Recovery of Rare Earth (i.e., La, Ce, Nd, and Pr) Oxides from End-of

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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Recovery of Rare Earth (i.e., La, Ce, Nd, and Pr) Oxides from End-ofLife Ni-MH Battery via Thermal Isolation Samane Maroufi,* Rasoul Khayyam Nekouei, Rumana Hossain, Mohammad Assefi, and Veena Sahajwalla

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Centre for Sustainable Materials Research and Technology (SMaRT), School of Materials Science and Engineering, University of New South Wales, High Street, Sydney, New South Wales 2052, Australia ABSTRACT: In this study, rare earth elements (REEs, i.e., La, Ce, Nd, and Pr) were thermally isolated in oxide form, from Nimetal hydride (Ni-MH) batteries via an oxidation−reduction process. The anode part of the batteries (metal hydride anode, MHA) (with chemical composition of 54 wt % Ni, 23.7 wt % La, 6.7 wt % Ce, 5.4 wt % Co, 3.6 wt % Nd, and 3.4 wt % Mn) sourced from e-waste was subjected to an oxidation process in air at 1000 °C for 60 min followed by reduction at 1550 °C for 90 min using waste developer kit (>99 wt % Fe) as a reducing agent. Oxides of nickel and cobalt were reduced and diffused into metallic iron, resulting in the formation of ferronickel alloy. In a separate process, pure hematite was mixed with the MHA as oxidizing agent, and the resulting mixture underwent a 90 min heat treatment at 1550 °C. Both processes resulted in a successful separation of Fe-based metal (ferronickel) and rare earth oxide (REO) phases. The mechanism of thermal isolation of REEs in both processes is explained in this paper. The distribution of elements between Fe-based metal and oxide phases was observed using energy-dispersive X-ray spectroscopy (EDS)/electron probe microanalysis (EPMA) elemental mapping. Oxide phase was rich in La, Ce, Nd, and Pr, and these elements did not remain in the metal phase. Iron and nickel were the main components of the metal phase. KEYWORDS: Ni-MH batteries, Recycling, Rare earth oxides, Pyrometallurgy, E-waste

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INTRODUCTION High global generation of e-waste has turned into a critical concern.1 In 2014, around 42 Mt of e-waste was generated globally which is estimated to increase up to 50 Mt by the end of 2018.2 A large proportion of the e-waste is spent batteries, as a result of their wide application in portable electronic devices. It is forecasted that the global battery demand rises 7.7% per year up to $120 billion in 2019.3 The growing trend of battery consumption is experienced in Australia where around 345 million hand-held batteries reach their end-of-life annually. Less than 6% of such batteries are recycled, and the remainder is destined for landfill or informally processed in developing countries where the manual dismantling by local people can cause serious health and environmental risks.4 It has been reported5 that in global scale, the destination for spent batteries are deposition in landfills, stabilization through incineration, or the recycling process.6 Ni-metal hydride (Ni-MH) batteries are currently one of the widely used rechargeable batteries which were developed and commercialized in Japan in 1989−1990.7 This type of battery had the advantages of low self-discharge rates, high electrochemical property, reasonable environmental compatibility, safety, and the feasibility to function efficiently within a wide range of temperatures.8 Before 1992, NiCd batteries were © XXXX American Chemical Society

dominantly sold in market as portable rechargeable batteries. In 1992, Ni-MH batteries entered the market, and more than half of NiCd batteries were replaced by such batteries.7 Ni-MH batteries did not have the issue of Cd toxicity9 and exhibited a better performance in terms of energy density and memory effect. In fact La, Ce, Pr, and Nd were used in Ni-MH batteries which are less harmful.10,11 In Ni-MH batteries, anodes are in form of AB5-type alloys, in which A is a component composed of rare earth elements (REEs, i.e., La and Ce), and B is a component composed of Ni, Co, Al, and Mn (the content of Co, Al, and Mn is limited compared to Ni). Therefore, the main elements present in the electrode of Ni-MH batteries are Ni, La, Ce, and also Co, Al, and Mn.12 While rechargeable batteries such as Ni-MH batteries are one of the dominant uses of REEs, the global supply of such resources is under considerable risk. In the reports published by European Union in 2010 and 2014, and the US Department of Energy in 2010, the supply of lanthanide group elements is under serious constrain.13 More than 90% of REEs is extracted Received: May 8, 2018 Revised: July 5, 2018

