Application of encapsulated and immobilized nano zero-valent iron for iron removal from roof-harvested rainwater

Most urban areas around the world have experienced rapid population growth, posing the challenge of supplying enough water to meet the demands of social consumption and economic activities (Kilonzo et al. 2019). Rainwater harvesting is a promising option for addressing water supply pressures in areas where water sources are scarce or polluted (Thomas 2009). Rainwater reuse reduces the consumption of potable water, thereby conserving fresh water resources. One of the most significant barriers to rainwater usage is public health (Igbinosa & Aighewi 2017). Heavy metals and microbiological contamination are two of the most commonly mentioned issues regarding the quality of water flowing down from roofs (Meera & Mansoor Ahammed 2006; Ahammed & Meera 2010). Iron contamination is often reported in roof-harvested rainwater (Igbinosa & Aighewi 2017). Although iron is a necessary element for humans, its presence in aqueous systems above a specific level renders the water unfit for drinking and other purposes due to colouring, metallic taste, odour, turbidity, and laundry stains (Khatri et al. 2017). Providing people with clean drinking water is a universal human need that necessitates efficient yet cost-effective methods. Various systems for providing clean drinking water are available, but they are either costly or ineffective without energy. Therefore, appropriate technology that makes use of low-cost, generally accessible materials which meet or surpass the capabilities of current water treatment methods is the need of the present-day water scenario.

Nano materials exhibit several unique properties compared to bulk materials due to their large surface area and high pore volume, and have received considerable interest in the scientific community over the past few decades (Rajan 2011; Rathor et al. 2017). The large surface area makes these materials effective adsorbents in environmental remediation. Among the nano materials, nano zero-valent iron (nZVI) has been extensively studied for heavy metal treatment, but nZVI is susceptible to agglomeration and rapid oxidation, causing recovery problems (Phenrat et al. 2007; Baalousha et al. 2008). Due to these difficulties, applications of nZVI can limit environmental clean-up to a large extent. However, many studies have been made and/or are in progress for the surface modification and immobilization of nZVI to overcome these challenges (Fatisson et al. 2010; Bhowmick et al. 2014; Fu et al. 2015; Liu et al. 2016; Gao et al. 2018). Among the modifiers, CaCO₃ is an important bio-compatible material having ideal biodegradability. At the same time, phases and the morphology of CaCO₃ can be readily modified and controlled to acquire a special structure (Galván-Ruiz et al. 2009; Wang et al. 2016; Zhang et al. 2019). Similarly, polyurethane (PU) is an organic polymer having C = O and N-H groups, which facilitate its binding with nZVI (Khan et al. 2019; Sasidharan et al. 2021). In this study, CaCO₃ was employed to encapsulate the nZVI, and the encapsulated nZVI was effectively immobilized on PU foam (PUF) for the first time. CaCO₃-nZVI and PUF/CaCO₃-nZVI were used to remove iron from harvested rainwater.

Merck Millipore supplied analytical grade ferrous sulphate heptahydrate (FeSO₄·7H₂O), CaCO₃, ethylene diamine tetraacetic acid (EDTA), sodium borohydride (NaBH₄), ethanol (CH₃CH₂OH), sodium hydroxide (NaOH), and hydrochloric acid (HCl).

For dispersing CaCO₃ in ethanol, a 20 kHz ultrasonic homogeniser sonicator with a 12 mm ultrasonic probe was utilized. Remi R-12M laboratory centrifuge and a Palintest PTBH-7500 photometer were employed for the separation of nanoparticles and analysis of iron, respectively.

4.17 g FeSO₄·7H₂O was dissolved in 150 mL distilled water and carefully mixed with 100 mL 0.05 M EDTA solution. CaCO₃ suspension was prepared by dispersing 0.51 g CaCO₃ in 150 mL ethanol using an ultrasonicator for 15 minutes. In a two-neck round bottom flask, CaCO₃ suspension was added and combined with a mixture of FeSO₄·7H₂O (CaCO₃/Fe mass ratio of 0.6) and EDTA solution. This mixture is named as CaCO₃ dispersed iron salt solution. Nitrogen purging was used to remove the dissolved oxygen. After that, 2.84 g NaBH₄ was dissolved in 100 mL distilled water and added into the flask drop by drop. The mixture was agitated for another 20 minutes after the addition of the reducing agent. Centrifugation was used to collect the black nanoparticles, which were then washed three times with ethanol. The particles were oven-dried at 50 °C and kept in a sealed container for future usage under normal circumstances (Zhang et al. 2019; Cheng et al. 2020; Antony et al. 2022).

