Modified Shrimp-Based Chitosan as an Emerging Adsorbent Removing Heavy Metals (Chromium, Nickel, Arsenic, and Cobalt) from Polluted Water

Abstract

Water quality is under constant threat worldwide due to the discharge of heavy metals into the water from industrial waste. In this report, we introduce a potential candidate, chitosan, extracted and isolated from shrimp shells, that can adsorb heavy metals from polluted water. The waste shrimp shell chitosan was characterized via Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS). The adsorption capacity of heavy metals on the modified shrimp shell was measured using an inductively coupled plasma mass spectrometry before and after adsorption. The highest adsorption of arsenic, nickel, and cobalt was 98.50, 74.50, and 47.82%, respectively, at neutral pH, whereas the highest adsorption of chromium was 97.40% at pH 3. Correspondingly, the maximum adsorption capacities of MSS for As, Cr, Ni, and Co were observed to be 15.92, 20.37, 7.00, and 6.27 mg/g, respectively. The application of Langmuir and Freundlich isotherm models revealed that the adsorption processes for the heavy metals were statistically significant (r² > 0.98). The kinetic studies of metal adsorption, using modified shrimp shell, were well explained by the pseudo-second-order kinetic model with linear coefficients (r²) of >0.97. The presence of a greater number of functional groups on the adsorbent, such as N–H coupled with H–O, –COO⁻, C–H, N–N, and C–O–C, was confirmed by FTIR analyses. Furthermore, SEM-EDX analysis detected the presence of elements on the surface of modified shrimp shell chitosan. This noteworthy adsorption capacity suggests that MSS could serve as a promising, eco-friendly, and low-cost adsorbent for removing toxic heavy metals including Cr, Ni, As, and Co and can be used in many broad-scale applications to clean wastewater.

1. Introduction

Water, an essential resource for life, is under constant threat by the pollution generated by human activities [1,2,3]. In particular, heavy metals present in the environment are hazardous to most living organisms, creating severe environmental issues impacting variety preservation [4]. The increased environmental pollution caused by heavy metals in rivers, lakes, soils, or sediments can be attributed to rapid growth in agricultural, industrial, and domestic activities [5]. For example, industrial wastewater contains different derivatives of heavy metals, for example, arsenic (As), chromium (Cr), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), manganese (Mn), silver (Ag), cadmium (Cd), lead (Pb), mercury (Hg), and organic dyes, among others. These pollutants are continuously discharged into the ecosystem, can accumulate in muscles, brain, bones, liver, and kidney, and may cause many severe diseases and disorders including kidney failure, nervous illness, anemia, encephalopathy, hepatitis, nephritic syndrome, and even death [3,6,7,8,9,10]. Furthermore, long-term exposure to arsenic via drinking water and contaminated foods has led to several lethal diseases, such as gangrene, keratosis, cancer, diabetes, and cardiovascular disorders, among others [2,11], while chromium is classified as a priority pollutant [12] and can be carcinogenic, toxic, and mutagenic to living organisms. A trace amount of this metal is essential in the human diet for glucose metabolism [13]. Nickel poisoning causes pulmonary fibrosis, skin dermatitis, vomiting, diarrhea, nausea, and neurological disintegration, particularly in children [14]. The long-term effect of cobalt may cause an asthma-like allergy resulting in shortness of breath, cough, chest tightness, and wheezing. In addition, cobalt may affect hearing and visual impairment, thyroid, kidneys, liver, and cardiovascular and endocrine systems [15]. Consequently, wastewater recycling becomes essential at the point of generation along with the removal of heavy metals from water for safe water consumption for various human activities [16].

The sustainable removal of heavy metals from polluted water has become a significant challenge for scientists. Several techniques have been developed for removing heavy metals and other pollutants from polluted water. Such techniques include microbial degradation, coagulation, bioaccumulation, ion exchange, chemical precipitation, evaporation, reverse osmosis, electrofloatation, membrane filtration, electrocoagulation, electrodialysis, solvent extraction, nanotechnology, and electrodeposition [17,18,19,20,21]. These approaches, however, require a huge quantity of chemical reagents and may not be ideal, as the techniques generate secondary byproducts along with slug which need additional processing, leave behind trace-level heavy metals, and have high operational costs [22,23]. Nevertheless, the application of biotechnology in removing heavy metals can be considered through the lens of some advantages including low cost, renewability, ready availability, high efficiency, wide suitability, easy handling, simplicity of use, surface area, various surface groups, and environmental friendliness. To this effect, agricultural-based wastes may be considered as the potential resources for producing bio-adsorbents for heavy metal remediation [24,25,26,27,28,29].