A

DOI: 10.1021/acssuschemeng.8b02097 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

This study aims to address the challenges of achieving environmentally friendly recovery of REEs from Ni-MH batteries via a novel two-step oxidation−reduction approach, using waste developer kit (DK) and Fe2O3 as reducing and oxidizing agents, respectively. The oxidation step was performed either via simple calcination at 1000 °C in air or by adding oxidizing agent (i.e., iron oxide). In one process, NiMH anode powder (MHA) was oxidized in air and then reduced by DK as a reducing agent at 1550 °C for 90 min. In another process, MHA was mixed with Fe2O3, and the mixture was heat treated at 1550 °C for 90 min. Both processes resulted in separation of REEs (i.e., La, Ce, and Nd) in oxide form and ferronickel alloy from Ni-MH batteries.

in China. However, due to high internal demand and usage by industries in China, this country gradually tightened its export around the world. As a result, supplies of REEs in market significantly dropped, and the price increased. Adding to this figure is the inefficiencies in REE ore mining and also the absence of suitable REE ore deposits in the territories of many countries. All this evidence proves the recovery of REEs through recycling should be urgently addressed.14 Despite its necessity, currently, only less than one percent of REEs are recovered via recycling, while growing stockpiles of e-waste containing REEs remain untapped. Waste Ni-MH batteries can be recycled via hydro- or pyrometallurgical processes. Because of high rate in the recovery of valuable elements, hydrometallurgical processes have been applied extensively. Several research groups around the world have investigated hydrometallurgical techniques for the recovery of nickel, cobalt, and REEs from Ni-MH batteries.14 Lyman and Palmer15 studied the leaching of NiMH scrap with different mineral acids (HCl, H2SO4, and HNO3) to transfer REEs into the solution. Another research group16 developed a hydrometallurgical process for the separation and recovery of nickel, cobalt, and REEs from NiMH batteries in which 2 M H2SO4 was used. The REEs were then recovered from leach liquor by solvent extraction with 25% bis(2-ethylhexyl) phosphoric acid. In many cases, REEs were dissolved from Ni-MH batteries by leaching with H2SO4 or HCl.17,18 Meshram et al. published a work in which they used a closed loop two-stage leaching process for the selective dissolution of metals (i.e., Ni, Zn, MN, Co, REEs, and Co) from electrode of Ni-MH batteries. The overall recovery in this work was reported 96% for REEs.19 The drawback associated with such processes is that they are time-consuming and in most cases very complex which might cause environmental pollution because of large amount of acid used.20 In ref 21, REEs were extracted from Ni-MH batteries based on superficial fluid extraction using CO2 as solvent. This process resulted in the 90% recovery of REEs. REEs and transition metals such as Fe and Ni exhibit different behaviors in terms of oxygen affinities, and this property attracts the attention of many researchers in recent years.22−27 Using this property, REEs can be separated in the form of oxides at high temperature from transition metals. Unlike the hydrometallurgical route, in this technique, acids are not used, and generation of waste solution is avoided. Therefore, this process can be considered environmentally friendly. Based on this concept, via a two-step oxidation− reduction approach, we were able to extract REEs from Nd− Fe−B permanent magnets in which waste-tire-rubber-derived carbon (WTR-DC) was utilized as reducing agent.28 The recovery of REEs from Ni-MH battery using high-temperature pyrometallurgical process has been investigated by a few researchers. In research carried out by Jiang et al.,12 SiO2 and Al2O3 powders were added into the starting material as fluxes which were then melted at 1550 °C to form a Ni−Co alloy and a REO−SiO2−Al2O3−MnO slag (REO, rare earth oxide). In another work published by Tang et al.,29 waste calcium silicate slags were used as the REO absorbent. The high-temperature process in this research resulted in the formation of calcium cerite and Ni−Co alloy. The use of stable oxides such as CaO, Al2O3, and SiO2 as fluxing oxides results in introducing more impurity to the slag phase. In fact, such oxides are not reduced to the metallic phase and will remain in the slag phase.

