regression correlation coefficient (R²) confirmed that pseudo-second-order reaction kinetics model was the best fitted model and the value of calculated adsorption capacity at equilibrium was 27.77 and 31.25 mg/g for pristine and derivatized material respectively. This value was very close to experimental value for both of the adsorbents.

Fruit pulp, seed coat and seed are the main parts of the P. emblica fruit. Fat, proteins, primary aliphatic or aromatic amino acids, carbohydrates, pectic substances, gallic acid and tannic acids (phenolic acids), and crude fibers are the main components of seed coat of P. emblica. Octadeca-9,12,15-trienoic acid, long chain fatty acid tetradecanoic acid and octadecadienoate (linoleates) were the major components in seed coat oil (Khan 2009; Rai et al. 2012; Hasan et al. 2016; Variya et al. 2016; Kulkarni & Ghurghure 2018). Carboxylic group of thioglycolic acid reacts with EDC to form an activated ester leaving group, which is displaced by nucleophilic attack of primary aliphatic or aromatic amino acids on P. emblica seed at pH 4.0–6.0. EDC is released as soluble urea by product derivative (Figure 1 (Scheme I)). Trivalent arsenic has high affinity for sulfhydryl groups. One arsenic trivalent ion binds with three sulfhydryl groups (Figure 1 Scheme II). There may be two possibilities of binding the arsenic trivalent ions with sulfhydryl groups, one is that it may bind with sulfhydryl groups of the same unit and in another case the three sulfhydryl groups are from three different units (Figure 1 Scheme II (A&B)). Removal of the arsenic from water depends on the availability of binding sites of the adsorbent. Removal of arsenic by the derivatized material was also facilitated by binding with the sulfhydryl groups (Mohan & Pittman 2007; Shen et al. 2013; Ko et al. 2017). Thus, derivatized material showed more adsorption capacity than the pristine material because of more favourable binding positions on the surface of the adsorbent (Duarte et al. 2021).

Recycling of applied adsorbents and recovery of the arsenic ions from the desorbing agents is one of the most important parts of adsorption study to keep the environment and human beings safe and healthy. It is also very important to reduce the cost of the method. Both adsorbents were regenerated by designing column study using 30% H₂O₂ in 0.1 M HNO₃ followed by 0.1M NaOH solution (Natale et al. 2013; Lata et al. 2014). After regeneration both materials were washed with double distilled water and dried at 120 °C overnight, and stored for further use. The Langmuir q_max after regeneration of the PPE was observed 62.11 and for DPE 77.51 mg/g respectively. Thus, 80.74% for PPE and 85.27% for DPE efficiency was achieved in the first cycle for desorbing process (Figure S3).

Column study was performed using PPE and DPE adsorbent as a fixed bed packing in the column of 25 mL capacity, having the length 79.75 cm, outer diameter 1.4 cm, inner diameter 0.8 cm at a constant flow rate of 5 mL/minute for 20 mL synthetic arsenic solution. 2 mg/L aqueous arsenic solution at pH 7 was passed through the column packed with PPE and DPE adsorbents to check the applicability of the used materials. Column was prepared by taking glass column packed with PPE/DPE up to 5 cm. Glass wool was placed at the end of the column and at the top of the column to prevent the loss of the adsorbent (Figure S2). The column was preconditioned with double-distilled water. More than 98% efficiency was observed for PPE and 100% efficiency for DPE in the consecutive steps. No arsenic was found after passing through column with both of the adsorbents. Optimization plays a key role for treatments of ground as well as wastewater. Various combinations of parameters can be set by using the optimization results for maximum removal of arsenic from various water samples. These materials can be applied as a packing material in the column for real water samples with knowing the other characteristics conditions of the water samples.

It is significant to evaluate the active performance of novel adsorbents used in the study with other reported materials keeping in view of accessibility, usability, and adsorption capacity. Numerous synthetic, natural, and modified materials have been applied as adsorbents for removal of heavy metals and arsenic trivalent ions. However, utilization of some materials is still less due to low adsorption capacities and lack of regeneration studies or reuse capacity of the materials.

Similarly, uptake capacity of modified adsorbents is specified below(Figure 8(b)): For modified adsorbents: DPE seed coat 90.90 mg/g > Fe-coated EFBB (FC-EFBB) 31.4 > Fe coated rice husk biochar (FC-RHB) 30.7 > thiol functionalized sugarcan bagasse (TFSCB) 28.57 > xanthated water melon rind (X-WMR) 3.96 > citric acid modified water melon rind (CA-WMR) 3.89 > Japanese oak wood-derived biochar (OW-BC) 3.21 > acid treated longifolia leaf powder (ATLP) 1.51 > modified Cassia fistula biochar (MCFB) 0.78 > nanoporous activated garlic stem carbon (AGSC) 0.19 > thioglycolated sugarcane carbon (TSCC) 0.08 (Mohan & Pittman 2007; Shakoor et al. 2016, 2019).

The study reveals that P. emblica seed coat and its modified form are very good adsorbents with very high adsorption capacity compared to many natural and modified materials shown in Figure 8. The reason of high adsorption capacity of these adsorbents are presence of more and more binding sites on the surface and high affinity of arsenic trivalent ions for sulfhydryl groups in modified form.

In this paper PPE seed coat were modified in the form of N-substituted derivative of 2-sulfanylacetamide by using thioglycolic acid and EDC. IR, SEM, EDX, and XRD techniques were applied for characterization of PPE and DPE products. A weak peak at 1,575 cm⁻¹ (N–H bending) in infra red is deformed at 1,608 cm⁻¹ in the modified product due to modification of the primary amine into amide with the interaction of thioglycolic acid. Both novel materials were used for adsorption or removal of trivalent arsenic from water medium. Significant change was observed in derivatized product DPE because of presence of high affinity groups towards trivalent arsenic ions. The Langmuir q_max for PPE and DPE were observed to be 76.92 and 90.90 mg/g respectively. These materials can be applied as a packing material in the column for real water sample as well as for wastewater treatment under a given set of conditions to remove arsenic along with other ions.

This work was supported by management of Manav Rachna International Institute of Research and Studies Faridabad, Haryana, India.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.