One such adsorbent is the shrimp shell [30]. The demand for shrimp has increased worldwide, especially in coastal cities or countries [31]. Therefore, shrimp processing and consumption generate millions of tons of waste shrimp shells which are discarded yearly [30]. Shrimp shell is an abundant natural resource of chitin and chitosan that may cause serious environmental problems if improperly disposed of. The shrimp chitosan products are rich in protein, minerals, polysaccharides, oxygen, and nitrogen-containing functional groups, effective in the adsorption of metal ions [30,32,33]. Shrimp shell waste product chitosan provides two benefits: (i) it can reduce the concerns related to waste material disposal and degradation, and (ii) it removes heavy metals from industrial effluent or any other contaminated aqueous solution [34]. The chitosan from waste shrimp shells can be developed for implementing the concept of “waste to treat waste” by an eco-friendly manner. Thus, it can reduce the overall cost of remediation technologies [34].

This study aimed to develop a sustainable technique for the removal of heavy metals from the wastewater. We report here the synthesis of an effective shrimp-based adsorbent from the modified waste shrimp shell (MSS), using cheap and readily available NaOH and H₂SO₄ solution as the modification reagent for removing Cr, Ni, As, and Co metals. The MSS chitosan, a modified and deacetylated form of chitin, has been demonstrated to be a heavy metal adsorbent. In addition, Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and inductively coupled plasma mass spectroscopy (ICP-MS) were utilized to measure the adsorption efficiency of Cr, Ni, As, and Co ions in the solution. Our results provide us with useful knowledge for the bioremediation of heavy metals. This study further elucidates new possibilities of a cost-effective technology for bioremediation of heavy metals from wastewater.

2. Materials and Methods

2.1. Reagents

Unless otherwise mentioned, all the chemicals used in this study were obtained from Wako Pure Chemical Corporation, Osaka, Japan. The chemicals were ACS grade and used as received. 1M stock solutions of potassium chromate (K₂CrO₄), nickel sulfate (NiSO₄), disodium arsenate (AsHNa₂O₄), and cobalt chloride (CoCl₂) were prepared and stored at 4 °C until further use. The standard working solutions of 100 mM and then 1.0 mM were prepared by diluting the stock solution using Milli Q water, Ʊ at 18.2 (Merck, Darmstadt, Germany). The pH of the metal solutions was adjusted using either 0.1 M NaOH or 0.1 M HCl. All adsorption experiments were performed at 28 °C temperature.

2.2. Preparation of Chitosan

The waste shrimp shells were collected from a local shrimp processing unit in Saga, Japan and washed with hot water to remove shrimp flesh residues, lipids, and other materials [4]. Then, the shells were filtered using a net, washed with distilled water, and dried in an air oven at 70 °C. Further preparation of chitosan is depicted in Figure 1.

2.2.1. Demineralization

The demineralization of shrimp shells was performed using a method previously described with some modifications [4]. In brief, the dried shrimp shells were submerged in 1.5 % HCl at 1:30 (w/v) for 20 h at room temperature to decalcify. The shrimp shells were then washed several times with deionized water to remove CaCl₂ and other water-soluble impurities until the waste solution reached neutral pH 7 ± 0.1.

2.2.2. Deproteinization

The deproteinization of mineralized shrimp shells was carried out as described previously with some modifications [4,33]. Briefly, to remove the remaining proteins and other organic materials, the demineralized shells were immersed in 5% NaOH for 24 h at 90 °C with a solvent-to-shell ratio of 12:1 (v/w). The shrimp shell was then washed with deionized water to neutrality and then dried in a vacuum oven at 60 °C for 12 h. The resulting product was chitin.

2.2.3. Decolorization

The decolorization of the chitin was performed by submerging it in acetone solution (Environmental Grade, 99.5%) at room temperature for 24 h to remove the pigments. The decolorized samples were cleaned with DI water to pH = 7 ± 0.1. Then, the chitin was dried in an oven at 60 °C for 12 h to obtain pure chitin chitosan [4].

2.2.4. Deacetylation

The deacetylation of chitin chitosan was performed using 50% NaOH (15%, w/v) for 8 h at 60 °C. The sample was rinsed with deionized water until the eluent reached pH of 7.0 and dried for 12 h at 80 °C. The dried sample of chitosan was finely grounded and sieved through 180 mesh, and the percentage yield of the chitosan was calculated using Equation (1) [35]:

$$\% \mathrm{yield}=\frac{mass of chitin \left(g\right)}{mass of dried shrimp shells \left(g\right)}\times 100\%$$

(1)

2.3. Fourier Transform Infrared (FT-IR) Spectroscopy

The FT-IR spectrometer provides detailed information on the functional groups in the modified shrimp shell (MSS) samples. FT-IR spectroscopy analysis of MSS samples was performed as previously described [36]. To identify the presence of chemical functional groups and binding mechanisms involved in the metal biosorption process, structural details of the MSS bio-sorbents were observed by Fourier transform infrared spectrometer (FT-IR) (model: VERTEX 70v). FT-IR spectra of untreated MSS and treated MSS with four metal ions (As⁵⁺, Cr⁶⁺, Ni²⁺, and Co²⁺) were recorded in absorbance mode in the range of 400–4000 cm⁻¹ [37].

