The use of nano zero-valent iron (nZVI) materials for groundwater Cr(VI) removal encountered challenges of agglomeration and low removal efficiency. Controlled release materials (CRMs) gradually release reactive substances or reducing agents, prolonging the release time. Here, we report the development of novel CRMs containing nZVI and activated carbon (AC). During the removal of Cr(VI) in groundwater, the prepared AC/nZVI/CRMs slowly released nZVI, greatly reducing the agglomeration of nZVI. The adsorption capacity of AC-containing CRMs prolonged the residence time of Cr(VI) in water, improving the removal efficiency of the AC/nZVI/CRMs. We found that lower pH enhanced the removal of Cr(VI) by the AC/nZVI/CRMs from simulated groundwater. The removal efficiency of the AC/nZVI/CRMs was also affected by the simulated groundwater environment and decreased with the increasing flow rate of the groundwater. Our results suggested that these novel nZVI-containing CRMs minimized agglomeration during the removal of Cr(VI) by nZVI, exhibited enhanced efficiency under acidic conditions, and facilitated Cr(VI) removal from similar groundwater environments.

  • Controlled-release materials (CRMs) based on activated carbon and nano zero-valent iron (nZVI) were prepared.

  • CRMs exhibit reduced nZVI aggregation and effectively remove Cr(VI) from groundwater over long periods.

  • Cr(VI) removal from groundwater by CRMs is more effective under acidic conditions.

  • Activated carbon improves the Cr(VI) removal efficiency of CRMs.

With the rapid progress in industrialization and urbanization, the problem of heavy metal pollution in groundwater has become increasingly prominent and one of the urgent environmental issues that needs to be addressed globally (Alsubih et al. 2021; Malik et al. 2023; Sanga et al. 2023). Common heavy metal pollutants include chromium (Cr), lead (Pb), mercury (Hg), and arsenic (As), which persist and accumulate in the environment, posing serious threats to the environment and public health (Saha et al. 2022; Hasan et al. 2023; Kumar et al. 2023). For example, wastewater generated from industrial activities, such as metallurgy, leather tanning, electroplating, and mining, contains chromium in high concentrations, which can easily migrate into groundwater (Tiwari et al. 2019; Khan et al. 2020; Hussain et al. 2021). Micro-polluted groundwater containing Cr(VI) ions mainly exists near industrial areas. Recently, trace heavy metal ions, including Cr(VI) ions, have been detected in groundwater in such areas (Saha et al. 2017; Hausladen et al. 2018; Khan et al. 2020; Guo et al. 2021a, b). Existing removal technologies for Cr(VI) in groundwater include permeable reactive barriers (PRBs) (Cao et al. 2022), ion exchange technology (Ren et al. 2020), and electrochemical removal (Wang et al. 2023c). Nano zero-valent iron (nZVI) is often used as a removal material due to its low cost, non-toxicity, and strong removal ability (Li et al. 2019). nZVI is strongly reactive, reducing Cr(VI) to Cr(III) while being oxidized to Fe(II) or Fe(III). However, only a few examples in the literature report on the remediation of heavy metal ions in micro-polluted groundwater that can be sustained over a long period.

Previously, related studies on Cr removal by nZVI have been published (Zhu et al. 2018, Liu et al. 2022). A green synthesis method has been used to combine active substances containing polyphenols and caffeine in green tea with nZVI and Cu(II), which improved the reactivity of nZVI and Cr(VI) (Zhu et al. 2018). Although the active substances in green tea were added to enhance the stability of nZVI, this material also contained Cu(II), which may lead to groundwater contamination (Tumampos et al. 2021). Mortazavian used a combination of annealing and borohydride reduction to prepare composite materials of activated carbon (AC) and nZVI. In the Cr(VI) removal experiment, AC/nZVI showed better performance at pH 4 and still reached 57.6% removal efficiency at pH 10 although AC had no adsorption effect under this condition (Mortazavian et al. 2018). Li et al. (2020a, b) used polyvinylpyrrolidone and sodium oleate to modify nZVI, resulting in a smaller particle size and better dispersion of the nZVI particles. However, nZVI often agglomerates during the removal process, which affects its removal efficiency and service life. Coating nZVI with a non-toxic magnesium hydroxide coating decreases the van der Waals force between nZVI and reduces agglomeration (Maamoun et al. 2022). Angaru et al. (2021) used zeolite as a carrier to prepare a bimetallic material containing nZVI and nickel. The surface area of the composite material increased, and nZVI particles were evenly dispersed on the surface of the zeolite. Liu et al. (2021) used xanthan gum to modify graphene material loaded with nZVI. Despite a limited lifetime, the modified material showed a good removal effect in simulated sandbox experiments.

