Shell biomass material supported nano-zero valent iron to remove Pb²⁺ and Cd²⁺ in water

Abstract

Nanoscale zero-valent iron (NZVI) has a high adsorption capacity for heavy metals, but easily forms aggregates. Herein, preprocessed undulating venus shell (UVS) is used as support material to prevent NZVI from reuniting. The SEM and TEM results show that UVS had a porous layered structure and NZVI particles were evenly distributed on the UVS surface. A large number of adsorption sites on the surface of UVS-NZVI are confirmed by IR and XRD. UVS-NZVI is used for adsorption of Pb²⁺ and Cd²⁺ at pH = 6.00 in aqueous solution, and the experimental adsorption capacities are 29.91 and 38.99 mg g⁻¹ at optimal pH, respectively. Thermodynamic studies indicate that the adsorption of ions by UVS-NZVI is more in line with the Langmuir model when Pb²⁺ or Cd²⁺ existed alone. For the mixed solution of Pb²⁺ and Cd²⁺, only the adsorption of Pb²⁺ by UVS-NZVI conforms to the Langmuir model. In addition, the maximum adsorption capacities of UVS-NZVI for Pb²⁺ and Cd²⁺ are 93.01 and 46.07 mg g⁻¹, respectively. Kinetic studies demonstrate that the determination coefficients (R²) of the pseudo first-order kinetic model for UVS-NZVI adsorption of Cd²⁺ and Pb²⁺ are higher than those of the pseudo second-order kinetic model and Elovich kinetic model. Highly efficient performance for metal removal makes UVS-NZVI show potential application to heavy metal ion adsorption.

1. Introduction

Rapid growth of industrialization has created severe heavy metal pollution, and the release of many kinds of heavy metal ions in the environment not only affect the aquatic system but also human health [1]. According to statistics, about 67 000 tons of Pb(II) ions and 40 000 tons of Cd(II) ions are discharged into the environment annually [2,3]. Most of these toxic and untreated ions reaching the soil, the surface and ground water, permanently exist and accumulate in the ecosystem [4]. Through the food chain, Pb(II) and Cd(II) may accumulate in internal organs of animals and humans, such as the kidneys and liver, and result in acute or chronic poisoning, causing various diseases [5]. Thus, in order to maintain ecological stability and public safety, effectively removing Pb(II) and Cd(II) ions from the environment have become an important research focus in environmental fields [6,7].

In previous studies, various conventional techniques have been developed for the treatment of heavy metal ions in wastewaters, such as chemical precipitation, membrane separation, ion exchange, adsorption and electrochemical methods [8–12]. The chemical precipitation method has been widely applied due to its simplicity and ease of operation. However, the effectiveness of treatment for wastewater with low concentration of heavy metal ions remains poor [13]. The membrane separation and ion exchange are very effective methods for removing heavy metal ions from water, but the operational costs are higher in comparison with other methods [14]. Of all the known methods, the adsorption technique has been regarded as a simple and effective tool for the removal of heavy metal ions from wastewater owing to its wide adaptability, environment-friendliness and low cost [15]. In recent years, nanoscale zero-valent iron (NZVI), an environmentally benign material, via its controllable particle size, non-toxic, high reactivity and abundant reactive surface sites, has been widely used to treat various metal ions in aqueous solutions [16,17]. However, direct use of NZVI is restricted due to its lack of stability, easy aggregation and facing difficulties in separating NZVI from treated effluents [18]. To address this issue, some researchers consider using stable materials as carriers to support NZVI, which could mitigate oxidization and aggregation of the NZVI, such as Zhang et al. used pillared clay as the carrier of NZVI, and Fu et al. used cashew nut shell to support NZVI [19–21]. Of all carriers used by previous researchers, most carriers are only considered for their stability and load capacity, but not for their surface structure and specific surface area. However, superior porous structure and high specific surface area can make the carrier material support more NZVI, hence, looking for such a stable carrier with a large specific surface area will promote the wide application of NZVI.

Recently, due to the development of fisheries, the production of shellfish has increased year by year, and the massive shells produced have been directly discarded due to its limited application, thus causing waste of shell resources and environmental pollution [1,22]. As a high yield biomass resource, if shellfish can be fully used in large quantities according to its characteristics, it can not only realize the reuse of waste, but reduce the damage to the ecological environment. Shell materials contain about 90% of inorganic substance and about 10% of organic substance, and the two components are evenly distributed [23]. If we remove the organic substance and keep the inorganic substance, it will form a porous structure and the specific surface area will increase significantly. Using the activated shell as a substrate to load nano-zero-valent iron can not only support the nano-zero-valent iron and prevent agglomeration, but also make full use of the porous structure of the shell, increase the specific surface area of the adsorbent, achieving physical adsorption and chemical adsorption to combine. Therefore, it is worth studying to load NZVI on the low-cost shells as an adsorbent and explore its adsorption mechanism for different heavy metals.