The novel nano-composite, PUF/CaCO₃-nZVI was prepared for the first time in this study. The PUF was cut into 25 mm × 25 mm × 6 mm pieces and submerged in a nitrogen purged, CaCO₃ dispersed iron salt solution (prepared as mentioned in section 2.2) for 1 hour. After 1 hour, the PUF pieces were lightly washed with ethanol and immersed for 30 minutes in 100 mL of 0.75 M NaBH₄ for full reduction. PUF/CaCO₃-nZVI sorbents were washed three times in absolute ethanol before being oven-dried at 50 °C.

The surface morphology and elemental composition of nano materials were studied by scanning electron microscope (CARL-ZEISS model DSM 960 A, Germany) equipped with energy-dispersive X-ray spectroscopy. Bruker D8 advance X-ray diffractometer with Cu-Kα radiation source operating at 40 kV/ 40 mA was used to investigate the crystalline properties of nano materials.

The surface area, pore size and pore volume distribution were measured by N₂ adsorption-desorption isotherms using Brunauer-–Emmett–Teller surface area analyser (BET, Belsorp Max, MicrotracBelsorp, Japan).

The Thermo Nicolet Avtar 370 model spectrometer was employed to record the FTIR spectrum at a resolution of 4 cm⁻¹. The spectrum was recorded in the transmission mode at room temperature using KBr pellets in the range 400–4,000 cm⁻¹. The sample was mixed with spectrally pure KBr and pressed to form thin plate, then subjected to IR spectroscopic analysis to detect the interacting functional groups for the formation of sorbents.

The harvested rainwater was collected in polyurethane storage tanks using PVC downpipe attached to two roof systems viz. TR and GISR. Roof runoff was collected for one week in the middle of rain event during September-October months of 2021. The roofs were of 12 years old and located in a residential area adjacent to Cochin International airport, Kerala, India and NH 65. Water samples were then collected carefully in cleaned high-density polythene bottles (1 L capacity). Bottles were sealed and brought to the laboratory for analyses which were carried out in accordance with the standard methods (APHA 2012, 2017). ICP-OES (ICAP 7000 series) was used to determine the concentration of heavy metals present in water samples.

The potential of CaCO₃-nZVI and PUF/CaCO₃-nZVI for iron remediation were explored by conducting experiments in batch equilibration mode. 200 mL of samples were taken in a series of conical flasks and agitated in an orbital shaker by adding 0.25 g/L nano sorbent at natural sample pH as well as at pH 10, which was found to be maximum in synthetic iron solution (Figure S2).

10 mL sample was withdrawn after 3 hours treatment (optimum reaction period found for synthetic iron solution- Figure S1) and allowed settling for 20 min. The centrifugation, as well as magnetic separation, was given for the collected supernatant and the total iron concentration in the treated sample was analyzed using Palintest PTBH-7500 photometer, which employs the principle of 1–10, phenanthroline method (APHA 2012).

The physico-chemical characteristics of the samples collected from different aqueous systems are listed in Table 1. The iron content in all water samples exceeds the drinking water standard. The presence of other metals (Ca, Mg, Pb, Cu, Cd, Cr, Zn and Mn) was also detected in which the concentration of lead exceeds the acceptable limit for the potable water as per Indian Drinking Water Standard (IS 10500, 2012).

The effluent characteristics presented in Table 2 shows that both CaCO₃-nZVI and PUF/CaCO₃-nZVI have the capacity to bring the iron concentration to the desired limits of drinking water. The reduced concentration of other metals present in the harvested rainwater viz. Ca, Mg, Pb, Cd, Cr, Cu, Zn and Mn are also shown. The presence of other metal cations detected in EDX results of spent nano sorbents further verifies the high potential of CaCO₃-nZVI and PUF/CaCO₃-nZVI in removing heavy metals (Table 3).

The nano zero-valent iron nanoparticle has a typical core shell structure produced during the synthesis process, which includes a shell of Fe(II), Fe(III), and core of zero-valent iron (Çelebi et al. 2007; Boparai et al. 2011). Because of its unique structure, nZVI has the abilities of reduction, surface sorption, stabilization, and precipitation of different pollutants. For metal ions with standard electrode potentials (E⁰) very close to, or more negative than, Fe⁰ (−0.44 V), the removal process by nZVI is mostly by sorption/surface complexation. In comparison, removal of metal ions which have E⁰ much more positive than Fe, occurs primarily via surface mediated reductive precipitation. However, sorption with partial chemical reduction has been demonstrated for metal cations that are only slightly more electropositive than iron (Li & Zhang 2007; Li et al. 2017). According to this concept, the sorption through electrostatic interaction/surface complexation is the only possible mechanism that can occur in iron sequestration by CaCO₃-nZVI and PUF/ CaCO₃-nZVI due to the same redox behaviour of the nanoparticles and the metal ions present in the aqueous medium.