2.4. Scanning Electron Microscopy and Energy Dispersive Spectroscopy (SEM-EDS)

The surface morphology of MSS was further examined with an SEM-EDS (SEM Hitachi 3400N, Tokyo, Japan) before and after exposure to metals to identify the presence of elements [38]. Briefly, the MSS was shocked in a 1.0 mM cocktail of a four-metal solution (Cr⁶⁺, Co²⁺, Ni²⁺, and As⁵⁺) for 2 h while continuously shaken at 160 rpm at room temperature. The MSS samples were collected and gently washed with deionized water. The MSS samples were then coated with platinum before imaging and processed as described previously [38] using a vacuum device MSP-1S Magnetron Sputtering Equipment (Magnetron Sputter, RT1195006, Hitachi, Tokyo, Japan) for the surface morphology analysis using a Hitachi S-3400N at 15~25.0 kV (Tokyo, Japan). To determine the presence of ions on the bio-sorbent, the MSS samples were processed for the Energy Dispersive Spectroscopy (EDS) [38]. High-resolution spectra were fitted and the analyses were quantified using the AVANTAGE software (Thermo Fisher Scientific, XPS, Wilmington, DE, USA), where Wt% is the concentration of the element in terms of the mass fraction of that element in the sample. The other elements were removed from the EDS analysis, whereas oxygen (O) and carbon (C) were kept as the signal elements. The EDS system provides information on the quantitative weight percentages of each element of interest.

2.5. Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) Analyses

Following the bio-adsorption of metals by MSS, the concentration of metal ions was measured in the liquid solution using an inductively coupled plasma mass spectrometer (ICP-MS) as reported previously [1]. Briefly, a mixture of four metal ions, Cr⁶⁺, Co²⁺, Ni²⁺, and As⁵⁺, with MSS was shaken at 160 rpm for 2 h at 28 °C. The control samples without MSS were also shaken at 160 rpm for 2 h at 28 °C. The MSS-free supernatant was harvested by centrifugation for 10 min at 10,000 rpm. All samples (supernatants) were filtered using a 0.45 μm filter to eliminate the small particles and acidified with supra pure 2M HNO_3. Each acidified sample was diluted and made to a constant volume in the cellulose-free liquid prior to measuring the concentration of metal ions using Agilent technologies 7900 quadrupoles ICP-MS (Santa Clara, CA, USA). All treatments were conducted in triplicates and the data were presented as the mean ± the standard error (S.E.). Bio-adsorption was calculated using the following Equations (2)–(4):

C_ad = C₀ − C_t

(2)

$$\mathrm{Bioadsorption} \%=\frac{\mathrm{Cad}}{{\mathrm{C}}_0}\times 100$$

(3)

where C₀ denotes the initial concentration and C_t denotes the concentration at a time ‘t’ (ppm); C_ad is the adsorbed concentration (ppm).

The adsorption capability q_e(mg/g) was calculated after equilibrium using the following equation:

$${\mathrm{q}}_{\mathrm{e}}=\frac{\mathrm{Cad}}{\mathrm{W} \left(\mathrm{g}\right)}. \frac{\mathrm{V}\left(\mathrm{mL}\right)}{1000}$$

(4)

where W denotes the amount (g) of MSS used, V denotes the volume (mL) of the metal solution, and q_e is the adsorption at equilibrium [39,40].

An adsorption isotherm offers information on the binding affinity, adsorption capacity, and surface characteristics of the adsorbent, which may help to understand the adsorption capacity along with the binding mechanism of the adsorbate with the adsorbent [41]. The Langmuir adsorption isotherm was applied in this study to clarify the equilibrium adsorption characteristics of shrimp-based chitosan for metal uptake, as depicted in Equation (5):

$${\mathrm{q}}_{\mathrm{e}}=\frac{{\mathrm{q}}_{\max }{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}{1+{\mathrm{K}}_{\mathrm{L}}{\mathrm{C}}_{\mathrm{e}}}$$

(5)

where q_max denotes the maximum adsorption capability (mg/g), and K_L is Langmuir’s isotherm constant, which shows the binding affinity between test beads and metal ions. The isotherm constants were calculated from the slopes of linear plots and intercepts, and the linear form of Langmuir’s isotherm equation is expressed in Equation (6).

$$\frac{1}{{\mathrm{q}}_{\mathrm{e}}}=\frac{1}{{\mathrm{K}}_{\mathrm{L}}{\mathrm{q}}_{\max }}.\frac{1}{{\mathrm{C}}_{\mathrm{e}}}+\frac{1}{{\mathrm{q}}_{\max }}$$

(6)

The separation factor (R_L) was calculated using Equation (7).

$${\mathrm{R}}_{\mathrm{L}}=\frac{1}{1+{\mathrm{C}}_{\mathrm{i}}{\mathrm{K}}_{\mathrm{L}}}$$

(7)

where R_L specifies the adsorption possibility is either linear (R_L = 1), unfavorable (R_L > 1), favorable (0 < R_L > 1), or irreversible (R_L = 0) [42].