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MATERIALS AND EXPERIMENTAL PROCEDURE

Ni-MH batteries sourced from the Reverse E-waste Company, Sydney, Australia, were dismantled manually. The plastic cases were manually separated, and the cell was cut longitudinally; the active components were separated from the plastic pieces. The dismantling of selected Ni-MH batteries exhibited two electrode plaques separated via a polymer sheet which was impregnated by KOH electrolyte. The materials of the exterior housing were Ni-plated steel surrounded with a plastic case. The anode metal (MHA) powder was separated from the supporting metal mesh and other parts. MHA powder was dried in oven overnight and then analyzed using XRD, XRF, and inductively coupled plasma (ICP) technique. MHA powder was then heated in a hot muffle furnace in air at 1000 °C for 1 h to oxidize all elements (i.e., La, Ce, Nd, Ni, Co, Mn, and Al) present in the anode alloy. Waste developer kit (DK) from printers, supplied by TES-AMM, Sydney, Australia, was used as a source of iron for reduction of oxides. Iron was selected as reducing agent and alloying solvent into which the Ni can diffuse and produce Ni−iron alloy. The oxide of MHA powder (MHAO) was then mixed with DK with ratio of 1:1, and the resulting mixture was placed in a graphite crucible heated up to 1550 °C in a tubular furnace with dimension of 100 cm length × 5 cm diameter in argon atmosphere (1 L min−1) for 90 min. After heat treatment, the sample was kept in the cold zone of the furnace in argon for 10 min. The composition and elemental distribution of the resulting phases were further analyzed by X’pert PRO multipurpose XRD (MPD system), ICP, scanning electron microscopy (SEM)/energy-dispersive X-ray spectroscopy (EDS), and electron probe microanalysis (EPMA). In a separate process, pure hematite (>99%) supplied by Aldrich was also utilized as oxidizing agent in the oxidation−reduction process. MHA powder was mixed with hematite with ratio of 1:1, agitated in rotating drum for 1 h for homogeneity, and the resulting mixture was then heat treated at 1550 °C in the same tubular furnace under argon purge (1 L min−1) for 90 min. After heat treatment, the sample was kept in the cold zone of the furnace in argon for another 10 min. The composition and elemental distribution of the resulting phases were further investigated. The results of the two processes were compared.

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RESULTS AND DISCUSSIONS Characterization of the Starting Materials. The chemical composition of the MHA powder was characterized by XRF. The metal composition of MHA powder is listed in Table 1. The precise chemical composition of the anode in Ni-MH batteries differs between manufacturers, but generally the main Table 1. Composition of MHA Powder Determined by XRF element weight percent B

Al 0.3

Mn 3.4

Co 5.4

La 23.7

Ce 6.7

Nd 3.6

Ni 54.1

DOI: 10.1021/acssuschemeng.8b02097 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering components in the anode are a mixture of rare earth elements (REEs) and Ni. Three main REEs identified are lanthanum (La), cerium (Ce), and neodymium (Nd) with the total REE weight percent being approximately 34 wt %. The major impurities are nickel (Ni), manganese (Mn), cobalt (Co), and aluminum (Al). MHA powder before and after calcination was subjected to XRD analysis. The sample before calcination consists of two basic phases: lanthanum nickel (La0.866Ni5.268) and cobalt neodymium (Co5Nd) as can be seen from Figure 1a. The XRD result confirms the anode material in this study is in AB5 form, where A is La, Nd, and Ce, and B is Ni, Co, Al, and Mn.

For LaNiO3 and Nd0.2Ce0.8O1.9, no heat capacity measurement is available from the literature. The Gibbs free energy of the reaction was estimated using HSC software. Given that anode material in this study is in AB5 form, Co5Ce is supposed to be present in MHA, but it was not detected by XRD. As shown in eqs 4a and 4b, dissociation of Co5Nd/Ce and further reaction with oxygen results in the formation of Nd0.2Ce0.8O1.9. Waste DK was also characterized using XRF and XRD. XRF analysis confirms that DK was pure iron, and no impurity was detected within the sample. Figure 2 exhibits the X-ray diffraction spectrum of DK. As can be seen metallic iron is the only phase detected by XRD.

Figure 1. XRD spectra of MHA powder (a) as received and (b) after oxidation at 1000 °C for 1 h. Figure 2. XRD spectrum of waste DK.