This iron adsorption on CaCO₃-nZVI is proposed to occur in the following ways in light of FTIR spectrum of nanoparticle (Figure 3) after treatment:

• (1)

  The broad absorption band at 3,410–3,128 cm⁻¹ corresponding to O-H stretching vibration is shifted to 3,352–3,350 cm⁻¹ and 3,371–3,369 cm⁻¹ after treatment of harvested rainwater from GI and tiled roof respectively, showing the possibility of contaminant iron adsorption on iron oxide/hydroxide shell. Moreover, emergence of a peak at 582 cm⁻¹ and 583 cm⁻¹ corresponding to Fe-O stretching vibration confirms the role of oxide shell.

• (2)

  The absorption band at 1,630 cm⁻¹ before treatment is shifted to 1,633 cm⁻¹ and 1,632 cm⁻¹ after treatment. This peak is attributed to the C = O stretching vibration present in the EDTA as well as in CaCO₃ and confirms the contaminant iron adsorption on these groups. Similar observations were found in studies by Cheng et al. (2020) for chromium removal by CaCO₃ coated nZVI.

• (3)

  The peak at 1,383 cm⁻¹ (C-H stretching vibration) has disappeared and the peak at 1,107 cm⁻¹ (C-O or C-O-C) has shifted to 1,109 cm⁻¹ and 1,102 cm⁻¹ after treatment. The above changes in the peaks of functional groups present in EDTA also reveal iron adsorption.

The characterization studies thus divulge the involvement of iron oxide/hydroxide shell, EDTA and CaCO₃ in the iron sorption process of CaCO₃-nZVI (Table 4). Similarly, the FTIR spectrum of spent PUF/ CaCO₃-nZVI (Figure 6) and the possible functional groups presented in Table 4 recommends that the iron sequestration process may occur as follows:

• (1)

  The shifting occurs in the region 3,296–3,293 cm⁻¹ (N-H stretching vibration), 2,970 cm⁻¹ and 2,929 cm⁻¹ (C-H stretching vibration) and 1,582 cm⁻¹ (N-H bending) unveil that the iron is adsorbed to the PUF/CaCO₃-nZVI by making interactions with the N-H and C-H group of PUF.

• (2)

  The peak at 3,410 cm⁻¹ (O-H stretching vibration) has disappeared and new peak at 582 cm⁻¹ and 574 cm⁻¹ (Fe-O stretching vibration) are observed in the spent PUF/CaCO₃-nZVI. These changes occurred after the treatment process proves the involvement of iron oxide shell in iron remediation.

• (3)

  The absorption band at 1,630 cm⁻¹ shifts to 1,639 cm⁻¹ and 1,640 cm⁻¹, after treatment of roof harvested rainwater from GI sheet and tiled roof respectively. The C = O stretching vibration found in EDTA and CaCO₃ is responsible for this peak, which verifies contaminant iron sorption on these groups.

• (4)

  Similar to the observations for CaCO₃-nZVI, the peaks corresponding to C-H (1,383 cm⁻¹) and C-O or C-O-C stretching vibration (1,107 cm⁻¹) are shifted after treatment, revealing the significant role of EDTA.

In comparison to previously reported techniques, PUF/CaCO₃-nZVI was found to have greater efficacy in iron remediation (Table 5). PUF/CaCO₃-nZVI outperformed both traditional sorbents/techniques and nano materials in iron removal. In comparison to study conducted in wastewater using a nano composite of carbon nanotubes (CNT) and nano iron oxide, the iron removal efficacy is found to be low. However, as Fe₃O₄/CNT nano composite is toxic, its application is limited.

The nZVI was successfully encapsulated by CaCO₃ and immobilized on PUF. The increased surface area and widely dispersed particles visible in the SEM image confirm that encapsulation and immobilisation completely eliminated aggregation issues. The functional groups which plays significant role in the stabilization of nZVI through encapsulation as well as impregnation were identified using the FTIR spectrum. Both CaCO₃-nZVI and PUF/CaCO₃-nZVI showed high efficiency to sequester contaminant iron from harvested rainwater. The sorption mechanism was confirmed by EDX and FTIR spectra of spent nano sorbents, which revealed the favourable functional groups for iron adsorption. This research provides a new way of immobilising CaCO₃-nZVI and demonstrates that surface encapsulated immobilized nZVI can be effectively used for iron removal from harvested water meeting potable standard.

The All India Council for Technical Education (AICTE) funded this research under the NDF-RPS initiative for NDF research scholars.

This study has not been published and is not being considered for publication anywhere. We have no potential competing interests to disclose.

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

The authors declare there is no conflict.