Controlled release materials (CRMs), which can stably release reactants over a long time in a controlled manner, have recently been developed, preventing agglomeration of nZVI (Wang et al. 2022). Most CRM research has focused on the removal of refractory organic pollutants with CRMs using potassium permanganate or sodium persulfate as oxidants (Chen et al. 2021; Liang & Weng 2022; Wang et al. 2022). Wen et al. (2020) used AC, nZVI, and alginate to prepare a composite gel that can slowly release iron ions to remove Cr(VI). However, there have been few studies on the application of nZVI CRMs for the treatment of heavy metal pollution in groundwater. AC is often used as an adsorbent for the removal of heavy metals from water due to its large specific surface area and high porosity (Mariana et al. 2021). Li et al. (2020a; b) prepared composite materials containing zeolite and AC and found that increasing the AC content improved the adsorption performance of the material. Asimakopoulos et al. (2021) prepared AC by pyrolysis and activation of Posidonia oceanica, achieving a Cr(VI) adsorption capacity of up to 120 mg/g. Applying AC as a component of CRMs can improve the adsorption capacity of CRMs and increase the time that Cr(VI) stays adsorbed on the CRMs, thereby prolonging the reaction between reagents and Cr(VI) and improving efficiency.

CRMs that are prepared in the form of small particles are easy to package and transport and have a long cycle of use. After reaching its service life, the material remains as solid particles, making it easy to replace. During the remediation stage, there is no need for electrical energy input like in electrochemical methods. Although the development and application of this material are currently in the laboratory research stage, it can be applied to wells or replaced with PRB materials in the later stage to reduce agglomeration and blockage problems and extend its service life. The costs of the technology combined with nZVI CRMs will be lower than those of the traditional treatment systems involving electrochemical methods and PRBs. In addition, the new technology will have advantages such as environmental friendliness and a simple production process.

In this work, CRMs based on AC and nZVI were fabricated to maintain stable performance in the removal of heavy metal ions from groundwater for a long time. Furthermore, the synergistic Cr(VI) removal performance of CRMs by the adsorption of Cr(VI) ions on AC and their reduction by nZVI was investigated. The adsorption on AC not only ensured the enrichment of Cr(VI) ions in CRMs to promote Cr(VI) reduction but also avoided the desorption of Cr3+ after the reduction step. Therefore, the AC/nZVI/CRMs constructed in this work have a promising application prospect for the remediation of heavy metal ions in micro-polluted groundwater. The effectiveness of these CRMs in the removal of Cr(VI) was verified by static and dynamic groundwater experiments, representing a breakthrough in their application to remove heavy metals from groundwater.

Materials and characterization

Concentrated hydrochloric acid, concentrated phosphoric acid, sodium sulfite, concentrated nitric acid, and concentrated sulfuric acid were purchased from Shanghai Lingfeng Chemical Co., Ltd (China). Absolute ethanol was purchased from Chengdu Kelong Chemical Reagent Co., Ltd (China). AC was purchased from Nanjing XFNANO Materials Tech Co., Ltd (China). Paraffin sections were purchased from Shanghai Kexing Trading Co., Ltd (China). Polyethylene glycol was supplied by Tianjin Xucheng Chemical Co., Ltd (China). Quartz sand and nZVI were bought from Shanghai Macklin Biochemical Co., Ltd (China). Diphenylcarbazide was purchased from Sinopharm Chemical Reagent Co., Ltd (China). Concentrated hydrochloric acid, AC, paraffin, and polyethylene glycol were chemically pure. nZVI was a guaranteed reagent. All other chemical materials were analytically pure. The Cr(VI) concentration was analyzed at a wavelength of 540 nm, using a UV–Vis spectrophotometer (Puxi, TU-1901). A standard electrode was used to measure the pH of the samples.