The XRD pattern of the MHA powder after oxidation in air (MHAO) at 1000 °C, in Figure 1b, displays the strong diffraction peaks corresponding to oxide phases. In fact, after oxidation at 1000 °C for 1 h, the XRD pattern shows diffraction peaks that belong to LaNiO 3 , NiO, and Nd0.2Ce0.8O1.9. However, the peaks assigned to the Co and Mn phases were not identified since they are below the detection limit of XRD. The presence of these components indicates that all elements (i.e., La, Ce, Ni, Nd) in the MHA sample were transformed to their oxide forms. The formation of LaNiO3, NiO, and Nd0.2Ce0.8O1.9 requires the initial dissociation of the La0.866Ni5.268 and Co5Nd intermetallic phases followed by the formation of oxides from their elements:

Oxidation−Reduction Process (Using DK and Iron Oxide). For the final separation of REEs, the reduction of the resulting oxide powder at temperature of 1550 °C was carried out. For this purpose waste DK (i.e., metallic iron) was selected as reducing agent and alloying solvent in which Ni, Co, and carbon can diffuse. The separation of the REEs (La, Ce, and Nd) from Fe, Ni, and Co via this technique is based on the difference in the nature of oxidation processes between REEs and Ni, Fe, and Co. Based on the Ellingham diagram, carbon can reduce nickel and cobalt oxides at temperatures below 1400 °C. However, oxides of La, Ce, Nd, Pr, and Al stay unaffected at such a temperature; it is not possible to reduce REOs into metallic form at this temperature. In addition carbon and iron can be applied to efficiently separate the Ni and Co from the REEs. Due to the diffusion of carbon into metallic iron and lowering the melting point of iron, the fluidity of iron improves which is critical for the separation process. For the recovery of REEs from MHA powder another technique of oxidation−reduction was applied in which iron oxide (i.e., hematite) was used as oxidizing agent and mixed with MHA powder. The mixture was heat treated in a graphite crucible and reduced by carbon from graphite crucible. The results of the two oxidation−reduction processes were compared. Mechanism of the Separation of REEs from MHA Powder. The concentrations of CO and CO2 gases released during the reduction of MHA−Fe2O3 and MHAO−DK at 1550 °C were monitored using an IR−gas analyzer connected to the furnace. Figure 3 illustrates the volume percentage of CO and CO2 from off-gas measurements over time. As can be seen in Figure 3, in the first 3 min of reaction time, the concentration of CO gas surged in both cases of MHA− Fe2O3 and MHAO−DK while concentration of CO2 gas had a slight increase during this time.

2LaNi5(s) + 7O2 ↔ 2LaNiO3(s) + 8NiO(s) ΔG1550* = −2124.1 kJ

(1)

where ΔG1550* is estimated Gibbs free energy. Breaking of LaNi5 is followed by the combination of La and Ni with O and further formation of a mixed oxide according to LaNi5(s) ↔ La + 5Ni

ΔG1550* = −31.9 kJ

(2)

La + 2Ni + 2O2 ↔ LaNiO3(s) + NiO(s) ΔG1550* = −798.5 kJ

(3)

Co5Nd(s) ↔ Nd(s) + 5Co(s)

(4a)

Co5Ce(s) ↔ Ce(s) + 5Co(s)

(4b)

Nd(s) + 4Ce(s) + 4.75O2 (g) ↔ 5Nd 0.2Ce0.8O1.9(s) ΔG1550* = −2185.5 kJ

(5) C

DOI: 10.1021/acssuschemeng.8b02097 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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(Figure 3) is very limited indicating that reaction consuming CO2 and reaction 10 is faster than reactions producing CO2. High concentration of CO favors reaction 9 and makes ΔG negative. (NiO) + Fe ↔ [Ni] + (FeO)

ΔG° = 735 − 39.98T

ΔG1550 = −72.162 kJ

(7)

(NiO) + CO(g) ↔ [Ni] + CO2 (g) ΔG° = −28 553 − 10.581T

Figure 3. Released gas concentration in terms of CO and CO2 as a result of reduction of (a) MHA−Fe2O3 and (b) MHAO−DK at 1550 °C.

ΔG1550 = −47.85 kJ (8)

(FeO) + CO(g) ↔ Fe(C) + CO2 (g)

In the case of MHAO−DK, first nickel oxide was reduced. The initial rapid stage of reduction of nickel/cobalt oxide occurs simultaneously both by carbon (sourced from crucible) and by iron via an exchange reaction. The overall NiO reduction from slag by carbon and the mechanism of separation of REEs from MHAO−DK are schematically presented in Figure 4.