In the study, the most common undulating venus shells (UVS) in hainan were processed into porous UVS carrier by impurity removal, pulverization, sieving and activating. In the chemical activation method, although strong acid conditions can remove organic matter, it will destroy the CaCO₃ skeleton structure of the shell. However, the removal of organic matter under alkaline conditions is not complete. Therefore, we choose high-temperature calcination as the activation process. Taking FeCl₂ as the raw material, we compound UVS loaded nanoscale zero-valent iron adsorbent (UVS-NZVI) using borohydride reacting. The composition and structure of UVS-NZVI were characterized by SEM, TEM, IR, XRD and BET. Finally, the adsorption properties of this material for Pb²⁺ and Cd²⁺ in water were investigated and the thermodynamic and kinetic mechanism for adsorption of Pb²⁺ and Cd²⁺ by UVS-NZVI was elucidated. This work provides an effective application approach for shellfish biomass resources, and promotes the wider application of NZVI adsorbent.

2. Material and methods

2.1. Materials

Undulating venus shells (UVS) were obtained from Haikou Yongshun seafood processing market. Absolute ethyl alcohol, FeCl₂ · 4H₂O (analytical grade), NaBH₄ (analytical grade), NaOH (analytical grade), HNO₃ (excellent grade), Cd(NO₃)₂ (analytical grade) and Pb(NO₃)₂ (analytical grade) reagents were purchased from Aladdin reagent (Shanghai) limited company. Deionized water was prepared by ET461275 ultra pure water machine (Lovibond, Germany).

2.2. Preparation of the undulating venus shell carrier

After washing with deionized water, the UVS were immersed in 0.01 mol l⁻¹ HNO₃ solution for 1 h to remove the soluble impurities attached to the surfaces. Then, the shells were cleaned with deionized water and placed in a drying oven for 24 h. The washed and dried UVS were crushed and passed through a 140 mesh sieve, and the resulting powder was placed in a muffle furnace and calcined at 450°C for 60 min [1]. Finally, the optimized UVS carrier was obtained.

2.3. Preparation of the supported adsorbent (UVS-NZVI)

The supported adsorbent was prepared by the liquid-phase reduction process, and the synthesis process compared with NZVI-GAC of Li et al. has been simplified and improved [24]. The whole synthetic process was performed in the helium atmosphere. Firstly, the UVS (6.00 g) and FeCl₂ · 4H₂O (5.49 g) were placed into a three-necked open flask, adding ethanol/water solution (200 ml, 30%) and stirring for 30 min. Subsequently, the NaBH₄ solution (100 ml, 1.00 mol l⁻¹) was added dropwise into the mixture, constantly stirring for 30 min. Finally, the black solid (UVS-NZVI) was isolated by suction filtration and dried. The reduction reaction is as formula (2.1) [25]

$$2\text{F}{\text{e}}^{2+}+\text{B}{\text{H}}_4^{-}+3{\text{H}}_2\text{O}+\text{UVS}\to 2E^0/\text{UVS}+{\text{H}}_2\text{B}{\text{O}}_3^{-}+4{\text{H}}^{+}+2{\text{H}}_2.$$ 2.1

2.4. Characterization of UVS-NZVI

The microstructures of the UVS and UVS-NZVI were imaged on S-4800 scanning electron microscopy (Hitachi, Japan) and H-9500 transmission electron microscopy (Hitachi, Japan). Fourier transform infrared (FTIR) spectra were obtained on a Tensor27 FTIR spectrometer (Bruker, Germany). X-ray diffraction (XRD) was recorded on a D/Max-2400 powder diffractometer (Rigaku, Japan), operating at 40 kV and 40 mA. The specific surface area and pore structure were measured on a specific surface area and porosity analyser (ASAP 2460 analyser, Micromeritics, USA). UVS-NZVI was degassed in vacuum and characterized by N₂ sorption at 77 K. The specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method. Pore diameter were calculated by Barrett–Joyner–Halenda (BJH) method [26,27].

2.5. Calculation method for adsorption amount

The total concentrations of Cd²⁺ and Pb²⁺ were determined by a flame atomic absorption spectrophotometer (TAS-990, Purkinje General, Beijing). The adsorption amount of Cd²⁺ and Pb²⁺ in aqueous solution by the UVS-NZVI is calculated using the formula (2.2)

$$Q=\frac{(C_0-C)\hspace{0.17em}\ast \hspace{0.17em}V}{m},$$ 2.2

where Q (mg g⁻¹) was the amount of adsorption, C₀ (mg l⁻¹) was the initial concentration of Cd²⁺ and Pb²⁺ before adsorption and C (mg l⁻¹) was the concentration of Cd²⁺ and Pb²⁺ after adsorption. V (l) was the volume of the solution, while m (g) was the mass of the UVS-NZVI.

Prepare lead standard series solutions with mass concentrations of 5, 10, 20, 30 and 40 mg l⁻¹ with 0.15 mol l⁻¹ dilute nitric acid solution. Measure the absorbance of the standard series of solutions by flame atomic absorption method; draw the standard curve of A_Pb (absorbance)–C_Pb (mass concentration of Pb²⁺). Measurement conditions: gas flow rate 1500 ml min⁻¹, height 5.00 mm, position 0.50 mm, working lamp current 2.00 mA, spectral bandwidth 0.40 nm and negative high voltage 300 V.

The standard curve of Pb²⁺ is shown in figure 1, the standard curve equation: y = 0.00906 + 0.02038x, R² = 0.99983. Within the range of Pb²⁺ mass concentration of 5–40 mg l⁻¹, the linear relationship is good.

Figure 1.

Standard curve for determining Pb²⁺ content.

Prepare cadmium standard series solutions with mass concentrations of 0, 1, 2, 3, 4 and 5 mg l⁻¹ with 0.15 mol l⁻¹ dilute nitric acid solution. Measure the absorbance of the standard series of solutions by flame atomic absorption method; draw the standard curve of A_Cd (absorbance)–C_Cd (mass concentration of Cd²⁺). Measurement conditions: gas flow rate 1300 ml min⁻¹, height 5.00 mm, position 0.50 mm, working lamp current 2.00 mA, spectral bandwidth 0.40 nm and negative high voltage 300 V.

The standard curve of Cd²⁺ is shown in figure 2, the standard curve equation: y = 0.01471 + 0.13171x, R² = 0.9968. Within the range of Cd²⁺ mass concentration of 0–5 mg l⁻¹, the linear relationship is good.

Figure 2.

Standard curve for determining Cd²⁺ content.

2.6. Adsorbent dosage experiment

Six samples of 100 ml 20 mg l⁻¹ Pb²⁺ solutions were prepared with the pH 6.00, and the adsorbent dosage set to 0.01 g, 0.02 g, 0.03 g, 0.04 g, 0.05 g and 0.06 g, respectively. The mixture was shaken at 30°C for 2 h. After filtration, the filtrate was measured for absorbance, the average value was determined with three parallel measurements, and the adsorption amount was calculated using formula (2.2).

Cd²⁺ solution was used for the above experiment, and other experimental conditions were the same.

2.7. PH gradient setting experiment

Six samples of 100 ml Pb²⁺ solutions were prepared with a concentration of 20 mg l⁻¹. After adjusting the value of pH from 4.00 to 6.50 (at 30°C, the solubility product of Pb(OH)₂ is about 1 × 10⁻¹⁷; according to the concentration of Pb²⁺ in the solution, calculations show that when the pH of the solution is slightly greater than 7.00, Pb²⁺ begins to precipitate), 0.04 g of UVS-NZVI was added into each solution. Then, the mixture was shaken at 30°C for 2 h. After filtration, the filtrate was measured for absorbance, the average value was determined with three parallel measurements, and the adsorption amount was calculated using formula (2.2).

Nine samples of 100 ml Cd²⁺ solutions were prepared with a concentration of 20 mg l⁻¹. The value of pH was adjusted from 4.00 to 8.00 and 0.04 g of UVS-NZVI was added into each solution. The subsequent processes were the same as Pb²⁺ solutions.

The pH of each group of solutions was adjusted with dilute solutions of NaOH and HNO₃ by pH meter (FE28-Standard, Mettler Toledo, Switzerland).

2.8. Coexistent ion experiment

Seven samples of 100 ml 20 mg l⁻¹ Pb²⁺ solutions and seven samples of 100 ml 20 mg l⁻¹ Cd²⁺ solutions were prepared with the pH 6.00, and the Cu²⁺ concentration set to 0, 2, 4, 6, 8, 10 and 12 mg l⁻¹, respectively. Of UVS-NZVI, 0.04 g was added into each solution. Then, the mixture was shaken at 30°C for 2 h. After filtration, the filtrate was measured for absorbance, the average value was determined with three parallel measurements, and the adsorption amount was calculated using formula (2.2).

Change Cu²⁺ to Ni²⁺ in the above process, and other experimental conditions are the same.

2.9. Adsorption thermodynamic experiment

Seven samples of 100 ml Pb²⁺ solutions and seven samples of 100 ml Cd²⁺ solutions were prepared, and the concentrations were 5, 10, 20, 30, 50, 75 and 100 mg l⁻¹, respectively. Then 0.04 g of UVS-NZVI was added to each solution and the mixture was shaken at 30°C for 2 h. After filtration, the filtrate was measured for absorbance, and the average value was determined with three parallel measurements. The adsorption amount was calculated using formula (2.2).

Also, seven kinds of different concentrations of mixed solutions of Pb²⁺ and Cd²⁺ were prepared and adjusted to pH of 6.00. The following steps are consistent with the above experiment. The Langmuir and Freundlich adsorption isotherm models were adopted to evaluate the isotherm experiment data.

Langmuir and Freundlich adsorption isotherm models were given as formulae (2.1) and (2.2) [28,29]

$$q_e=\frac{q_{max}b{C}_e}{1+b{C}_e}$$ 2.3

and

$$q_e=K_f{C}_e^{n_f},$$ 2.4

where q_e (mg g⁻¹) is the equilibrium adsorption capacity, q_max (mg g⁻¹) is the maximum adsorption capacity, C_e (mg g⁻¹) is the equilibrium concentration, b is the constant, K_f is the Freundlich constant, and n_f is the concentration index.

Five samples of 100 ml 20 mg l⁻¹ Pb²⁺ solutions and five samples of 100 ml 20 mg l⁻¹ Cd²⁺ solutions were prepared. Then 0.04 g of UVS-NZVI was added to each solution and the mixture was shaken at 20, 30, 40, 50 and 60°C for 2 h, respectively. After filtration, the filtrate was measured for absorbance, and the average value was determined with three parallel measurements. The adsorption amount was calculated using formula (2.2).

The Van't Hoff equation was used to fit the adsorption capacity at different temperatures to obtain the thermodynamic parameters ΔH and ΔS of Pb²⁺ and Cd²⁺. Calculate ΔG at different temperatures by Gibbs equation. Van't Hoff and Gibbs equations were given as formula (2.5) and (2.6) [30,31]

$$\text{ln}{K}_C=-\left(\frac{\mathrm{\Delta }H}{R}\right)\frac{1}{T}+\frac{\mathrm{\Delta }S}{R},$$ 2.5

$$\mathrm{\Delta }G=\mathrm{\Delta }H-T\mathrm{\Delta }S$$ 2.6

$$\text{and}{K}_C=\frac{C_s}{C_e},$$ 2.7

where C_s is the concentration of the solid surface at the adsorption equilibrium and C_e is the concentration in the solution at the adsorption equilibrium.

2.10. Kinetic adsorption experiment

A 50 mg l⁻¹ Pb²⁺ solution and 50 mg l⁻¹ Cd²⁺ solution were prepared. Then 0.02 g of UVS-NZVI was added to 100 ml of Pb²⁺ or Cd²⁺ solution and the mixture was shaken at 30°C. Determining the concentration of Pb²⁺ and Cd²⁺ every 5 min within the adsorption time of 5–60 min, the adsorption amount was calculated using formula (2.2). The pseudo first-order, pseudo second-order and Elovich kinetic models were used to determine the rate of the adsorption process.

Pseudo first-order and pseudo second-order kinetics models were given as formula (2.8) and (2.9) [32,33]

$$q_t=q_e(1-\exp (-k_{1}t))$$ 2.8

and

$$q_t=\frac{q_e^2{k}_{2}t}{1+q_e{k}_{2}t},$$ 2.9

where q_t (mg g⁻¹) is the adsorption capacity at time t (min), and q_e (mg g⁻¹) is the equilibrium adsorption capacity. And k₁ (min⁻¹) was the pseudo first-order kinetic rate constant, and k₂ (g min mg⁻¹) was the pseudo second-order kinetic rate constant.

Elovich kinetic model was given as formula (2.10) [34]

$$q_t=(1+{\beta }_E)\ln (1+{\alpha }_E{\beta }_{E}t),$$ 2.10

where β_E (g mg⁻¹) is the desorption constant related to the activation energy of chemisorption and α_E (mg (g min)⁻¹) is the initial adsorption rate.

2.11. Recyclability of UVS-NZVI

The reusability of UVS-NZVI was tested by repeated Pb²⁺ and Cd²⁺ adsorption and desorption cycles for five consecutive cycles. First, 100 ml of 50 mg l⁻¹ Pb²⁺ or Cd²⁺ solution was prepared. Then 0.02 g of adsorbent was added. The mixture was shaken at 30°C for 2 h. The desorption process was in 30 ml ultrapure water for 10 h.

The filtrate during each cycle was measured for absorbance. The average value was determined with three parallel measurements, and the adsorption amount was calculated using formula (2.2).

The preparation and adsorption processes of UVS-NZVI are shown in figure 3.

Figure 3.

Preparation and adsorption processes of UVS-NZVI.

3. Results and discussion

3.1. Characterization of adsorbents

The porous lamellar structure of UVS is shown in figure 4a,b. More channels inside the UVS might be opened due to the acidic conditions in the preparation process, which increased the pore volume and internal specific surface area of UVS [35]. The SEM images of NZVI and UVS-NZVI are presented in figure 5. From figure 5a,b the NZVI particles can be seen clearly and had an average diameter of 60 nm. However, these particles are aggregated due to lack of support material [36]. According to figure 5c,d, NZVI particles are uniformly dispersed on the UVS surface without obvious accumulation, and the size of particles is relatively small, indicating that NZVI loaded onto the UVS successfully [37].

Figure 4.

SEM images of UVS.

Figure 5.

SEM images of NZVI (a,b) and UVS-NZVI (c,d).

The structural information provided by TEM is in agreement with SEM analysis. From figure 6a,b, massive NVZI particles are aggregated, while in figure 6c,d, flocculation (NZVI or derivatives of Fe⁰) is distributed around the UVS structure evenly [38].

Figure 6.

TEM images of NZVI (a,b) and UVS-NZVI (c,d).

The XRD patterns of UVS and UVS-NZVI are shown in figure 7. For UVS (a), the characteristic peaks are attributed to aragonite CaCO₃ and calcite CaCO₃, and most of the characteristic peaks are calcite CaCO₃ [39]. For UVS-NZVI (b), except for the 2θ = 57.48° attributed to calcite CaCO₃ characteristic peaks, the rest are characteristic peaks of Fe and its compounds. As shown, the strong peaks at 44.81° and 50.93° are present on UVS-NZVI composite, corresponding to Fe⁰ [40]. In addition, some minor peaks of Fe compounds, for example, Fe₂O₃ (55.23°) and Fe(OH)₃ (64.01°), are also observed, which might result from oxidation and corrosion during the synthesis and drying process [41,42].

Figure 7.

XRD pattern of UVS (a) and UVS-NZVI (b). 1–3, 5, 7, 8, 12, 13 = calcite-type CaCO₃; 4, 6 = aragonite-type CaCO₃; 9, 10 = Fe⁰; 11 = Fe₂O₃; 14 = Fe(OH)₃.

Figure 8 exhibits FTIR of the UVS and UVS-NZVI. For UVS (a), the 710.58, 860.18 cm⁻¹, etc. are the characteristic peaks of $\text{C}{\text{O}}_3^{2-}$, indicating that the main component of UVS is CaCO₃ [43]. The peak at 860.18 cm⁻¹ is blue shifted by 5.68 cm⁻¹ compared with the peak at 854.50 cm⁻¹ of aragonite CaCO₃, which shows that UVS also contains a small amount of calcite CaCO₃, and the remaining peaks are essentially identical with those of aragonite CaCO₃. Based on the FTIR spectrum of UVS, there are no other functional groups in addition to $\text{C}{\text{O}}_3^{2-}$ [39,44]. Therefore, calcium carbonate did not decompose at 450ۜ°C in this experiment.

Figure 8.

FTIR spectra of UVS (a) and UVS-NZVI (b).

After UVS was loaded with NZVI (b), various peaks of iron compounds are all obvious. The peak at 3353.91 cm⁻¹ can be attributed to O–H association with Fe or its oxyhydroxide [45]. The peaks at 1276.71 and 1344.56 cm⁻¹ can be attributed to bands associated with Fe₃O₄, Fe₂O₃ and FeOOH formation surrounding Fe⁰, as well as ethanol added in synthesizing the adsorbents [46]. In addition, 1130.26, 946.31 and 823.57 cm⁻¹ are the characteristic peaks of −OH in Fe(OH)₃, and 618.56 cm⁻¹ is the characteristic peak of Fe₂O₃ [47,48]. The FTIR spectrum indicates the slight oxidation of Fe⁰, and the abundant oxygen functional groups could act as available adsorption sites, which could provide free pairs of electrons to interact with the empty orbital of metal ions (Pb²⁺ and Cd²⁺). This characterization result is basically consistent with the XRD, indicating the existence of zero-valent iron on the surface of UVS.

Nitrogen sorption isotherm and pore size distribution are shown in figures 9 and 10. The specific surface area and pore diameter of UVS-NZVI calculated by BET method and BJH method are 83.77 m² g⁻¹ and 3.54 nm, respectively, which indicates that the porous structure of shell material makes nano-zero-valent iron have a larger contact area with the solution, and the material may have higher adsorption performance.

Figure 9.

Nitrogen sorption isotherm of the sample recorded at 77 K.

Figure 10.

Pore size distribution.

3.2. Adsorption studies

3.2.1. Influence of adsorbent dosage on the adsorption of Pb²⁺ and Cd²⁺ ions

The effects of different adsorbent dosage on the adsorption of Pb²⁺ and Cd²⁺ by UVS-NZVI are shown in figure 11. For Pb²⁺ and Cd²⁺, when the amount of adsorbent is less than 0.04 g, the adsorption capacity remains almost unchanged. When the amount of adsorbent is greater than 0.04 g, the adsorption capacity drops significantly. This may be due to the ion concentration being too small, the added adsorbent has not reached its optimal adsorption effect. Therefore, in order to achieve the best removal effect and the best utilization of the adsorbent, the amount of adsorbent in subsequent experiments will be 0.04 g.

Figure 11.

The effect of adsorbent dosage on adsorption of Pb²⁺ and Cd²⁺.

3.2.2. Influence of solution pH on the adsorption of Pb²⁺ and Cd²⁺ ions

According to figure 12, the pH of the solution significantly affected the adsorption of UVS-NZVI to heavy metal ions.

Figure 12.

The effect of pH on adsorption of Pb²⁺ (a) and Cd²⁺ (b).

In strong acidic conditions, Fe⁰ was corroded to some extent, which made UVS-NZVI poor adsorption effect on metal ions. As a result, within 4.00 < pH < 5.50, the equilibrium adsorption capacity was less than 20 mg g⁻¹, whether it is Pb²⁺ or Cd²⁺, and the pH increased significantly after adsorption. Within pH ≥ 5.50, adsorption performance improved obviously.

In the adsorption of Pb²⁺ by UVS-NZVI, compared with Fe²⁺/Fe⁰, Pb²⁺/Pb⁰ has the higher electrode potential. Therefore, Pb²⁺ can be reduced to Pb⁰ by Fe⁰ and the Pb⁰ was continually adsorbed onto the iron oxide layer. Within pH ≥ 5.50, the chemical equation of the reaction is as formula (3.1) [6,49]

$$\text{P}{\text{b}}^{2+}+\text{F}{\text{e}}^0+2\text{O}{\text{H}}^{-}\to \text{P}{\text{b}}^0+\text{Fe}{(\text{OH})}_2.$$ 3.1

According to the chemical equation, increasing OH⁻ concentration is conducive to the reaction, and the adsorption process reduces the pH of the solution. As a result, for Pb²⁺ (figure 12a), the equilibrium adsorption amount increased with increasing pH in the pH range of 5.5–6.5. The adsorption effect of UVS-NZVI for Pb²⁺ was optimal at pH = 6.00, and the pH of each solution decreased after adsorption.

For Cd²⁺, due to the ion exchange between Cd²⁺ and H⁺ on the surface of UVS-NZVI, pH of the solution decreased [50]. In addition, Cd²⁺ could coprecipitate with Fe²⁺ to form metal hydroxides over the external surfaces of adsorbents. Within pH ≥ 5.50, the chemical equation of the reaction is as formula (3.2) [51,52]

$$n\text{C}{\text{d}}^{2+}+(1-n)\text{F}{\text{e}}^{2+}+2\text{O}{\text{H}}^{-}\to \text{C}{\text{d}}_n\text{F}{\text{e}}_{(1-n)}{(\text{OH})}_2.$$ 3.2

Therefore, increasing pH facilitated the adsorption of Cd²⁺ by UVS-NZVI (figure 12b). The equilibrium adsorption amount first raised and then stayed the same approximately in the pH range of 5.50–8.00. The adsorption effect of UVS-NZVI for Cd²⁺ was optimal in pH ≥ 6.00.

Due to the acidic environment, the Fe⁰ reacted with H⁺, which limited its adsorption effectiveness. As a result, the optimal pH for the adsorption by UVS-NZVI is selected as pH = 6.00 in subsequent experiments.

3.2.3. Influence of coexisting ion on the adsorption of Pb²⁺ and Cd²⁺ ions

The effects of different concentrations of Cu²⁺ and Ni²⁺ on the adsorption of Pb²⁺ and Cd²⁺ by UVS-NZVI are shown in figures 13 and 14.

Figure 13.

The influence of Cu²⁺ concentration on the adsorption of Pb²⁺ and Cd²⁺.

Figure 14.

The influence of Ni²⁺ concentration on the adsorption of Pb²⁺ and Cd²⁺.

According to figure 13, the presence of Cu²⁺ has a great influence on the adsorption of Pb²⁺ and Cd²⁺. Since the oxidability of Cu²⁺ is similar to that of Pb²⁺ and slightly greater than that, as the concentration of Cu²⁺ increases, the adsorption performance of UVS-NZVI for Pb²⁺ decreases to a certain extent. The oxidability of Cu²⁺ is much greater than that of Cd²⁺, so as the concentration of Cu²⁺ increases, the adsorption performance of UVS-NZVI on Pb²⁺ decreases significantly, and when the concentration of Cu²⁺ reaches 12 mg l⁻¹, the adsorption capacity of Cd²⁺ is only 1.93 mg g⁻¹, which shows that Cd²⁺ is hardly adsorbed.

The effects of Ni²⁺ on the adsorption of Pb²⁺ and Cd²⁺ by UVS-NZVI are shown in figure 14. Since the oxidability of Pb²⁺ is much greater than that of Ni²⁺, with the increase of Ni²⁺ concentration, the adsorption capacity of Cd²⁺ hardly changes, and there is almost no adsorption competition between the two ions. The oxidability of Ni²⁺ is close to that of Cd²⁺. As the concentration of Ni²⁺ increases, the adsorption capacity of Cd²⁺ decreases to a certain extent, but UVS-NZVI has still a significant adsorption capacity for Cd²⁺.

3.2.4. Adsorption thermodynamic analysis

The parameters and fitted plots of Langmuir and Freundlich adsorption isotherm models are listed in table 1 and figure 15a,b. The maximum adsorption capacity of UVS-NZVI for Pb²⁺ in two different situations was 93.01 and 86.99 mg g⁻¹ (figure 15a), respectively. In the solution containing only Pb²⁺, based on the higher determination coefficient (R²), the Pb²⁺ adsorption by UVS-NZVI was more in line with the Langmuir model, which indicates that the adsorption sites on the adsorbent are homogeneously distributed with a monolayer coverage of adsorption products [45,53,54]. The adsorption capacity of UVS-NZVI to Pb²⁺ was hardly affected by the coexisting Cd²⁺ in mixed solution and the adsorption of UVS-NZVI to Pb²⁺ was still more consistent with Langmuir model compared to Freundlich. In addition, the Freundlich constant n_f for single and mixed adsorption experiment were all between 0.1 and 1, indicating that the adsorption process of Pb²⁺ was favourable [55,56].

Table 1.

Langmuir and Freundlich adsorption isotherm parameters for Cd²⁺ and Pb²⁺ on UVS-NZVI.

metal ions	adsorption types	Langmuir			Freundlich
		qmax (mg g−1)	b (l mg−1)	R2	Kf	nf	R2
Pb2+	single	111.88	0.103	0.9911	20.273	0.395	0.9234
	mixed with Cd2+	114.928	0.057	0.9909	13.08	0.477	0.9499
Cd2+	single	66.016	0.035	0.9797	5.307	0.515	0.9344
	mixed with Pb2+	3.501	—	—	—	—	—

Figure 15.

Adsorption isotherm model for Pb²⁺ (a) and Cd²⁺ (b) on UVS-NZVI.

The maximum adsorption capacity of UVS-NZVI for Cd²⁺ was 46.07 mg g⁻¹ in the solution containing only Cd²⁺. Because the correlation coefficient of Langmuir model (0.9797) fitting is greater than that of Freundlich model (0.9344), the Cd²⁺ adsorption by UVS-NZVI was more in line with the Langmuir model. Nevertheless, the presence of Pb²⁺ observably decreased the Langmuir adsorption capacity of Cd²⁺ to 3.31 mg g⁻¹ (figure 15b), which indicates the competition for active sites between them. The compound Pb-hydroxide on the surface of UVS-NZVI inhibited the formation of Cd–Fe-hydroxide in mixed solution and the surface cation exchange sites were completely occupied by Pb-hydroxide precipitation and Pb [57–59]. Hence, there was almost no Cd²⁺ adsorbed on the UVS-NZVI composite. In summary, Pb²⁺ is far more competitive than Cd²⁺.

In separate adsorption experiment for Pb²⁺ or Cd²⁺, the correlation coefficients of Langmuir fitting are 0.9911 and 0.9797, respectively, which are higher than the correlation coefficients of Freundlich fitting (0.9234 and 0.9344). Since the important assumption of the Langmuir model is monolayer adsorption, and typical chemical adsorption is also monolayer adsorption, it can be inferred that the adsorption of Pb²⁺ or Cd²⁺ by UVS-NZVI may be chemical adsorption.

Since the activated shell (UVS) has a porous structure and a larger specific surface area than other biomass materials, the adsorption capacity of UVS-NZVI has certain advantages. The maximum adsorption capacities of Pb²⁺ and Cd²⁺ by nano-zero-valent iron supported by different biomass carriers are listed in table 2 [7,60–63].

Table 2.

Maximum adsorption capacity of different biomass materials.

ion species	biomass carrier	maximum adsorption capacity (mg g−1)
Pb2+	UVS	93.01
	Posidonia oceanica seaweed	49.63
	cassava fibre	52.97
	sepiolite	33.42
Cd2+	UVS	46.07
	Posidonia oceanica seaweed	33.15
	sugarcane fibre	45.36
	chitosan	19.13

Van't Hoff's fitting results and parameters for temperature changes are shown in figure 16 and table 3. According to the calculation results of thermodynamic parameters, the ΔH of UVS-NZVI for Pb²⁺ and Cd²⁺ adsorption are 29.33 and 53.89 kJ mol⁻¹, respectively, which means that the adsorption processes of UVS-NZVI for Pb²⁺ and Cd²⁺ are endothermic. According to the study of Ma et al. [64], the enthalpy change range of physical adsorption is between 2.10 and 20.90 kJ mol⁻¹, and the enthalpy change range of chemical adsorption is between 20.90 and 418.40 kJ mol⁻¹. Therefore, the adsorption of Pb²⁺ and Cd²⁺ by UVS-NZVI is chemical adsorption in this study, which is consistent with the results obtained by Langmuir model fitting. In the temperature range of 293.15–333.15 K, the ΔG range of UVS-NZVI for Pb²⁺ adsorption is between −4.84 and −0.74 kJ mol⁻¹, and the ΔG range for Cd²⁺ adsorption is between −7.99 and −0.57 kJ mol⁻¹. With the increase of temperature, the two ΔG values both decrease, indicating that the adsorption process is spontaneous.

Figure 16.

Fitting results of K_c and 1/T of Pb²⁺ and Cd²⁺.

Table 3.

Adsorption thermodynamic parameters of Pb²⁺ and Cd²⁺.

ion species	T (K)	ΔG (kJ mol−1)	ΔH (kJ mol−1)	ΔS (J mol−1 K−1)	R2
Pb2+	293.15	−0.74	29.33	102.56	0.9955
	303.15	−1.76
	313.15	−2.79
	323.15	−3.81
	333.15	−4.84
Cd2+	293.15	−0.57	53.89	185.77	0.9917
	303.15	−2.43
	313.15	−4.82
	323.15	−6.14
	333.15	−7.99

The calculation results of the above thermodynamic parameters fully prove that the adsorption process of Pb²⁺ and Cd²⁺ by UVS-NZVI is a spontaneous chemical adsorption process.

3.2.5. Adsorption kinetics analysis

The effect of adsorption time for the removal of Pb²⁺ and Cd²⁺ ions by the UVS-NZVI was studied with ion concentration = 50 mg l⁻¹, and the results are shown in figure 17a,b. It was observed that the adsorption amount of metal ions was increased with an increase of the time in the range of 5–60 min whether it was Pb²⁺ or Cd²⁺, and the adsorption amount was increased quickly in the first 30 min, then increased gradually. As a result, Pb²⁺ reached adsorption equilibrium on the UVS-NZVI at 50 min and Cd²⁺ at 45 min. The adsorption of ions increased rapidly at the initial stage due to the accessibility of huge amount of binding sites on the surface of the UVS-NZVI. Thus the percentage removal of ions significantly increased at lesser adsorption time. At higher adsorption time, the rate of adsorption increased slowly until equilibrium due to the available binding sites being gradually decreased; for that reason, it took a long time for the adsorption process to reach equilibrium [18].

Figure 17.

Adsorption kinetic model for Pb²⁺ (a) and Cd²⁺ (b) ions on UVS-NZVI.

The adsorption mechanism of the Pb²⁺ and Cd²⁺ ions on UVS-NZVI was analysed by using three adsorption kinetic models, namely pseudo first-order kinetic model (the reaction rate is linearly related to the concentration of a reactant, and this model is based on the fact that rate-determining step is a physical process), pseudo second-order kinetic model (the reaction rate is linearly related to the concentration of two reactants, and this model is based on the fact that rate-determining step is a chemical reaction) and Elovich kinetic model (the Elovich model is suitable for processes with irregular data or with large activation energy) [65]. The fitting results are shown in figure 17a,b, and the kinetic parameters for the adsorption of Cd²⁺ and Pb²⁺ are listed in table 4. The fitted adsorption capacity (q_e) was compared with the equilibrium adsorption amount (q_e,exp) in the experiment. It can be observed from table 4 that the q_e of pseudo first-order kinetic for the adsorption of Cd²⁺ and Pb²⁺ by UVS-NZVI was much closer to the q_e,exp compared with other kinetic models. In addition, the determination coefficient of the pseudo first-order kinetic was higher than that of pseudo second-order and Elovich kinetic model for both Cd²⁺ and Pb²⁺, which indicates that the physical adsorption is the rate-determining step [66]. It may be possible that the huge specific surface area of UVS material increases the contact area significantly between NZVI and the solution compared with other carrier materials and this advantage plays a decisive role in the adsorption process, which agrees with Huang et al. [67].

Table 4.

Adsorption kinetic parameters for Pb²⁺ and Cd²⁺ ions on UVS-NZVI.

adsorption kinetic model	parameters	metal ions
		Pb2+	Cd2+
pseudo first-order	qe (mg g−1)	43.79	23.75
	k1 (min−1)	0.0655	0.0676
	R2	0.9976	0.9938
pseudo second-order	qe (mg g−1)	55.56	29.99
	k2 (min−1)	0.0012	0.0023
	R2	0.9874	0.9782
Elovich	αE (mg (g min)−1)	0.0259	0.0551
	βE (g mg−1)	13.6274	6.8208
	R2	0.9616	0.9506

3.2.6. Recyclability analysis

Figure 18 shows the adsorption amount variation trend of multiple adsorption of Pb²⁺ or Cd²⁺ by UVS-NZVI in aqueous solution. After five recycles, the adsorption of Pb²⁺ decreased from the initial 41.26 to 14.34 mg g⁻¹ and the Cd²⁺ from 23.09 to 9.02 mg g⁻¹. The reason was that the NZVI particles were exhausted by chemical reduction and the absorbed Pb²⁺ and Cd²⁺ ions were not completely resolved [42]. In adsorption of the fourth recycle, the UVS-NZVI adsorption amount of Pb²⁺ was 20.39 mg g⁻¹ and the Cd²⁺ adsorption amount was 11.90 mg g⁻¹, which indicates that UVS-NZVI can still achieve about 50% effect of the initial adsorption after four recycles.

Figure 18.

The effect of utilization times on adsorption of Pb²⁺ and Cd²⁺.

4. Conclusion

In summary, UVS material-supported NZVI was successfully synthesized. NZVI particles can be evenly distributed on the porous layered structured UVS surface, which can effectively prevent NZVI from reuniting. A large number of adsorption sites can be derived from a large amount of Fe⁰, a small amount of Fe₂O₃, Fe(OH)₃ and FeOOH on the surface of UVS-NZVI. The adsorption amounts of Pb²⁺ and Cd²⁺ both increased with the initial pH and then remained unchanged, and the adsorption effect of the two ions reached the peak at pH = 6.00 and the experimental maximum adsorption capacities of Pb²⁺ and Cd²⁺ were 29.91 and 38.99 mg g⁻¹, respectively. The presence of Cu²⁺ has a significant effect on the adsorption of Pb²⁺ and Cd²⁺. Ni²⁺ has a great influence on the adsorption of Cd²⁺, but there is almost no adsorption competition with Pb²⁺. Thermodynamic studies showed that the adsorption of Pb²⁺ and Cd²⁺ by UVS-NZVI is a chemical adsorption. Kinetic studies demonstrated that the physical adsorption was the rate-determining step. Further, the recyclability experiment suggested that UVS-NZVI had a good recycling effect in treating Pb²⁺ and Cd²⁺ in wastewater. Overall, this study not only provides an effective application approach for shellfish biomass resources, but also promotes the wider application of NZVI adsorbent, which makes UVS-NZVI have certain application potential in the heavy metal ions adsorption field.

Data accessibility

The data supporting the findings of this study are available at the Dryad Digital Repository: https://doi.org/10.5061/dryad.gqnk98sjz [68].

Authors' contributions

Z.W., X.W., S.L., Y.W., Z.T. and Q.D. undertook the research. Z.W. and X.W. participated in every part of the experiment and wrote the manuscript, they should be considered co-first authors.

Competing interests

We declare we have no competing interests.

Funding

This research was supported by the Natural Science Foundation of China (grant no. 21767008) and the Natural Science Foundation of Hainan Province (grant no. 219000).