CRM preparation

AC was pretreated, ground to a particle size of 180 mesh, and then soaked in a 1-M aqueous hydrochloric acid solution. After 24 h, the material was retrieved, washed with deionized water, soaked in anhydrous ethanol for 12 h, and then placed in an oven at 105 °C for 15 min. Paraffin was placed in a 50-mL beaker and heated in a water bath until a temperature of 80 °C was reached. The appropriate amounts of quartz sand, polyethylene glycol, nZVI, and AC were combined and stirred under exposure to ultrasound for 20 min until the components were completely mixed. Then, the CRMs were poured into a mold (volume of 1 cm3) while in a molten state and left for about 15 min until the shape formation of a CRM cube with a side length of 1.0 cm was complete (Figure 1). The error of each side length was approximately ±0.05 cm, as shown in Figure S1. The particle size range of the nZVI was 400–800 nm. The specific surface area of AC was 1878.18 m2/g.
Figure 1

Preparation of CRMs containing nZVI and AC.

Figure 1

Preparation of CRMs containing nZVI and AC.

Close modal

Various components were used in the preparation of the CRMs and evaluated using static removal experiments (Table 1). In the CRMs, the mass ratio of nZVI to paraffin, quartz sand, AC, and polyethylene glycol was 1:3.6:6.75:0.36:2.54. As a binding agent, paraffin provided a foundation for the structural support of CRMs due to its low solubility in water. Quartz sand could adjust the pore size of CRMs and enhance permeability. Adding polyethylene glycol effectively improved the dispersibility of CRMs.

Table 1

Composition of materials used in static experiments

nZVI (g)Paraffin (g)Quartz sand (g)AC (g)Polyethylene glycol (g)
CRMs 3.6 6.75 2.54 
AC 0.36 
nZVI 
AC/CRMs 3.6 6.75 0.36 2.54 
nZVI/CRMs 3.6 6.75 2.54 
AC/nZVI/CRMs 3.6 6.75 0.36 2.54 
nZVI (g)Paraffin (g)Quartz sand (g)AC (g)Polyethylene glycol (g)
CRMs 3.6 6.75 2.54 
AC 0.36 
nZVI 
AC/CRMs 3.6 6.75 0.36 2.54 
nZVI/CRMs 3.6 6.75 2.54 
AC/nZVI/CRMs 3.6 6.75 0.36 2.54 

Removal experiments in static groundwater

Static removal experiments were performed in a glass bottle, the temperature of the system was controlled at 25 °C, and the Cr(VI) concentration of the solution was 10 mg/L (Figure 2). To explore the influence of the different components in the CRMs, several CRMs were prepared for the static removal experiments (Table 1). The component ratio of the CRM was optimized through experiments, and subsequent experiments were conducted using CRM No. 11, as presented in Table S1. The effect of pH on Cr(VI) removal was investigated at pH 2, 4, 6, and 8. Under the conditions of pH 2, Cr(VI) solutions with initial concentrations of 5, 10, 15, and 20 mg/L were prepared to assess the effect of Cr(VI) concentration on removal efficiency. In the cyclic removal experiment, four experiments were performed in sequence at pH 2 with an initial Cr(VI) concentration of 10 mg/L. Considering that groundwater also contains various anions, the influence of Cl, , and on the removal efficacy of the CRMs was explored.
Figure 2

Schematic diagrams of static (a) and dynamic (b) experiments.

Figure 2

Schematic diagrams of static (a) and dynamic (b) experiments.