ΔG° = −29 288 − 29.403T [C] + CO2 (g) ↔ 2CO ΔG1550 = −146.199 kJ

ΔG1550 = 24.31 kJ

(9)

ΔG° = 161 150 − 168.58T (10)

Therefore, by a combination of carbon and iron, NiO is fully reduced from oxide phase in a few minutes. After reduction of NiO and FeO, ferronickel is formed while the remaining slag is rich in rare earth oxides (i.e., La, Ce, and Nd). It is worth noting that FeO particles are reduced at the gas−slag interface from which Fe particles can be transported to the upper slag on rising bubbles of carbon monoxide. It has been shown in the literature that the surface tension forces of slag can be able to hold some particles (below a certain size) on the surface of slag, regardless of the fact that they might be denser than the slag phase. Although these iron particles are very limited, however, once transported to the slag surface, they remain there.31 Small iron particles of approximately up to 10 μm were found trapped within the solidified slag (Figure 10). In the case of MHA−Fe2O3, the mechanism of the separation of REEs is schematically shown in Figure 5. In

Figure 4. Mechanism of separation of REEs from MHAO−DK during heat treatment at 1550 °C in argon.

In the overall reaction (eq 6), four phases are involved, and therefore it is unlikely that the whole reaction occurs at a single site.30 (NiO) + [C] ↔ [Ni] + CO(g) ΔG = 132 597 − 179.163T (ΔG1550 = −194.04 kJ) (6)

In this process, at the initial stage a film of gas is formed at the slag−metallic iron interface. Therefore, slag−metallic iron interface will be partially replaced by gas−slag and gas−metal interfaces. NiO in slag phase is reduced by metallic iron at slag−metal interface (eq 7) followed by introducing FeO into the slag. The ΔG° associated with reduction of NiO by iron (NiO + Fe = Ni + FeO) is negative and small. The metallic Ni produced from reduction of NiO is then dissolved into iron which lowers its activity and makes ΔG° more negative.30 As a result, NiO is easily reduced by iron and introduces FeO into the slag phase. After this stage, oxygen is present in the oxide phase in form of remaining NiO and FeO. FeO and NiO then will be reduced by carbon via CO at the gas−slag interface producing Ni, Fe, and CO2. This CO2 will be reduced back to CO at the gas−metal interface via Boudouard reaction (eq 10). It is worth noting that, in eq 9, the Gibbs free energy of the equation depends on partial pressure of CO and CO2. CO2 concentration in the exit gas

Figure 5. Mechanism of separation of REEs from MHA−Fe2O3 during heat treatment at 1550 °C in argon.

this process, the oxidation−reduction step occurs simultaneously. The initial contact between solid carbon and Fe2O3 results in the formation of Fe3O4 and CO gas (eq 11). The produced CO gas in turn reacts with Fe2O3 and reduces it to Fe (eq 12). The generated CO2 reacts with REEs present in MHA and oxidizes them (eq 14). The produced CO2 also partially reacts with carbon from crucible according to the Boudourad reaction (eq 13). When CO is generated from the Boudouard reaction and REE oxidation, the gaseous reduction of Fe2O3 in eq 12 increases, producing more CO2 gas. D

DOI: 10.1021/acssuschemeng.8b02097 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering 3Fe2O3(s) + C(s) ↔ 2Fe3O4 (s) + CO(g) ΔG° = 23 557 − 225.1T

ΔG1550 = −286.837 kJ (11)

Fe2O3(s) + 3CO(g) ↔ 2Fe(s) + 3CO2 (g) ΔG° = −4324 + 7.436T C(s) + CO2 (g) ↔ 2CO(g)

ΔG1550 = −17.88 kJ

(12)

ΔGo = 161 150 − 168.58T

ΔG1550 = −146.2 kJ

(13)

REE + CO2 ↔ REO + CO

(14)

2La + 3CO2 ↔ La 2O3 + 3CO ΔG° = −946 785 + 26.102T

ΔG1550 = −899.2 kJ (15)

Figure 7. (a) Photograph of MHA−Fe2O3 after reduction at 1550 °C. (b) Photographs of the oxide and metal phases after separation. (c) SEM images of metallic phase and (d) SEM image of the oxide phase after separation from MHA−Fe2O3.