Freundlich isotherm is also most commonly used to represent adsorption data from solutions [2]. The logarithmic linear Freundlich model can be expressed in Equation (8) [30].

$${\ln \mathrm{q}}_{\mathrm{e}}=\frac{1}{\mathrm{n}}\ln {\mathrm{c}}_{\mathrm{e}}+\ln {\mathrm{K}}_{\mathrm{F}}$$

(8)

where K_F (mg/g) represents the Freundlich characteristic constants related to adsorption intensity and n denotes the adsorption intensity. In particular, when 1/n is in the range of 0.1–1.0, it indicates the favorable adsorption of metals [30,43].

To further evaluate adsorption kinetics parameters, the pseudo-first-order and pseudo-second-order models stated in Equations (9) and (10) were introduced [30,43].

$$\log \left({\mathrm{q}}_{\mathrm{e}}-{\mathrm{q}}_{\mathrm{t}}\right)={\mathrm{logq}}_{\mathrm{e}}-\frac{{\mathrm{k}}_1}{2.303}\mathrm{t}$$

(9)

$$\frac{1}{{\mathrm{q}}_{\mathrm{t}}}=\frac{1}{{\mathrm{K}}_2{\mathrm{q}}_{\mathrm{e}}}+\frac{1}{{\mathrm{q}}_{\mathrm{e}}}\mathrm{t}$$

(10)

where q_t (mg/g) and q_e (mg/g) represent the adsorption capacity of MSS at time t (min) and equilibrium, respectively. k₁ (min⁻¹) is the rate constant of pseudo-first-order adsorption process. Values of k₁ were calculated from the plots of log (q_e − q_t) versus time (t). On the other hand, k₂ is the rate constant of pseudo-second-order adsorption process. The values of q_e and k₂ were calculated from the slope and intercept of the linear plot of t/q versus t, respectively.

2.6. Effects of pH on Bio-Adsorption

The effects of pH on the bio-adsorption of the metal ions by MSS were assessed [40]. Briefly, in a 3 mL tube, 10 mg of MSS was added into a mixture of 1.0 mL of 1 mM solution of each metal. The pH of each metal solution was then adjusted individually to either 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, or 7.0 ± 0.1 pH. A pH of more than 7 was not considered in this study, as some metals are precipitated at basic pH [44]. The biosorption was performed at 28 °C for 2 h with continuous shaking at 160 rpm. According to the World Health Organization guideline, the pH of drinking water should preferably be less than 8.0. It is hypothesized that the MSS could be used to purify drinking water and wastewater. Therefore, all experiments in this study were conducted in triplicates at pH 7 in a cocktail solution of the metal ions. Following adsorption, the concentrations of each metal ion in the solution were measured by ICP-MS.

2.7. Effects of Contact Time on Adsorption

To investigate the effect of contact time on adsorption, 10 mg of MSS was added to a 1 mL solution containing 1.0 mM of each of the Cr⁶⁺, Co²⁺, Ni²⁺, and As⁵⁺ ions. The pH was then adjusted to 7.0 ± 0.2. The adsorption process was conducted at 28 °C by shaking at 160 rpm for the following seven different time points: 0, 30, 60, 90, 120, 150, and 180 min. The samples were collected at each time point for ICP-MS analyses.

2.8. Effects of Bio-Adsorbent Dosages

The amount of adsorbent used plays a crucial role in adsorption [45]. Therefore, to determine the optimum bio-adsorbent dosages, each sample of 1 mL solution adjusted at pH 7.0 ± 0.2, containing 1.0 mM of each of the Cr⁶⁺, Co²⁺, Ni²⁺, and As⁵⁺ ions with an increasing amount of MSS of 2.5, 5.0, 7.5, 10, and 12.5 mg, was incubated at 28 °C by shaking at 160 rpm for 24 h.

2.9. Statistical Analysis

The statistical analyses were performed using OriginPro 2021 software from OriginLab Corporation (Northampton, MA, USA). The results were presented using the mean value with error bars representing the standard deviation of three replicate experiments (n = 3). The goodness of fit to a bio-adsorption model was assessed using the coefficient of determination (r²). A linear fit correlation was run to assess the relationship between metal concentration and adsorbent amount. There was a significant positive correlation between metal concentration and adsorbent amount r² = 0.999. At the 0.05 level, the slope is significantly different from zero. Reference management was performed using Mendeley software (Elsevier, Mendeley Ltd., London, UK).

3. Results

3.1. Elemental Analysis

A previous study found that shrimp shells contain 20–30% chitin, 20–30% protein, 30–40% calcium carbonate, and other smaller substances [46]. The EDS elemental analysis of MSS and the contents of elements are presented in

Table 1. SEM-EDX quantification of elements present in MSS.

Element	Wt%
C	49.6
O	26.49
N	21.33
Na	0.88
Mg	0.52
I	0.48
S	0.21
Cl	0.18
P	0.17
Ca	0.16
Total	100

and Figure 2. The results illustrated that the major elements present in the MSS are C, O, and N, as much as 49.6%, 26.49%, and 21.33%, respectively, whereas the minor elements, including Na, Mg, I, S, Cl, P, and Ca, are presently less than 1% in the shrimp shell (Table 1).