Close modal

Removal experiments in dynamic groundwater

Dynamic experiments were conducted in a glass column (diameter of 5 cm, length of 18 cm) (Figure 2). Before the experiment, the columns were packed with sand and then rinsed with deionized water. CRMs filled the upper part of the column. A peristaltic pump was used to induce a flow of water containing 10 mg/L Cr(VI) from the top of the column to the bottom, and the effluent was collected from the outlet for analysis at the end of the experiment. To explore the influence of AC components on removal efficacy, experiments were carried out with CRMs containing AC and nZVI and CRMs without AC. The influence of the flow rate on the removal efficacy of the CRMs was investigated by experiments conducted at flow rates of 2, 4, and 6 mL/min. Under the conditions of pH 6 and a flow rate of 4 mL/min, Cr(VI) solutions with initial concentrations of 5, 10, 15, and 20 mg/L were prepared for removal experiments to evaluate the effect of Cr(VI) concentration on removal efficiency.

Removal of Cr(VI) from static groundwater by CRMs

The ratio of paraffin and quartz sand, which were employed in the construction of stable CRMs, had a significant impact on removal performance. The higher the paraffin:sand ratio, the slower the release rate of the CRMs. The change in removal time from No. 1 to No. 8 indicated that as the quartz sand content decreased, the time of Cr(VI) removal by the CRMs also decreased (Table S1), which can be explained by a higher proportion of paraffin wax covering a larger amount of nZVI, preventing its release. When the content of quartz sand was high, the CRMs became looser and less firmly wrapped, making it easier for nZVI to be released and even preventing its formation (such as No. 8), thereby affecting the removal time. Polyethylene glycol, as a pore-forming agent, can be added to CRMs to increase porosity and regulate release performance. Therefore, with an increasing proportion of polyethylene glycol, the number of pores increased and the removal time decreased. Based on the results, the following experiments to assess the effect of pH, the presence of anions, and dynamic conditions were conducted using CRM No. 11 (Table S1).

In the static experiment with a Cr(VI) concentration of 10 mg/L, pH 2, and a temperature of 25°C, the removal time of Cr(VI) was compared for six different reaction systems (Table 2). In Figure 3(a), the Cr removal of AC and AC/CRMS, but not of the other CRMs, conformed to the first-order kinetic model. In the absence of AC and nZVI, the CRMs containing only quartz sand, paraffin, and polyethylene glycol had almost no effect on Cr(VI) removal, and the concentration of Cr(VI) in the solution remained unchanged (Table 2, Figure 3(a)), indicating AC and nZVI as the components effective for Cr(VI) removal. Pure AC removed more than 90% Cr(VI) after 48 h with a maximum adsorption capacity of 13 mg/mg, and the removal time of AC/CRMs was 72 h (Table 2). Thus, compared with pure AC, the Cr(VI) removal time can be extended by using AC/CRMs (Figure 3(a)). The removal efficiency of nZVI alone was 90% after only 10 min, while nZVI/CRMs required a removal time of 180 min (Table 2, Figure 3(b)). According to Table S2, when using pure CRMs, there was no change in Cr(VI) concentration before and after desorption. When applying AC/CRMs, the concentration of Cr(VI) in the solution after desorption was 0.92 mg/L, and almost all Cr(VI) absorbed by AC was completely desorbed. However, for AC/nZVI/CRMs, the Cr(VI) content after desorption was zero, indicating that Cr(VI) was completely removed by nZVI.
Table 2

Cr(VI) removal times for various materials

CRMsACnZVIAC/CRMsnZVI/CRMsAC/nZVI/CRMs
Adsorption (≥90%) – 48 h – – – – 
Removal (≥90%) – – 10 min 72 h 180 min 300 min 
CRMsACnZVIAC/CRMsnZVI/CRMsAC/nZVI/CRMs
Adsorption (≥90%) – 48 h – – – – 
Removal (≥90%) – – 10 min 72 h 180 min 300 min 
Figure 3

Removal of Cr(VI) in simulated static groundwater experiments using different CRMs.

Figure 3

Removal of Cr(VI) in simulated static groundwater experiments using different CRMs.