(16)

Both the metallic and oxide phases after separation were further examined using different techniques. Figure 7c,d shows SEM images of the cross sections of ferronickel alloy and oxide phase, respectively. In the SEM image of the oxide phase, two phases with different color (dark and bright gray) can be observed. The separated oxide powder from both MHA−Fe2O3 and MHAO−DK mixtures was subjected to XRD analysis, and the presence of the three phases of La0.88MnO2.92, CeO1.66, and Nd2O3 was confirmed (Figure 8). Although the concentration of manganese in the initial MHA powder is below the detection limit of XRD, and X-ray diffraction spectrum of MHA powder did not identify the phases corresponding to Mn, however, it was detected in the recovered oxide phase. The oxide phase is rich in REEs (La, Ce, and Nd), and Al and Mn are the impurities detected by XRD. The impurities can be removed through a facile leaching process. To study the distribution of the elements (i.e., La, Ce, Nd, Mn, Fe, Ni, Co, and C) between the metal and oxide phases, the sample was further investigated by EDS distribution element mapping, as shown in Figures 9−11. Figures 9 and 10 display EDS distribution element mapping in iron-based metal and oxide separated from MHA−Fe2O3 mixture after heat treatment at 1550 °C in argon for 90 min. As can be seen from Figure 9, Fe and Ni are the dominant elements of the metallic phase. The REEs such as La, Ce, Pr, and Nd were almost undetectable in the metal phase (Figure 9). In contrast, in the oxide phase, La, Ce, Nd, and Pr were dominantly present while a small trace of Al and Mn can be also observed (Figure 10). It proves that REEs are concentrated in the oxide phase. Figure 11 shows EDS mapping of elements distributed between ironbased metallic phase and oxide phase separated from MHAO− DK mixture after heat treatment in argon at 1550 °C for 90 min. While Fe and Ni are the main elements of the Fe-based metal phase, La, Ce, Nd, Pr, and Al entered the oxide phase. It can be also observed from the EDS distribution element mappings of the oxide phase that some iron droplets (99 wt % iron) as reducing agent and heat treated for 90 min in argon at 1550 °C. Oxides of cobalt and nickel were reduced by iron and separated from the oxide phase which led to the formation of Fe−Ni alloy and oxide phases. In a separate process, pure hematite was mixed with MHA followed by 90 min of heat treatment at 1550 °C. Iron oxide was reduced and resulted in the formation of CO2 which oxidized REEs followed by diffusion of Co and Ni into metallic iron. In this process REEs were isolated in oxide phase and separated from Fe−Ni alloy. The resulting Fe−Ni Alloy and oxide phases were further examined for their elemental distribution. The oxide phase mainly contained REEs (i.e., La, Ce, and Nd), and a minor amount of Mn was detected. REEs were not present in the Ni−Fe alloy. As a comparison both processes exhibited different behaviors in terms of purity of the separated REOs. Thermal isolation of MHO−DK resulted in separating oxide phase with much less impurity and high concentration of REEs. Given that DK is sourced from e-waste, the use of this waste material as reducing agent could demonstrate a new lowcost means of recovering REEs from Ni-MH batteries and the benefits of transforming two wastes to value.

Figure 12. EPMA elemental mapping of REOs separated from thermal isolation of (a) MHAO−DK and (b) MHA−Fe2O3.

indicate elemental mapping and concentration of metal and oxide phases separated from thermal isolation of MHAO−DK. The EPMA elemental mapping of the oxide phase separated via thermal isolation of MHAO−DK, Figure 12a, shows that the average weight percentages of La, Ce, Nd, and Pr in selected areas were 47, 25, 10, and 5 wt %, respectively. The content of Al as impurity was around 3.5 wt %. The oxide phase separated from MHA−Fe2O3 was also analyzed using EPMA, and the results are shown in Figure 12b. The average

Corresponding Author

*E-mail: s.maroufi@unsw.edu.au. ORCID

Samane Maroufi: 0000-0001-5553-8519 Notes

The authors declare no competing financial interest. G

DOI: 10.1021/acssuschemeng.8b02097 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

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DOI: 10.1021/acssuschemeng.8b02097 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX