Exaggerated arsenic removal efficiency and pH adaptability by adsorption using monodispersed porous pinecone-like magnesium hydroxide

adsorption, arsenic, industrial wastewater, porous magnesium hydroxide, pH symmetry

Arsenic compounds are classified as Class I carcinogens due to their high toxicity to the organism. Also, they are easily accumulated in water bodies, and both H₂AsO₄⁻ and HAsO₄²⁻ are present simultaneously and convert to each other in a wide pH range. Based on the strategy of simultaneous removal of protons to immobilize AsO₄³⁻, a monodispersed porous pinecone-like Mg(OH)₂ (PLMH) was prepared via a facile and environmentally friendly ultrasound-assisted precipitation route for deep As(V) removal. The PLMH presents a porous and stable framework structure formed by crossed lamellae, and the As(V) solution can be completely immersed inside, which gives a ‘surface effect’ inside the microsphere and makes the As(V) capture performance much higher than the general adsorbents by the removal of protons to immobilize AsO₄³⁻. In addition, the PLMH has an extremely wide pH applicability range (pH 3–12), special pH effects, and symmetry phenomena. These performances indicate that the PLMH can be a good candidate for the treatment of real arsenic industrial wastewater.

HIGHLIGHTS

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The PLMH exhibits exaggerated adsorption properties for water contaminated with As (945.8 mg·g⁻¹).
The first discovery of interesting pH symmetry of the PLMH in wastewater contaminated with As.
The PLMH can be mass-produced and has great prospects for real wastewater applications.

INTRODUCTION

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Arsenic is a known toxic substance (70–80 mg of arsenic trioxide can cause human death) (Sharma & Sohn 2009; Wei et al. 2019b) and one of the top 20 high-priority hazardous substances (Yu et al. 2011). Frequent human activities (Yu et al. 2019) such as the discharge of industrial wastewater have led to the accumulation of arsenic in the environment and caused serious water pollution (Yang et al. 2022b). Generally, the arsenic concentrations for groundwater and surface water are in the range of 1 μg·L⁻¹–73.6 mg·L⁻¹ and 1–263 μg·L⁻¹, respectively (Hao et al. 2018). Arsenic in water easily enters the human body through the biological chain and long-term direct consumption, causing various health problems, such as respiratory irritation, nausea, skin effects, and cancer (Yan et al. 2021). Generally, arsenic is present in four chemical valence states, As(–III), As(0), As(III), and As(V) (Liu et al. 2020). But in an aqueous environment, it usually exists in trivalent (arsenite) and pentavalent (arsenate) forms (Li et al. 2016). Though arsenite is more toxic than arsenate owing to the high affinity for the active site of the enzyme (the thiol group in the organism), it will eventually be oxidized to arsenate (As(V)) in an aerobic environment gradually (Yang et al. 2022a). Therefore, the treatment of pentavalent arsenic has more significance in actual practice.

For the standard drinking water, the World Health Organization (WHO) proposed a maximum concentration of 0.01 mg·L⁻¹ (Santoyo-Cisneros et al. 2020). How to easily and efficiently remove arsenic in water has become an urgent concern. To date, tremendous efforts have been made in the removal of arsenic, such as coagulation-–flocculation (Choong et al. 2007), electrochemical treatment (Radic et al. 2016), precipitation (Battaglia-Brunet et al. 2012), ion exchange (Pessoa-Lopes et al. 2016), membrane separation (Harisha et al. 2010), and adsorption (Liu et al. 2015). Among them, as an effective physicochemical treatment process, adsorption has attracted great attention for its easy-to-manage and far-ranging source. However, the traditional adsorbents have their own imperfections for arsenic removal, which are high preparation costs, complex synthesis processes, difficulty in mass production, and pH limit (Gallios et al. 2017; Liu et al. 2019; Wei et al. 2019a). In particular, with the increasingly severe industrial arsenic wastewater discharge (typically, the concentration of arsenic for the untreated industrial wastewater is above 500 mg·L⁻¹) (Li et al. 2021), few adsorbents can achieve efficient removal of arsenic from wastewater with a high arsenic concentration or provide near-complete removal of arsenic from wastewater with a low arsenic concentration to meet the discharge standards for arsenic in drinking water. Thus, developing new adsorbents with high efficiency, adaptability, and industrial application is an important issue in the field of arsenic removal.

Classically, the adsorption performance is dependent on the contact area and surface defects of the adsorbing material. Some adsorbents (Atallah et al. 2018), with high specific surface area, seem to satisfy these conditions. Yet their adsorption performances do not meet the requirements, attributed to the fact that these materials tend to agglomerate and the pores are easy to be blocked with the deposition of As, making it difficult to perform a complete ‘surface action’. Meanwhile, it is most important that for As(V), there is a mutual transformation between and in an aqueous environment when the pH changes, and both and ions are present simultaneously in a wide pH range (Yang et al. 2022a). Even some adsorbents with excellent dispersion or porous structure (Moraga et al. 2019) are still far from expectation and it is difficult to remove both and effectively at various pH values. Therefore, the development of materials for As(V) adsorption should not only take into account the contact area, dispersion, or porous structure, but more importantly, the removal of both and should also be fully considered in terms of the adsorption mechanism.

Since both and are acid salts, simultaneous removal of protons to immobilize is a feasible strategy. Accordingly, alkaline magnesium hydroxide (Mg(OH)₂) promises to be a potential candidate for the adsorption removal of and from an aqueous environment effectively. To meet the contact area, dispersion, or stable porous structure requirements, most of the current synthetic approaches are still costly and difficult for mass production. However, the economical precipitation method can hardly meet the above requirements. Here, the development of monodisperse and porous Mg(OH)₂ with a stable structure through the precipitation route may be the key for overcoming this challenge.

Based on the above conception, for the first time, a facile ultrasound-assisted precipitation method is reported for the synthesis of a monodisperse porous pinecone-like Mg(OH)₂ (PLMH) for deep As(V) capture. In particular, the synthetic route is economical and environmentally friendly, which can be easily scaled up. The chemical composition and structure of the PLMH were analyzed by SEM, TEM, XRD, and BET. The arsenic adsorption performance and applicability range of PLMH were investigated in depth by the influencing factors of initial concentration, coexisting ions, and initial pH value. The mechanism of arsenic removal was analyzed in combination with the results of adsorption thermodynamic characterization and changes in the morphological structure. Finally, the adsorption application performance of PLMH on actual industrial wastewater was explored.

METHODS

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Materials

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Magnesium sulfate (MgSO₄) was provided by Macklin Biochemical Co., Ltd. (Shanghai, China) Sodium hydroxide (NaOH) and hydrochloric acid (HCl) were acquired from Xilong Science Co. Ltd. (Shantou, China). Sodium arsenic dibasic heptahydrate (HAsNa₂O₄·7H₂O) was supplied by Sigma-Aldrich (Shanghai, China). Mannitol (C₆H₁₄O₆) was purchased from Yuanye Bio-Technology Co., Ltd. (Shanghai, China). Zhejiang Chemical Engineering Geological Survey Institute Co., Ltd. (Hangzhou, China) provides actual industrial wastewater taken from the Changshan Chemical-Contaminated Site Risk Control Project (Quzhou, China). Unless otherwise specified, ultrapure water was used for all experiments. All the chemicals were used without further purification.

Synthesis of PLMH

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Monodisperse porous PLMH was synthesized as follows: Appropriate amounts of anhydrous magnesium sulfate (12 g) and mannitol (2.4 g) were dissolved in ultrapure water (400 mL) and sonicated at room temperature accompanied by vigorous stirring. Under these conditions, the alkali (0.1 mol·L⁻¹ NaOH) was gradually added via a peristaltic pump at a constant rate (400 rpm) and the mixture was left for 1 h at the end of the reaction. The sediment was separated by centrifugation and washed three times with ultrapure water to remove extra mannitol. Finally, white PLMH powder was obtained by freeze-drying for 2 days in an Alpha 2-4 LSC basic freeze-dryer.

Formulation of arsenic solutions

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As(V) solution with ultrapure water as the solvent: 5.0 g of sodium arsenic dibasic heptahydrate was dissolved in ultrapure water and the volume was fixed to 1,000 mL, to prepare an arsenic solution with a concentration of 1,254.0 mg·L⁻¹. This is an original solution and the rest of the arsenic solutions with different concentrations were diluted from it.

As(V) solution with industrial wastewater as the solvent: 2.288 g of sodium arsenic dibasic heptahydrate was dissolved in industrial wastewater and the volume was fixed to 100 mL, to obtain industrial wastewater with an arsenic concentration of 550.045 mg/L.

Adsorption experiments

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To explore the arsenic adsorption removal by PLMH in the ideal state, sodium arsenic dibasic heptahydrate and ultrapure water were employed to prepare certain concentrations of As(V) solutions for the preliminary investigation. Subsequently, PLMH was used for the treatment of actual industrial wastewater and compared to the treatment of arsenic solutions in an ideal condition.

The initial pH of As(V) solutions was adjusted through the dropwise addition of NaOH or HCl solutions as required. In general, 0.05 g of PLMH was added to 100 mL of As(V) solution/real industrial wastewater, and the mixture was stirred magnetically at room temperature. Then, 3 mL of the reaction solution was extracted at set intervals and centrifuged immediately. In a volumetric flask, the supernatant was diluted 100-fold and strained through a 0.22-μm needle filter into vials. Concentrations of arsenic ions before and after adsorption were subsequently determined according to the arsenic standard solution curve using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP).

The arsenic adsorption capacity is calculated by the following equation:

The arsenic removal rate (E) is calculated by the following equation:

and E are the mass of arsenic adsorbed on the unit weight of PLMH at equilibrium and the As(V) removal efficiency at a specific time (min), respectively. and denote the original concentration and balance arsenic concentration of the solution. (L) represents the volume of the solution. (mg·L⁻¹) is the concentration of the arsenic solution after adsorption, and m (mg) is the mass of PLMH.

The Langmuir and the Freundlich adsorption isotherm models describe the correlation between the amount of arsenic adsorbed on PLMH and its equilibrium concentration in the solution. The Langmuir and the Freundlich adsorption isotherm equations are described as follows:

where

(mg·g⁻¹) represents the maximum adsorption capacity of PLMH and b (L·mg⁻¹) is the Langmuir constant related to the nature and temperature of PLMH and As,

⁠, and

are Freundlich empirical constants covering all factors affecting the adsorption process.

To study the adsorption mechanism of As(V) on PLMH, the experimental data were analyzed using pseudo-first-order and pseudo-second-order fits. The formulas are as follows:

where

(min⁻¹) represents the relevant rate constant for the pseudo-first-order kinetic model, while

(g·mg⁻¹·min⁻¹) is the relevant rate constant for the pseudo-second-order kinetic model.

Characterization

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The crystalline phases of the obtained adsorbents were identified through powder X-ray diffraction (XRD, Holland Panalytical company) equipped with Cu Kα radiation (40 kV, λ = 1.54 Å) at a scan rate of 10° min⁻¹. The porous structure of the material has been characterized by a BET surface analyzer (ASAP 2460, Micromeritics Instrument Corporation, USA) and a N₂ adsorption apparatus with a degassing temperature of 160 °C. The morphology and surface characteristics of PLMH were obtained by a scanning electron microscope (SEM) with an energy dispersive X-ray spectroscopy (EDS) system (JEOL JSM-7500F). FTIR (Thermo Nicolet 5700) was applied to observe the changes of functional groups before and after the adsorption of PLMH with scans ranging from 500 to 4,000 cm⁻¹. Arsenic concentrations in aqueous solutions were analyzed by ICP (Perkin Elmer, Optima 8000). X-ray photoelectron spectroscopy (XPS) was performed to analyze the chemical composition of material surfaces on a Thermo Scientific™ K-Alpha™⁺ spectrometer equipped with a monochromatic Al Kα X-ray source (1,486.6 eV) operating at 100 W. Cations such as K⁺, Na⁺, Ca²⁺, and Mg²⁺ in real industrial wastewater were detected by ICP (Perkin Elmer, Optima 8000), while anions such as Cl⁻, ⁠, ⁠, ⁠, and in industrial wastewater were determined by Ion Chromatography (America, ThermoFisher, Aquion). I⁻ in industrial wastewater was measured by Inductively Coupled Plasma Mass Spectrometry (ICP-MS, America, ThermoFisher, Icap RQ).