Close modal
The above results demonstrated that the preparation of the CRMs can prolong the effective removal time. When AC and nZVI were present in the CRMs, the time required for 90% Cr(VI) removal was extended from 180 to 300 min, indicating that the addition of AC in nZVI/CRMs can effectively regulate the release rate of nZVI and prolong the release time of AC/nZVI/CRMs. Compared with AC/CRMs, both nZVI/CRMs and AC/nZVI/CRMs can finally completely remove Cr(VI) (Figure 3(b)). Before CRM application, the CRM surfaces were relatively smooth without obvious pores (Figure 4(a)). After the static Cr(VI) removal experiment, the surface of the CRMs became rough and included some pores (Figure 4(b)). The results indicated that the CRMs can still maintain their basic skeletal shape after the static and dynamic experiments. The CRMs performed well regarding their mechanical stability. Figure S2 presents a surface scanning electron microscope image of CRMs before and after use for 10 h, which shows that the surface of CRMs was relatively smooth with fewer surface pores, indicating that paraffin can better encapsulate the reducing agent. After use for 10 h in the static groundwater experiment, the surface of the CRMs became rough and showed obvious concave and convex morphology.
Figure 4

Appearance of AC/nZVI/CRMs before (a) and after (b) the Cr(VI) removal experiment.

Figure 4

Appearance of AC/nZVI/CRMs before (a) and after (b) the Cr(VI) removal experiment.

Close modal

The effect of Cr(VI) concentration on Cr(VI) remediation by CRMs was explored (Figure S3). The increase in initial Cr(VI) concentration had a relatively small effect on the final removal rate of the CRMs. At the beginning of the reaction, the outermost nZVI surface had abundant active sites to ensure high Cr(VI) removal efficiency. As the reaction progressed, the number of active sites gradually decreased, and the removal efficiency decreased as a consequence. Higher Cr(VI) concentrations will also result in the formation of oxide barriers on the surface of nZVI, reducing the contact efficiency between Cr(VI) and the active sites on the surface of the CRMs, thereby affecting the removal efficiency for Cr(VI). However, due to the sustained release of nZVI by CRMs, the impact of concentration on removal efficiency was slowed down.

The cyclic removal experiment showed that the CRMs still maintained a high Cr(VI) removal rate (100% removal effect) after four successive experiments (Figure S4). The results indicated the high removal efficiency of CRMs after repeated use.

Effect of AC on the removal performance of AC/nZVI/CRMs

Figure 5 shows the removal results using pure nZVI in a static experimental column. The small particles of nZVI agglomerated during the reaction process, illustrating the severe agglomeration effect when using nZVI alone. When AC/nZVI/CRMs were used, there were no nZVI particles visible before the experiment, and no obvious agglomeration phenomenon was observed (Figure 6). Because the structure of AC/nZVI/CRMs was relatively tight, the release was slowed down and agglomeration was avoided. At the same time, the addition of polyethylene glycol increased the porosity of the CRMs and regulated their release performance. During the removal experiment, nZVI was well dispersed within the AC/nZVI/CRMs. When AC/nZVI/CRMs came into contact with the solution, nZVI in the matrix was gradually released from the pores. At the same time, Cr(VI) in solution was adsorbed on the AC surface, reacted with nZVI, and was removed, and bubbles appeared on the surface of AC/nZVI/CRMs. Overall, the agglomeration phenomenon was substantially reduced. During the entire reaction process, nZVI was never released at once in large amounts.
Figure 5

Appearance of nZVI in the column before (left), during (middle), and after (right) the removal experiment.

Figure 5

Appearance of nZVI in the column before (left), during (middle), and after (right) the removal experiment.

Close modal
Figure 6

Appearance of the AC/nZVI/CRMs in the column before (left), during (middle), and after (right) the removal experiment.

Figure 6

Appearance of the AC/nZVI/CRMs in the column before (left), during (middle), and after (right) the removal experiment.

Close modal
Figure 7

Cr(VI) removal process by AC/nZVI/CRMs in static groundwater experiments.

Figure 7

Cr(VI) removal process by AC/nZVI/CRMs in static groundwater experiments.

Close modal

Effect of pH on the removal performance of AC/nZVI/CRMs

To further evaluate the removal performance of AC/nZVI/CRMs, experiments were conducted at pH 2, 4, 6, and 8 while other experimental conditions remained unchanged. The conceptual diagram is shown in Figure 7 and the experimental results are shown in Figure 8. With increasing pH, the Cr(VI) removal rate gradually decreased. Aqueous Cr(VI) mainly exists in the form of HCrO4; its reactions with nZVI are as follows (Qu et al. 2017; Pei et al. 2020):
(1)
(2)
Figure 8

Influence of initial pH on Cr(VI) removal in static groundwater experiments.

Figure 8

Influence of initial pH on Cr(VI) removal in static groundwater experiments.

Close modal

At pH 2, the removal effectiveness was the greatest with nearly 100% Cr(VI) removal after 100 min. Under acidic conditions, the main form of Cr(VI) is . AC surfaces carry positive charges, while negatively charged Cr(VI) can be adsorbed and reduced. Due to the presence of a large amount of H+ in the solution under strong acid conditions, nZVI and AC surfaces carry a large number of positive charges, which attract Cr(VI) through electrostatic attraction. Furthermore, a large amount of H+ can effectively inhibit the formation of surface oxides on nZVI, thereby promoting the removal of Cr(VI). At the same time, H+ participates in the reaction between nZVI and Cr(VI), promoting electron transfer from ZVI. According to chemical Equations (1) and (2), the reaction proceeds to the right, and the generated Fe2+ also promotes the reduction of Cr(VI). At the same time, lower pH can inhibit Fe2+ or Fe3+ from forming Fe(OH)2 or Fe(OH)3 precipitates on the particle surfaces, which is beneficial for the reaction of Cr(VI) on the active sites of nZVI. Furthermore, there are oxygen-containing groups such as hydroxyl, ester, and carboxyl groups on the surface of AC, which can form hydrogen bonds with in solution, increasing the adsorption effect of the slow-release agent and promoting the removal efficiency of zero-valent iron. Table 3 lists the removal rates and times of the CRMs synthesized in this study and materials containing nZVI prepared in other studies. Compared with other nZVI composite materials, CRMs have a longer removal time and can continuously remove Cr for extended periods. As shown in Figure S5 and Table S3, all removal processes of Cr(VI) by AC/nZVI/CRMs at different pH followed first-order reaction kinetics, consistent with the literature (Zhou et al. 2015; Zhu et al. 2016).

Table 3

Comparison of the performance of CRMs prepared in this study with those of materials containing nZVI prepared in other studies

MaterialpHRemoval rate (%)Time (min)References
Others SA-nZVI 96.4 20 Li et al. (2019)  
Fe@LDH/rGO ∼100 15 Lv et al. (2019)  
CMC-nFe0 99 Wu et al. (2021)  
Fe/Cu-MCM-41 ∼99 40 Guo et al. (2021a, b)  
CRMs nZVI/AC/CA 100 360 Wen et al. (2020)  
nZVI@SA/MH-NH 2 –1 96.95 15 M Wang et al. (2023a
nZVI-CRM 90 180 This work 
AC/nZVI/CRM ∼100 300 This work 
MaterialpHRemoval rate (%)Time (min)References
Others SA-nZVI 96.4 20 Li et al. (2019)  
Fe@LDH/rGO ∼100 15 Lv et al. (2019)  
CMC-nFe0 99 Wu et al. (2021)  
Fe/Cu-MCM-41 ∼99 40 Guo et al. (2021a, b)  
CRMs nZVI/AC/CA 100 360 Wen et al. (2020)  
nZVI@SA/MH-NH 2 –1 96.95 15 M Wang et al. (2023a
nZVI-CRM 90 180 This work 
AC/nZVI/CRM ∼100 300 This work 

To further explain the effect of materials on the adsorption and reduction processes, the Zeta potential value of AC and nZVI was measured at different pH, and the results are presented in Figure S6. AC exhibited a maximum positive potential at pH 2.0. As the pH in the groundwater increased, the Zeta potential of AC gradually approached zero. In water, Cr(VI) exists in the form of , so AC in the CRMs demonstrated the best adsorption performance for Cr(VI) at pH 2.0. The Zeta potential of nZVI at different pH was close to zero or negative, suggesting that the Zeta potential has little effect on the reduction process.

Effect of anions on the removal performance of AC/nZVI/CRMs

Due to the complex composition of groundwater, the influence of other single anions on Cr(VI) removal was evaluated (Figure 9). Static removal experiments were conducted under conditions of a Cr(VI) concentration of 10 mg/l, pH 2, and a temperature of 25 °C. Compared with other anions, the Cr(VI) removal efficiency was the fastest in the presence of . The adsorption of onto the surface of nZVI can promote electron transfer and accelerate the reaction between Fe0 and Cr(VI) (Wen et al. 2020). Compared with , the presence of considerably reduced Cr(VI) removal, possibly due to the competition between and Cr(VI) for active sites and impeding the combination of Cr(VI) and functional groups (Gheju et al. 2016). However, the effect of Cl on the removal of Cr(VI) was relatively small. All removal processes of Cr(VI) by AC/nZVI/CRMs in the presence of different anions also followed first-order reaction kinetics.
Figure 9

Influence of various anions on the removal of Cr(VI).

Figure 9

Influence of various anions on the removal of Cr(VI).

Close modal

Removal of Cr(VI) by CRMs in dynamic groundwater experiments

In the dynamic experiments, the efficacy of the CRMs with or without AC in the removal of Cr(VI) was explored (Figure 10). The flow rate in the dynamic experiments was 4 mL/min, the pH was 6, and other conditions remained unchanged. The removal performance of the AC-containing CRMs was stronger than the CRMs without AC. In the dynamic experiments, the residence time of Cr(VI) in the CRMs was reduced due to the flowing water. The contact between Cr(VI) and the nZVI released from the CRMs without AC was also reduced, resulting in lower Cr(VI) removal efficiency compared to AC-containing CRMs. Since AC has more pores, a high specific surface area, and oxidized functional groups with adsorption properties, it promoted the adsorption of heavy metals (Neolaka et al. 2023; Wang et al. 2023b). At the beginning of the Cr(VI) removal experiment, the surface of the CRMs contained nZVI. As nZVI reacted with Cr(VI) and was consumed, gaps appeared on the surface of the CRMs, water diffused into the gaps, and nZVI in the matrix began to dissolve in contact with water. At the same time, AC in CRMs adsorbed Cr(VI) on active sites through chemical groups, and a large amount of Cr(VI) occupied the gaps between AC and CRMs. These gaps gradually increased as the nZVI released from the matrix continuously removed Cr(VI) adsorbed on the active sites. In addition, AC also acted as a dispersant (Qu et al. 2017), reducing the agglomeration of nZVI particles. Therefore, adding AC improved the removal efficiency of the CRMs. The removal processes of Cr(VI) by nZVI/CRMs and AC/nZVI/CRMs in simulated dynamic groundwater both followed first-order reaction kinetics.
Figure 10

Removal of Cr(VI) by nZVI/CRMs and AC/nZVI/CRMs in simulated dynamic groundwater experiments (flow rate of 4 mL/min).

Figure 10

Removal of Cr(VI) by nZVI/CRMs and AC/nZVI/CRMs in simulated dynamic groundwater experiments (flow rate of 4 mL/min).

Close modal

In Figure S7, under a constant flow rate, the initial concentration of Cr(VI) had a certain impact on the removal efficiency of CRMs. When the pollutant concentration was low, the active sites on nZVI were not completely occupied, and Cr(VI) could be entirely removed from the groundwater. When the pollutant concentration increased and exceeded the removal capacity of the system, there were not enough active sites for a complete reaction. The removal percentage of Cr(VI) decreased to around 80% when the initial Cr(VI) concentrations increased to 20 mg/L.

The following experiments employed CRMs containing 0.36 g AC and 1.00 g nZVI and were conducted at pH 6 under otherwise same conditions. In the dynamic experiments, the effect of different flow rates on the release rate of nZVI from CRMs was investigated using flow rates of 2, 4, and 6 mL/min. With deionized water as the flowing groundwater, no iron ions were detected. This experimental result proved that when the CRMs do not contact reactive substances, the nZVI contained in the material will not leach. Figure 11 shows the effect of dynamic groundwater flow rates on Cr(VI) degradation by CRMs containing nZVI and AC. As the flow rate increased, the removal efficiency gradually increased (Figure 11). In the early stages of removal, the flow rate had little effect on the initial removal rate of the AC/nZVI/CRMs due to adsorption by the AC/nZVI/CRMs. As adsorption reached saturation, the AC/nZVI/CRMs began to release nZVI. Subsequently, a faster flow rate increased the removal rate of Cr(VI). The rapid water flow likely promoted the dissolution of the AC/nZVI/CRMs, and more nZVI was released from the AC/nZVI/CRMs to participate in the reaction. Compared with the static experiments, the removal efficiency of the AC/nZVI/CRMs was improved in the dynamic experiments. The AC/nZVI/CRMs quickly eroded under the action of water flow, the surface of the CRMs quickly dissolved, and more nZVI was released into the water. Cr(VI) was continually introduced by the water flow and then adsorbed and removed by the AC/nZVI/CRMs. All the removal processes of Cr(VI) by AC/nZVI/CRMs in simulated dynamic groundwater at different flow rates on Cr(VI) also followed first-order reaction kinetics.
Figure 11

Influence of the flow rate on Cr(VI) removal.

Figure 11

Influence of the flow rate on Cr(VI) removal.

Close modal

Novel AC/nZVI/CRMs prepared with paraffin and polyethylene glycol and containing nZVI and AC were developed to remove Cr(VI) from simulated groundwater. The CRMs demonstrated a greatly reduced aggregation of nZVI and better stability, allowing longer-term Cr(VI) removal when compared to the use of nZVI alone. The optimal mass ratio of the CRM components, i.e., nZVI:paraffin:quartz sand:AC:polyethylene glycol was 1:3.6:6.75:0.36:2.54. During release, a portion of the nZVIs diffuse into the water and are adsorbed by the soil medium, while the majority remains on the surface of the CRMs for Cr(VI) removal. In the static experiments, lower pH improved the efficacy of the AC/nZVI/CRMs for Cr(VI) removal from simulated groundwater. In the dynamic experiments, as the flow rate increased, the Cr(VI) removal efficiency of the AC/nZVI/CRMs decreased. AC in the AC/nZVI/CRMs prolonged the residence time of Cr(VI) in the dynamic groundwater experiments due to its strong adsorption performance, increasing the reaction contact time between nZVI and Cr(VI), thereby improving the removal efficiency of the AC/nZVI/CRMs.

These novel AC/nZVI/CRMs containing nZVI overcame the primary problem of agglomeration in the removal of heavy metals from groundwater and demonstrated stable and effective removal of Cr(VI). Such AC/nZVI/CRMs have great application potential in the removal of heavy metals from groundwater. This treatment technology combined with the use of AC/nZVI/CRMs will have the advantages of environmental friendliness and simple production processes, and the costs of this technology will be lower than those of traditional treatment systems involving electrochemical methods and PRBs.

This work was supported by the Open Funding of Zhejiang Key Laboratory of Ecological and Environmental Big Data (No. EEBD-2022-05) and the National Natural Science Foundation of China (42077178).

Z.F. wrote the original draft and reviewed and edited the manuscript. J.C. participated in data curation, formal analysis, and wrote the original draft. G.C. reviewed the manuscript. C.H. wrote the original draft and reviewed and edited the manuscript. T.W. developed the methodology, wrote the original draft, and reviewed and edited the manuscript. X.Q. wrote and reviewed the manuscript. H.Z. wrote and reviewed the manuscript. H.C. conceptualized the whole article, developed the methodology, investigated and supervised the work, involved in resource preparation, validated the process, wrote the original draft, and reviewed and edited the manuscript.

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

The authors declare there is no conflict.

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Supplementary data