EDS results further showed that the surface of each sample of shrimp-based chitin and chitosan mainly consisted of C, N, and O. Nevertheless, other inorganic elements, such as Ca, Al, Mg, P, and Si, were found in MSS because of their high acidic affinities forming strong stabilities (Figure 2). Furthermore, the amount of minerals, especially calcium, is highly reduced in shrimp shells compared to the unmodified shrimp shell due to the modification of shrimp shells by the strong base-to-acid ratio (5% NaOH and 1.5 % HCl). This finding is consistent with a previously reported study [30]. The deacetylation of chitin results in a cationic polysaccharide, chitosan, with abundant amino groups which can serve as a platform to adsorb anionic metals extensively, in particular at a low pH—a milieu that enhances the adsorption of anionic pollutants. Consequently, chitosan can serve as a value-added bio-adsorbent [30].

3.2. Yield of MSS

Table 2. Yield and percentages of MSS.

Shrimp Shells	Weight	% of Yield
Wet weight	100 g	100
Dry weight	35 g	35% of wet weight
Demineralization	15.4 g	44% of dry weight
Deproteinization	9.5 g	27.14% of dry weight
Decolorization	9.0 g	25.72% of dry weight
Deacetylation	8.4 g	24% of dry weight

illustrates the yields and percentages of modified shrimp shell chitosan. A total of 100 g of wet clean shrimp shells were dried until a constant weight was observed. The dry weight measured as much as 35 g (Table 2). Therefore, the water content in the wet shrimp shells was determined to be 65%. Following demineralization, the weight was measured, and it measured as much as 44% of its dry weight (35 g). Similarly, the weight measurement was recorded and expressed as the percentage of the dry weight (35 g) of shrimp shells at each step of the chitosan modification (Table 2). The data also demonstrated that chitosan content represented 24% of dry weight after demineralization, deproteinization, decolorization, and deacetylation of shrimp shells. The low chitosan content could be attributed to the removal of calcium carbonate, other mineral components, and water-soluble impurities under the strong base-to-acid conditions (5% NaOH and 1.5% HCl). However, many washing steps were involved in the demineralization, deproteinization, decolorization, and deacetylation, which may affect the chitosan yield. Several studies have extracted chitin from shrimp shells and obtained different yield percentages [4,47,48]. In this study, the yield percentages differed from previously reported studies, which may be due to varying conditions, such as the washing steps and using different acid and base concentrations.

3.3. SEM-EDS Analysis

The SEM micrographs of MSS showed non-homogenous and non-smooth surfaces, as depicted in Figure 3. This analysis was performed before and after exposure of MSS to the cocktail solution of four metals containing Cr⁶⁺, Co²⁺, Ni²⁺, and As⁵⁺ to determine the adsorbents’ distribution and physical morphology of MSS. At higher magnifications of 500×, 1000×, and 2000×, the MSS displayed irregular shapes, with more small chips and uneven surface, showing many small hills and valleys with varying shapes in the presence of the metals (Figure 3a–c). Furthermore, the structure of the MSS treated with metals showed a distinct network structure and exhibited clear porous structures, suggesting that most of the protein and fat on the surface had been removed [47] and were favorable for metal adsorption [49].

EDS analysis of MSS following exposure to metal solutions revealed that the element composition of the chitin and chitosan consisted of carbon (C), nitrogen (N), and oxygen (O). The chitin was identified throughout the surface (Figure 3d) only when MSS was exposed to the metal solutions compared to that of control MSS exposed to solutions without any metal ions. Image mapping and overlapping were performed to verify how the metal ions were distributed over the surface. Image mapping, conducted using 64 images of corresponding metals with 256 pixels, helped detect how the metal ions were distributed on the surface layers of MSS (Figure 4). To this effect, the result showed that different metal ions were in different colors (Figure 4). Furthermore, EDS image mapping revealed that MSS adsorbed all four metals at the surface with a constant level of intensity throughout the mapping. This finding is further validated by the ion imaging on the surface of the MSS at the surface when exposed to the metal solution (Figure 4). These figures demonstrated that all metals’ intensity levels remained constant throughout the mapping. The image mapping further confirms the results obtained in the ion imaging on the surface of the MSS.

In contrast, EDS identified the Co²⁺ and Ni²⁺ ions with significantly greater intensity, but the biosorption remained smaller as quantified by the ICP-MS. Thus, the EDS spectra are reliable qualitative measurements but not a quantitative measure of the bio-adsorption efficiency. The image mapping detects the presence and distribution pattern of metal ions on the surface of the adsorbents.

3.4. FTIR Analysis

The functional groups present on the surface of MSS were characterized by the FTIR analysis. The oxygen-containing functional groups such as phenolic hydroxyl, carbonyl, hydroxyl, and carboxyl play an essential role in metal ion adsorption [50]. Details of the functional groups help assess the likely attachment of sorbate on the surface of the sorbents [51]. The FTIR spectra of MSS treated with a cocktail of metal ions revealed the widest and strongest peak at 3309 cm⁻¹, which may be due to the extensional vibration of N–H coupled with H–O on the surface of MSS (Figure 5). It refers to the presence of ketones and phenolic groups of cellulose, pectin, hemicellulose, and lignin, which assist the metal ion binding in an aqueous solution [50].

The alteration of peaks at 1629 cm⁻¹ revealed the presence of the carboxylate anions (Figure 5). This characteristic feature of chitosan polysaccharides suggests the occurrence of deacetylation [52]. The peaks at 1383 cm⁻¹, 1317 cm⁻¹, and 1100 cm⁻¹ are attributed to C–H, N–N, and C–O–C groups, respectively. The vibrational peaks at 2927 cm⁻¹ and 2850 cm⁻¹ may be due to the C–H symmetric and asymmetric stretching vibration, respectively [53]. Additionally, this could be attributed to aliphatic structures originating from the lipid in MSS [54] or to the deprotonation of carboxyl and hydroxyl groups of proteins coordinate with metal ions [55]. These changes are associated with carboxylate and hydroxylate ions, contributors to metal uptake [50]. The FTIR analysis further suggested that the metal binding ability of the adsorbent was enhanced due to the increased number of carboxylate ligands [56]. Therefore, due to the chemical modification, hydroxyl, carbonyl, and carboxyl groups enhanced the adsorption process, which is consistent with the findings by Abd-Talib et al. [56]. A significant amount of metal ion removal by MSS could be attributed to the presence of a high number of functional groups detected on the chemically pre-treated adsorbent.

3.5. Effects of pH

The initial pH of the solution influences the efficiency of bio-adsorbent to remove pollutants and plays a vital role that affects the protonation of functional groups, hence affecting the metal chemistry for the bio-adsorption of metal ions [30,57]. In this study, the bio-adsorption was conducted individually at varying pHs ranging from 1–7 (Figure 6). The maximum removal of the chromium ions was observed at pH 3, with the least effective removal noted at pH 7. At acidic pH, the electronegative functional groups on the surface were protonated, creating a suitable atmosphere for chromium ions to bind [40]. Another study suggested that pH 1 is the optimum condition for the removal of Cr (VI) from solution [4].

Several other studies demonstrated that the highest biosorption of Cr⁶⁺ was achieved at different pH values of 2 [32,58,59], 3 [60], and 5 [61] using different adsorbent. This might be because of the different procedure of the sample preparation compared with the present study. There are two forms of chromium ions at acidic pH: chromic acid (H₂CrO₄) at pH 1–2 and chromate ions (HCrO₄) at pH 3–7 [62]. However, the maximum bio-adsorption was observed at pH 7 for Ni²⁺, As⁵⁺, and Co²⁺. The bio-adsorption is noticeably low at pH 1 and 2 but increases linearly up to pH 7. An almost similar pattern of metal ion accumulation was observed at different pHs (Figure 6). A similar observation was noted for arsenic removal using shrimp shells at pH 6.5 [63], pH 7 [64], and for nickel removal using shrimp chitosan at pH 7 [33]. The removal of metal cations by chitosan at low-to-neutral pH can be attributed to its amino groups along with the increased level of electrostatic attraction as chitosan becomes more protonated [48,64,65]. At low pH, the high concentration of H⁺ and H₃O⁺ protonates the hydroxyl and carbonyl groups, and they compete for the aqueous heavy metal ions for the available binding sites in bio-sorbents, resulting in little or no adsorption [66].

3.6. Effects of Contact Time

The contact time required for bio-adsorption of heavy metals may be the most critical factor for the efficient remediation, as depicted in Figure 7. The bio-adsorption pattern of Cr⁶⁺ and As⁵⁺ was nearly the same in all time intervals from 30 min to 180 min, while nickel adsorption was initially slow and then gradually rose at 180 min. On the other hand, the absorption of cobalt was much higher during the first 30 min, followed by a slight decrease through 120 min and then a gradual increase by 180 min (Figure 7). The optimum time for Cr⁶⁺ removal was noted at 30 min, with a 96.42% reduction. A similar observation was reported in removing chromium using shrimp shells, where chromium was reduced by as much as 96.6% in 20 min [4]. The adsorption of As⁵⁺ was 84.9% during the first 30 min, which is consistent with the previously reported data demonstrating that the arsenic adsorption reached the optimum level after 20 min [67]. The adsorption percentage of As⁵⁺ in a cocktail solution is lower (85.21%) than an individual exposure (98.50%). Chen and Chung [68] showed that 71% of As⁵⁺ removal was performed using biopolymer chitosan.

The adsorptions of Co²⁺ and Ni²⁺ in a mixture were 28.84% and 53.57%, respectively, which are lower compared to individual exposure. Like previous reports [30,38,47], we also observed a faster initial removal rate followed by a slower biosorption of ions, which could be due to the accessibility of the binding sites on the MSS surface occupied by the metal ions at the initial phages. Therefore, 30 min could be considered the optimum contact time for removing the Cr⁶⁺, Ni²⁺, As⁵⁺, and Co²⁺ using MSS in an aqueous solution.