RESULTS AND DISCUSSION

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Characterization of material

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The morphology of PLMH samples before and after the adsorption of As(V) was observed by SEM and TEM instruments. The morphology of PLMH samples in SEM (Figure 1(a)–1(c)) are monodispersed pinecone-like spheres with around 3 μm in size (consistent with the results of laser particle size measurement (Supplementary material, Figure S1), self-assembled through the cross-growth of lamellas. Also, the internal structure of PLMH captured by TEM (Figure 1(d)), confirms the existence of longitudinal and horizontal channels inside the porous microspheres. Generally, the adsorption effect is mainly based on ion exchange or complexation on the surface. The presence of these large and stable pore channels makes the interior of the particles exhibit a ‘surface effect’. In other words, this novel structure allows for more active sites and higher storage capacity, which facilitates the adsorption of more harmful substances. The apparent change in particle morphology after the adsorption of arsenic (Figure 1(e) and 1(f)) verifies the internal capture through the internal channels of pinecone-like porous microspheres.

Figure 1

SEM and TEM images of PLMH before and after As(V) adsorption.

To further investigate the structure of PLMH, N₂ adsorption–desorption measurements were carried out. As shown in Figure 2, a typical type-IV with a distinct hysteresis loop was observed on the N₂ adsorption–desorption isotherm, indicating the presence of mesopores in PLMH. Also, the two branches of the hysteresis loop do not reach parallelism, implying the presence of macropores in the product. These pore characteristics of PLMH are in agreement with the microscopic morphology observed from SEM and TEM. The Barret, Joyner and Harenda (BJH) method pore size distribution obtained from isotherms (inset in Figure 2) shows that the pores are widely distributed in the sample. Specifically, the surface area of the PLMH spheres is 63.1 m²·g⁻¹ and the total pore volume is 0.16 cm³·g⁻¹.

Figure 2

Nitrogen adsorption–desorption isotherms and corresponding pore size distributions for PLMH.

Figure 3(a) shows the XRD patterns of PLMH before and after the adsorption of As(V). Before adsorption, according to the standard data of JCPDS 44-1482, all diffraction peaks were characteristic of magnesium hydroxide. No additional phase diffraction peaks of other substances were detected in the spectra, indicating that PLMH has a nice crystal structure and high purity. After the adsorption of arsenic ions, the diffraction peaks of magnesium hydroxide completely disappear and only the characteristic peak of Mg₃(AsO₄)₂·8H₂O is present, corresponding to JCPDS 33-0856, indicating that the adsorption process is relatively complete. Also, the FTIR characterization (Figure 3(b)) shows that after the PLMH adsorption of As(V), strong peaks attributed to the stretching vibrations of As–O groups appeared at about 842.49 and 799.18 cm⁻¹, respectively, verifying the strong adsorption of As(V) on PLMH.

Figure 3

XRD patterns (a) and FTIR spectra (b) for PLMH before and after As(V) adsorption.

Changes in arsenic and magnesium functional groups before and after the adsorption of PLMH were also measured by XPS. As shown in Figure 4(a), there is no obvious characteristic peak near 45.6 eV before adsorption, while a characteristic peak belonging to arsenic was observed after adsorption, indicating that PLMH contains arsenic after adsorption. In the high-resolution XPS fine spectrum of Mg (Figure 4(b)), the binding energy of Mg1s orbital shifts from 1,303.53 to 1,304.58 eV after As(V) adsorption, implying that the chemical bonding to Mg has changed. In Figure 4(c), the Mg2p peak binding energy is located at 49.34 eV attributed to the Mg–O–H species of Mg before adsorption. After the adsorption of As(V), two new peaks appeared. One with a binding energy of 45.23 eV can be inferred to be As(V)–O. The other with a binding energy of 50.19 eV is attributed to Mg–O, while the characteristic peak of Mg–O–H almost disappeared, indicating that the only product after adsorption is Mg₃(AsO₄)₂. These results correspond to the characterization results of XRD and FTIR, confirming the excellent adsorption effect of PLMH.

Figure 4

XPS spectra before and after As(V) adsorption by PLMH (a), XPS fine spectrum of Mg before and after As(V) adsorption (b), and arsenic before and after As(V) adsorption (c) by PLMH.

The SEM-EDX spectra of PLMH after adsorption of As(V) are shown in Figure 5. The pinecone-like morphology of PLMH changed after the saturation of arsenate adsorption, but the structure remained intact and did not fall apart into sheets due to the collapse. The mapping diagram shows that elemental arsenic is abundantly and uniformly distributed in the adsorption products, which confirms the superb arsenic adsorption performance of the PLMH.

Figure 5

SEM image of PLMH after adsorbing arsenic (a) and elemental mapping diagrams corresponding to (b) Mg, (c) O, and (d) As.

Adsorption properties PLMH

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Experiments with different initial As(V) concentrations were used to evaluate the removal effect of PLMH. The PLMH adsorbent removed nearly 38% with the initial 1,254 mg·L⁻¹ As(V), corresponding to 945.8 mg·g⁻¹ at equilibrium (Figure 6(a)), approximately 45% of 920.1 mg·L⁻¹ As(V) (Figure 6(b)), corresponding to 835.7 mg·g⁻¹ at equilibrium. The removal rate of As(V) at 550.0 mg·L⁻¹ was about 70% (Figure 6(c)), corresponding to 764.8 mg·g⁻¹ at equilibrium. The removal rate of As(V) at 323.3 mg·L⁻¹ was about 88% (Figure 6(d)), corresponding to 587.3 mg·g⁻¹ at equilibrium. The removal rate of As(V) at 90.6 mg·L⁻¹ was nearly 92% (Figure 6(e)), corresponding to 166.5 mg·g⁻¹ at equilibrium. The removal rate of As(V) at 1.1 mg·L⁻¹ was nearly 100% (Figure 6(f)), corresponding to 2.2 mg·g⁻¹ at equilibrium, and the residual arsenic concentration was nearly 0 mg·L⁻¹. It was observed that the removal of arsenic by PLMH increases in a gradient with a decreasing initial arsenic concentration. At high concentrations, the PLMH adsorbent has an adsorption capacity (945.8 mg·g⁻¹) that has never been reported in the literature (Supplementary material, Table S1). Also, at low concentrations, the PLMH adsorbent can quickly adsorb arsenic within 10 minutes, reducing the arsenic concentration in the arsenic solution to below the health standard of 0.01 mg·L⁻¹ for arsenic in drinking water, which means that the PLMH adsorbent can be used not only for the removal of high concentrations of arsenic in industrial wastewater, but also directly for daily drinking water treatment to ensure that arsenic does not exceed the WHO standard.

Figure 6

Removal of different arsenic concentrations (1,254.0 mg·L−1(a), 920.1 mg·L−1(b), 550.0 mg·L−1 (c), 323.3 mg·L−1(d), 90.6 mg·L−1(e), and 1.1 mg·L−1 (f)) by PLMH.

Removal of different arsenic concentrations (1,254.0 mg·L⁻¹(a), 920.1 mg·L⁻¹(b), 550.0 mg·L⁻¹ (c), 323.3 mg·L⁻¹(d), 90.6 mg·L⁻¹(e), and 1.1 mg·L⁻¹ (f)) by PLMH.

Isothermal adsorption model

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The adsorption performance of PLMH was investigated using adsorption isotherms. The saturation adsorption of As(V) solutions with initial concentrations of 1.1– 1,254 mg·L⁻¹ was carried out using PLMH at room temperature, and the results are displayed in Figure 7 with the corresponding parameters listed in Table 1. The higher correlation between the adsorption of PLMH on As(V) and the Langmuir model indicates that the adsorption is monolayer, which may be due to the formation of chemical bonds between the inner and outer surfaces of the adsorbent and the adsorbate. In addition, the maximum saturation capacity of PLMH for As(V) fitted by the Langmuir model was 957.1 mg·g⁻¹, close to the real experimental adsorption capacity of 945.8 mg·g⁻¹, corroborated with kinetic fitting deduction.

Table 1

Parameters of an isothermal adsorption model for PLMH

Sample	Langmuir			Freundlich
Sample	q_m(mg·g⁻¹)	K_L	q_e	R₁²	K_F(mg·g⁻¹)	R₂²
PLMH	957.1	0.0345	945.8	0.9909	187.2	0.9337

Sample	Langmuir			Freundlich
Sample	q_m(mg·g⁻¹)	K_L	q_e	R₁²	K_F(mg·g⁻¹)	R₂²
PLMH	957.1	0.0345	945.8	0.9909	187.2	0.9337

Figure 7

Isotherm adsorption model fitting of PLMH for As(V) adsorption.

Adsorption kinetics

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Adsorption kinetics is one of the important analyses to study the reaction mechanism of adsorbents. Herein, PLMH was fitted with pseudo-first-order (Figure 8(a)) and pseudo-second-order kinetic models (Figure 8(b)) for the adsorption process of As(V) in aqueous solutions. Also, the corresponding adsorption kinetic parameters are listed in Table 2.

Table 2

Adsorption kinetic parameters of PLMH

	Pseudo-first-order kinetic model			Pseudo-second-order kinetic model
Sample	(min⁻¹)	q_th(mg·g⁻¹)	R₁²	(min⁻¹)	q_e(mg·g⁻¹)	R₂²
PLMH	−0.00016	773.98364	0.9925	0.00106	698.68996	0.9246

	Pseudo-first-order kinetic model			Pseudo-second-order kinetic model
Sample	(min⁻¹)	q_th(mg·g⁻¹)	R₁²	(min⁻¹)	q_e(mg·g⁻¹)	R₂²
PLMH	−0.00016	773.98364	0.9925	0.00106	698.68996	0.9246

Figure 8

Fitted kinetic curves of (a) pseudo-second-order and (b) pseudo-first-order adsorption for As(V) solutions.

Combining the kinetic fitting model fits with the corresponding kinetic parameters and comparing their correlations, it is found that the kinetics of adsorption of As(V) by PLMH is more suitable for the pseudo-first-order kinetic model, implying that PLMH has higher activity and the removal of As(V) is chemisorbed (ion exchange) (Islam et al. 2011; Lee et al. 2017), while the rate of arsenic removal is controlled by the diffusion step.

Compared to real industrial wastewater with complex compositions, the adsorption kinetics using arsenic solutions seems to be difficult to apply. However, the subsequent coexisting ions and real industrial wastewater adsorption experiments indicate that the coexisting impurities have a limited effect on the arsenic adsorption efficiency. Herein, the adsorption kinetics and mechanism using arsenic solutions are informative for the study of real industrial wastewater.

The pH effect

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The solution pH is important to study the interaction of reactants with adsorbents and to deal with real-life arsenic contamination. Figure 9 shows the removal efficiency curves of arsenic wastewater at various initial pH values ranging from 2 to 13. Interestingly, the following pH symmetry phenomenon was found (never been reported before): in the range of pH 3–12, the arsenic removal efficiency under acidic and basic conditions was about centrosymmetric after adsorption equilibrium. Namely, the removal efficiency at pH 2 is the same as that at pH 13 and the removal efficiency at pH 3 agrees with that at pH 12. Compared to other pH values, the removal efficiency is a little lower at pH 3, due to the acid–base neutralization of part of the adsorbent (Mg(OH)₂), resulting in a little lower removal efficiency. Especially, at pH 2, arsenic concentration does not decrease owing to the complete reaction of PLMH by H⁺. Besides, the adsorption efficiency of PLMH at different pH values was also investigated, the lower the pH is, the faster the initial adsorption rate is, during the first few hours. Then, As(V) leaching occurs. Similarly, the lower the pH is, the faster the dissolution rate is, until a minimum value, followed by adsorption until an equilibrium state is reached. This is largely determined by the arsenic ionic state, where As(V) exists in solution mainly as

under acidic conditions (Yang et al. 2022a). For the acid solution (pH ≥ 3), a part of the adsorbent first reacts with H⁺ at the initial stage (Equation ()), and the solution pH rises to 9.28 within 10 min (Supplementary material, Table S2). Although a part of PLMH is consumed, it almost maintains its original morphology, bringing little effect on the subsequent adsorption (Supplementary material, Figure S2). The solution pH increases and

gradually decomposes into

and H⁺ (Equation ()). However, due to the small equilibrium constant (10^−6.99) of

to

and H⁺ (Li et al. 2018),

is still the predominant species in the solution. The adsorption reaction occurs on the surface of PLMH as shown in Equation (), and a large amount of Mg₃(AsO₄)₂ is rapidly generated until saturation (Supplementary material, Figures S3 and S4), corresponding to the removal rate within 4 h. As the adsorbent keeps releasing OH⁻, Mg₃(AsO₄)₂ keeps converting to

according to the inverse reaction of Equation (), corresponding to the stage of dissolution in the curve. Namely,

is completely converted into

in this stage, corresponding to the lowest point of dissolution. Subsequently, the reaction as shown in Equation () is carried out in an alkaline environment. Since

is less active, the reaction reaches equilibrium slowly compared to the beginning stage:

(1)

(2)

(3)

(4)

(5)

(6)

Figure 9

Effect of pH on 550.0 mg·L−1 As(V) wastewater using the PLMH adsorbent.

Effect of pH on 550.0 mg·L⁻¹ As(V) wastewater using the PLMH adsorbent.

In alkaline conditions, As(V) is mainly in the form of (Yang et al. 2022a). Also, adsorption is conducted following Equation (4) and completed in Equation (5), presenting a trend, that the higher the pH is, the more the early adsorption is inhibited. Especially, when the pH value exceeds 13, As(V) is mainly in the form of primarily reacting as shown in Equation (5) (Yang et al. 2022a), and the adsorption reaction can only occur as shown in Equation (6), which is difficult to carry out. Due to the presence of a large amount of free OH⁻ in the solution, the reaction cannot proceed, and the adsorbent has almost no adsorption effect.

Competitive adsorption

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To investigate the effect of coexisting anions in practical applications, the removal efficiency of PLMH for As(V) was examined in the presence of

⁠,

⁠, and Cl⁻ (all of the above ions in a concentration ratio of 1:1 to arsenic ions) (Figure 10). The results show that chloride, nitrate, and sulfate had little effect on the adsorbent, while carbonate and phosphate significantly hindered the removal of arsenate. Carbonate and phosphate have a strong affinity for the adsorbent, which means that they compete with arsenic for adsorption sites on the surface of PLMH microspheres, resulting in competitive adsorption (Li et al. 2012). Though carbonate and phosphate bring a reduction of arsenic removal (Tiberg et al. 2020), they can easily be removed preferentially through precipitation.

Figure 10

A highly stable indium based metal organic framework for efficient arsenic removal from water

Effect of Cl⁻, ⁠, ⁠, and ions on As(V) adsorption efficiency by the PLMH adsorbent.

Practical applications

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PLMH was used for the treatment of actual industrial wastewater at the contaminated site of Changshan Chemical, and the ions in the wastewater before and after adsorption are measured as shown in Table 3. After adsorption, the arsenic concentration in industrial wastewater reduced from 45 to 2 μg·L⁻¹ with a removal efficiency of 96%, which is lower than the groundwater quality standard of 10 μg·L⁻¹. This result indicates that the PLMH treatment of actual industrial wastewater (low arsenic content) is as effective as that under ideal laboratory conditions, attributed to the fact that the number of active sites exposed on the surface of the PLMH is much higher than the number of harmful ions that can be adsorbed. In addition, PLMH showed good removal of other toxic and harmful ions, such as and ⁠, especially ⁠.

Table 3

Concentrations of various ions in industrial water used in this study and the groundwater quality standard (GB/T 14848-2017)

Species	Concentration before removal (mg·L⁻¹) (this study)	Concentration after removal (mg·L⁻¹) (this study)	Maximum permissible limits (mg·L⁻¹)
As	0.045	0.002	0.01
K⁺	124.6	119.4	–
Na⁺	156.3	161.2	200
Ca²⁺	26.59	17.51	–
Mg²⁺	36.34	195.2	–
I⁻	0.100	0.986	0.1
Cl⁻	388.6	396.7	250
	13.22	11.69	10
	59.49	43.75	250
	22.87	4.53	–
	0.063	0.005	–
As(V) solution with ultrapure water as the solvent	550.000	164.9	–
As(V) solution with industrial wastewater as the solvent	550.045	197.1	–

Species	Concentration before removal (mg·L⁻¹) (this study)	Concentration after removal (mg·L⁻¹) (this study)	Maximum permissible limits (mg·L⁻¹)
As	0.045	0.002	0.01
K⁺	124.6	119.4	–
Na⁺	156.3	161.2	200
Ca²⁺	26.59	17.51	–
Mg²⁺	36.34	195.2	–
I⁻	0.100	0.986	0.1
Cl⁻	388.6	396.7	250
	13.22	11.69	10
	59.49	43.75	250
	22.87	4.53	–
	0.063	0.005	–
As(V) solution with ultrapure water as the solvent	550.000	164.9	–
As(V) solution with industrial wastewater as the solvent	550.045	197.1	–

Owing to the low arsenic content in this real case of wastewater, the arsenic concentration of industrial wastewater was increased to 550 mg·L⁻¹ by adding sodium arsenic dibasic heptahydrate into raw industrial wastewater, to better evaluate the industrial application of PLMH. Table 3 shows the removal effect of PLMH with ultrapure water and industrial wastewater as a solvent for 550 mg·L⁻¹ of arsenic, respectively. The results show that the removal efficiency of actual industrial wastewater as the solvent decreased only by 6% and the adsorption efficiency remained high. In other words, the effect of competing ions on the adsorption of PLMH in actual industrial wastewater was not significant. Therefore, PLMH is an excellent adsorbent that can be used in actual industrial wastewater treatment.

CONCLUSION

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In summary, a monodisperse porous PLMH has been successfully prepared via a facile and environmentally friendly ultrasound-assisted precipitation route. Due to the porous and stable framework structure, the material shows a ‘surface effect’ inside by the removal of protons to immobilize ⁠, which greatly enhances the adsorption performance. When used as an adsorbent for the deep removal of arsenic (⁠ and ⁠) from wastewater, the maximum adsorption capacity is up to 945.8 mg·g⁻¹, and the final arsenic concentration is well below 0.01 mg·L⁻¹, which is the highest among all reported adsorbents. In the pH range of 3–12, the removal efficiency of arsenic after adsorption equilibrium is neutrally symmetric. In addition, the PLMH has a specific pH effect. Under acidic conditions, the lower the pH value is, the faster the initial adsorption rate, and the maximum adsorption capacity is approached in a short time (2 h). Under alkaline conditions, the higher the pH value is, the slower the initial adsorption rate is. Finally, the PLMH shows good arsenic removal in real industrial wastewater applications with a removal efficiency of 96%. These advantages make it a good candidate for the removal of arsenic from industrial wastewater.

ACKNOWLEDGEMENTS

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This work was financially supported by the National Natural Science Foundation of China (Grant Nos U1805234 and 21706033), Natural Science Foundation of Fujian Province of China (Grant No. 2021J01156), Program for Innovative Research Team in Science and Technology in Fujian Province University, 100 Talents Program of Fujian Province, and Scientific Research Start-up Fund for High-Level Talents in Fujian Normal University (004828) Fujian-Taiwan Science and Technology Cooperation Base of Biomedical Materials and Tissue Engineering (2021D039).

DATA AVAILABILITY STATEMENT

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

CONFLICT OF INTEREST

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The authors declare there is no conflict.

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