ABSTRACT
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.
HIGHLIGHTS
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.
INTRODUCTION
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 METHODS
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
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.
. | nZVI (g) . | Paraffin (g) . | Quartz sand (g) . | AC (g) . | Polyethylene glycol (g) . |
---|---|---|---|---|---|
CRMs | / | 3.6 | 6.75 | / | 2.54 |
AC | / | / | / | 0.36 | / |
nZVI | 1 | / | / | / | / |
AC/CRMs | / | 3.6 | 6.75 | 0.36 | 2.54 |
nZVI/CRMs | 1 | 3.6 | 6.75 | 0 | 2.54 |
AC/nZVI/CRMs | 1 | 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 | 1 | / | / | / | / |
AC/CRMs | / | 3.6 | 6.75 | 0.36 | 2.54 |
nZVI/CRMs | 1 | 3.6 | 6.75 | 0 | 2.54 |
AC/nZVI/CRMs | 1 | 3.6 | 6.75 | 0.36 | 2.54 |
Removal experiments in static groundwater
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.
RESULTS AND DISCUSSION
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).
. | CRMs . | AC . | nZVI . | AC/CRMs . | nZVI/CRMs . | AC/nZVI/CRMs . |
---|---|---|---|---|---|---|
Adsorption (≥90%) | – | 48 h | – | – | – | – |
Removal (≥90%) | – | – | 10 min | 72 h | 180 min | 300 min |
. | CRMs . | AC . | nZVI . | AC/CRMs . | nZVI/CRMs . | AC/nZVI/CRMs . |
---|---|---|---|---|---|---|
Adsorption (≥90%) | – | 48 h | – | – | – | – |
Removal (≥90%) | – | – | 10 min | 72 h | 180 min | 300 min |
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
Effect of pH on the removal performance of AC/nZVI/CRMs
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).
. | Material . | pH . | Removal rate (%) . | Time (min) . | References . |
---|---|---|---|---|---|
Others | SA-nZVI | 6 | 96.4 | 20 | Li et al. (2019) |
Fe@LDH/rGO | 3 | ∼100 | 15 | Lv et al. (2019) | |
CMC-nFe0 | 5 | 99 | 5 | Wu et al. (2021) | |
Fe/Cu-MCM-41 | 2 | ∼99 | 40 | Guo et al. (2021a, b) | |
CRMs | nZVI/AC/CA | 2 | 100 | 360 | Wen et al. (2020) |
nZVI@SA/MH-NH 2 –1 | 3 | 96.95 | 15 | M Wang et al. (2023a) | |
nZVI-CRM | 2 | 90 | 180 | This work | |
AC/nZVI/CRM | 2 | ∼100 | 300 | This work |
. | Material . | pH . | Removal rate (%) . | Time (min) . | References . |
---|---|---|---|---|---|
Others | SA-nZVI | 6 | 96.4 | 20 | Li et al. (2019) |
Fe@LDH/rGO | 3 | ∼100 | 15 | Lv et al. (2019) | |
CMC-nFe0 | 5 | 99 | 5 | Wu et al. (2021) | |
Fe/Cu-MCM-41 | 2 | ∼99 | 40 | Guo et al. (2021a, b) | |
CRMs | nZVI/AC/CA | 2 | 100 | 360 | Wen et al. (2020) |
nZVI@SA/MH-NH 2 –1 | 3 | 96.95 | 15 | M Wang et al. (2023a) | |
nZVI-CRM | 2 | 90 | 180 | This work | |
AC/nZVI/CRM | 2 | ∼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
Removal of Cr(VI) by CRMs in dynamic groundwater experiments
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.
CONCLUSION
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.
FUNDING
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).
AUTHOR CONTRIBUTIONS
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.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.