3.7. Effects of Adsorbent’s Amount

Among all parameters considered in this study, the amount of adsorbent significantly influences the adsorption process involving removal efficiency and adsorption capacity. In this study, the amount of MSS (adsorbent) used ranged from 2.5 mg to 12.5 mg per experiment, containing 1.0 mM each of the four metal ions (Cr⁶⁺, Co²⁺, Ni²⁺, and As⁵⁺). The results of adsorbent dosages presented in Figure 8 depict that the biosorption of all metal ions increased gradually with the increasing bio-sorbent amount from 2.5 to 12.5 mg. The measurement ceased at 12.5 mg of MSS, demonstrating 97.72%, 58.86%, 89.10%, and 28.70% removal of Cr, Ni, As, and Co, respectively. It is evident that the adsorption increased with the increased absorbent [4,63], which corresponds with the findings of the present study. The adsorption capacity of shrimp shells is directly proportional to its surface area exposed to metal ions to interact. For example, the increased units of surface area enhance the metal removal capacity of MSS by exposing the binding sites for metals [45,69]. With the increasing amount of MSS (2.5 mg to 12.5 mg), the removal capacity of As⁵⁺ is increased from 43.14% to 89.10%. A similar study showed that by increasing the amount of shrimp shell from 1.0 g to 3.0 g, the removal capacity was increased from 76.39% to 91.47% [63]. Similarly, in this study, the removal capacity of Co²⁺, Ni²⁺, and Cr⁶⁺ was also increased from 21.43% to 28.70%, 25% to 58.86%, and 81.78% to 97.72%, respectively (Figure 8). To this effect, several other studies also observed that increased adsorbent dosage increases removal efficiency [40,60,61,70].

As stated above, the higher adsorbent dosage increased the total adsorption of metals by providing more surfaces and functional groups on the adsorbent [45,69]. While the Langmuir adsorption isotherm is applied to create a linear model for monolayer adsorption and is frequently used to calculate the adsorption, the Freundlich isotherm is used to model multilayer adsorption on heterogeneous surfaces [39]. These isotherms are most frequently used to represent the data of sorption from solution. The adsorption isotherms were fitted to the equilibrium values and the projected model parameters together with the linear regression coefficient (r²). Fitting curves of Langmuir and Freundlich isotherm models are plotted in Figure 9a–f, respectively. The corresponding adsorption parameters are listed in

Table 3. Isotherm and kinetic model parameters for the removal of metal ions by MSS.

Models	Parameters	Cr6+	Ni2+	As5+	Co2+
Langmuir	qmax (mg/g)	20.37	7.00	15.92	6.27
	KL(l/mg)	0.2857	0.2438	0.4124	0.8269
	RL	0.6428	0.6523	0.4569	0.8410
	r2	0.9999	0.9973	0.9999	0.9979
Freundlich	KF (mg/g)	19.489	6.69	14.88	5.99
	1/n	0.3440	0.2299	0.1852	0.4420
	r2	0.9989	0.9837	0.9950	0.9944
Pseudo-first-order	qe, cal (mg/g)	1.87	1.89	1.90	1.82
	k1 (min−1)	0.353 × 10−4	0.358 × 10−4	0.211 × 10−4	0.194 × 10−4
	r2	0.9801	0.9856	0.9435	0.9333
Pseudo-second-order	qe, cal (mg/g)	6.0309	3.8232	7.8150	2.1182
	k2 (g mg−1 min−1)	0.8683	0.0253	0.6659	0.0586
	r2	0.9999	0.9914	0.9995	0.9761

.

r² values calculated to be near 1 (r² > 0.95) for the observed linear relationships for all four metal ions were found to be statistically significant for both Langmuir and Freundlich isotherm analysis (Figure 9), demonstrating the excellent applicability of the adsorption process [71]. As indicated by the higher correlation coefficients of the Langmuir isotherm model, this model might be more accurate than the Freundlich isotherm model in describing the adsorption process. Moreover, the Freundlich isotherm model for Ni revealed that it neither touched all the dots nor properly fit for the adsorption. The expressions of straight lines were found by means of a mathematical transformation of the isotherm equation. Based on the Langmuir isotherm model, the maximum adsorption capacities of MSS for As, Cr, Ni, and Co were observed to be 15.92 mg g⁻¹, 20.37 mg g⁻¹, 7.00 mg g⁻¹, and 6.27 mg g⁻¹, respectively, whereas according to the Freundlich isotherm model, the maximum adsorption capacities of MSS for As, Cr, Ni, and Co were observed to be 14.88 mg g⁻¹, 19.49 mg g⁻¹, 6.69 mg g⁻¹, and 5.99 mg g⁻¹, respectively. The details shown in Table 3 illustrate the linear regression coefficient values, Langmuir’s constant, and adsorption possibilities. The values of R_L were found to be in the range of (0–1), which supported the favorable uptake of the heavy metal ions [39]. Furthermore, 1/n values from the Freundlich isotherm are within the range of 0.1–1.0, indicating the favorable adsorption and strong affinity towards the metals [30].

The fitting curves of pseudo-first-order and pseudo-second-order at six varied temperatures are plotted in Figure 10, and their individual kinetic parameters are simplified in Table 3. The overall correlation coefficient (r²) values of the pseudo-first-order kinetic model for Cr, Ni, As, and Co are 0.9801, 0.9856, 0.9435, and 0.9333, respectively, lower than those of the pseudo-second-order kinetic model for Cr, Ni, As, and Co, which are 0.9999, 0.9914, 0.9995, and 0.9761, respectively (Table 3). The data revealed that the pseudo-second-order kinetic model fitted well and was more suitable to describe the adsorption process. Several similar findings were observed for the adsorption of cadmium [72], nickel [16] on dead and live biomass of Bacillus subtilis, arsenic adsorption on chitosan biosorbent [41] and shrimp shells [43], and methyl orange adsorption on shrimp-shell-derived hydrochar [30].

The unit adsorption of metal ions was decreased with an increase in adsorbent dosage. For example, the unit adsorption of Cr was reduced from 20.37 to 4.87 mg g⁻¹ as the test solution’s adsorbent dosage increased from 2.5 to 12.5 mg/1 mL (Figure 11). Similarly, the unit adsorption of Ni was reduced from 7 to 3.29, As from 15.92 to 6.58, and Co from 6.27 to 1.68 mg g⁻¹ (Figure 11). This may be due to competitive adsorption, overlapping or aggregating adsorbent surface area available to ions in the solution. Obviously, the adsorption rate ratio is not equal to the adsorbent dosages.

Therefore, increasing the adsorbent dosage could reduce the availability of metal ions to be adsorbed. On the other hand, enough sites are available to interact with metal ions in the solution at the optimal amount of adsorbent. Therefore, the additional adsorbent is not appropriate for bio-adsorption. The adsorption capacity of chitosan on some heavy metal ions may be unstable, and several studies selected the optimal dosage for different bio-sorbent materials to remove metal ions from contaminated water [4,34,63,73,74]. However, these optimal dosages differ from the results obtained in this work because different studies were performed with varying bio-sorbents, metal ions, and metal ion concentrations.

The highest percentage of Cr⁶⁺ reduction was recorded at as much as 97.40% after 120 min of 10 mg of MSS exposure individually at pH 3 (Figure 11). At the same time, the highest adsorption of arsenic, nickel, and cobalt was 98.50%, 74.50%, and 47.82%, respectively, at neutral pH (Figure 12).

The reduction in arsenic is relatively higher comparing to that of other metals. Furthermore, chitosan and its derivatives displayed good adsorption capacities toward arsenic [68]. However, the preparation parameters of chitosan varied largely without any agreement. Therefore, the heavy metal adsorption behavior of chitosan varied greatly among published reports [75].

Table 4. Summary of the metal types, maximum adsorption capacity, and types of adsorbent.

Biosorbents	Cr (mg/g)	Ni (mg/g)	As (mg/g)	Co (mg/g)	Reference
Shrimp-based chitosan		45.1334 μg/g			[33]
Shrimp shells			0.125–0.126		[43]
Thiol-modified chitin nanofibers			149		[76]
Modified chitosan capsule			12.8 μmol/m2		[77]
Chitosan/clay/magnetite			5.9		[78]
Chitosan clay biocomposite	73				[79]
Chitin/magnetic nanocomposite	10.7				[80]
Chitin	4.6				[81]
Chitin-g-ethylenediamine	17.5				[82]
Chitosan/sporopollenin	0.99 mmol/g				[83]
Chitin nanofibrils	16.28				[84]
Chloroacetic acid chitosan				59.1	[85]
Chemically modified chitosan	20.37	7.00	15.92	6.27	This study

provides a quick summary of the maximal adsorption capabilities reported for the absorption of several metals along with the relevant references.

4. Conclusions

Shrimp-based chitosan is an effective natural scavenger of heavy metals. This study proposes a shrimp-based chitin and chitosan adsorbent employing several procedures, including acid washing, alkaline pretreatment, acetone washing, and deacetylation, with a great promising outcome of the efficient removal of toxic heavy metals from polluted water. Adsorption of arsenic (98.50%), chromium (97.40%), nickel (74.50%), and cobalt (47.82%) by chitosan are such examples. Correspondingly, the maximum adsorption capacities of MSS for As, Cr, Ni, and Co were observed to be 15.92 mg g⁻¹, 20.37 mg g⁻¹, 7.00 mg g⁻¹, and 6.27 mg g⁻¹, respectively. The FTIR analysis confirmed the presence of a greater number of functional groups on the surface of shrimp shell chitosan, such as N–H coupled with H–O, –COO⁻, C–H, N–N, and C–O–C. In addition, SEM-EDS analysis confirmed the presence and distribution of metal ions (Cr⁶⁺, Ni²⁺, As⁵⁺, and Co²⁺) on the surface of the MSS exposed in a metal solution. This technique is low-cost and environmentally friendly. The use of shrimp-based chitin and chitosan as a bio adsorbent may play an essential role in reducing the pollution caused by direct anthropogenic activities and in bringing significant economic benefits. The reusability and stability of chitosan could be checked for a better understanding of the nature of the adsorbent to refine the application of MSS in the bioremediation of heavy metals from polluted water.