Exploring ions selectivity of nanofiltration membranes for rare earth wastewater treatment

Highlights

• The roles of different valence anions and cations in the NF separation process are discussed in detail.

• Specific adsorption of rare earth ions on the membrane surface will cause membrane charge reversal.

• NF270 can be used for the separation of monovalent cations from multivalent cations.

• NF5 can be used for the separation of divalent cations from multivalent cations.

Abstract

The separation of metal ions with different valences is an intractable challenge in the treatment of rare earth (RE) ore leaching solutions, and nanofiltration (NF) is one of the most promising approaches to separate ions with different valences. Therefore, the separation behaviors of three commercial NF membranes (NF5, NF270 and NF90) were investigated in this study. The results showed that NF technology could be used for the separation of cations with different valences from nitrate and chloride; the NF270 membrane could be used for the separation of Na⁺/Ca²⁺ due to its appropriate pore size and negative charge. The NF5 membrane could be used for Ca²⁺/Nd³⁺ separation due to its large pore size and suitable zeta potential. Increasing the operating pressure and cross-flow velocity was beneficial to increase the separation factor of Ca²⁺/Nd³⁺. Increasing the solution pH and salt concentration facilitated the adsorption of metal cations on the membrane surface, resulting in the separation factor of Ca²⁺/Nd³⁺ increasing first and then decreasing. This work may provide new strategies to recycle valuable metals from RE wastewater or other complex mediums.

Introduction

Rare earth elements (REEs) are honoured as future materials because of their importance and significance in clean energy technologies, such as batteries, electric vehicles, wind turbines, hydrogen storage and so on [1], [2]. Global rare-earth reserves are limited and unevenly distributed across the globe. Most of these RE resources are located in a few countries. China accounts for 36.7% of global RE reserves and dominates the global rare-earth market [3]. With the development of the rare earth industry, the exploration and development of RE resources have generated increasing RE wastewater, resulting in serious environmental problems and wasted resources. Therefore, the separation of REEs was listed as one of “the seven chemical separation processes” [4]. Recently, the increasing applications of REEs in high-tech products have resulted in a significant increase in the demand for REEs [5]. Consequently, research and development toward sustainable REEs recovery from secondary sources are necessary to reduce the environmental impacts and replenish the resources required to satisfy future technological needs [2], [6], [7].

In-situ leaching mining is widely used to extract REEs because ion-adsorbed rare earth ores occupy an important position and have vital comprehensive utilization value in China. The most original leaching process is to use sodium chloride vat leaching and then use oxalic acid to precipitate and recover rare earth elements, which finally obtain a large amount of rare earth chloride. (NH₄)₂SO₄ is the second generation of leaching solution because ammonia nitrogen pollution is gradually replaced by the new leaching agents MgSO₄ and Al₂(SO₄)₃, which do not contain ammonium, and finally obtain a high concentration of rare earth sulfate [8], [9]. At present, the nitric acid leaching method has been proposed to efficiently extract RE from rare earth ores and finally obtain a high concentration of rare earth nitrate [10], [11], [12].

Purification of water contaminated with rare earth elements requires the ability to selectively remove rare earth ions without destructing or modifying them so that the metals can be recovered and recycled. Usually, the separation or removal of RE is carried out by chemical precipitation, ion exchange, solvent extraction, adsorption and chemical vapour transportation [13]. These methods most utilize additional solvents, which would increase cost and cause environmental concerns. Compared with these conventional techniques, membrane technologies have higher selectivity for small molecules and salts, lower energy requirements and easy operation. Moreover, membrane technologies offer the possibility of zero pollution discharge. As a consequence, membrane technology has received great attention in metal separation and resource recovery as a sustainable green strategy [5], [14], [15].

The membrane technology firstly used for rare earth separation is the liquid membrane technique, which can achieve extraction and stripping simultaneously [16]. It is an efficient way to separate and enrich metal ions on account of its large mass transfer interfacial areas [17], and it can extract heavy metal ions from acidic aqueous solutions well. Tahmasebizadeh et al. [18] used the emulsification liquid membrane (ELM) technique to separate zinc from a bioleaching solution of a low-grade lead/zinc sulfide ore, and more than 71% zinc was recovered from the bioleaching solution under optimal conditions. Ramakul et al. [16] used a microporous hydrophobic hollow fiber-supported liquid membrane to separate yttrium ions from the mixture of rare earths in lanthanide series. However, commercial applications are limited by several problems, such as short lifetimes and low stability [14], [19]. There is no doubt that more membrane separation techniques need to be exploited and innovated for rare earth separation in the future.

Nanofiltration (NF) is a relatively new pressure-driven membrane process with molecular weight cut-offs (MWCO) in the range of 100 to 2000 Da located between ultrafiltration (UF) and reverse osmosis (RO), and it is suitable for separation or removal of ions and small molecules [20], [21], [22]. The separation mechanism of NF is normally explained in terms of Donnan, dielectric effects and steric hindrance [23]. The selective layer of the NF membrane is a three-dimensional network of polymer chains, and its separation property is mainly governed by the effective pore size and charge on/in this layer. Different pH values and ionic strengths would cause these polymer chains to transform under various surrounding conditions. Therefore, even a minor change in the pore size or charge pattern would have a clear impact on the membrane permeability and the passage of molecules. These characteristics of NF membranes offer some degree of retaining ability to metal cations with different valences [15], [22]. Consequently, NF has great potential to separate inorganic salts with different valences. Many studies showed that the selective separation of monovalent and divalent ions could be realized by NF. Yang et al. [24] investigated the separation of lithium from salt lake brines by a dual-skin layer NF membrane. The separation factor of Mg²⁺/Li⁺ was up to 33.4 due to size sieving and electrostatic exclusion of NF membrane. Sun et al. [20] discussed the effect of operating conditions on the separation of magnesium and lithium by DL-2540 NF membrane. They found that the separation of magnesium and lithium became more efficient by increasing the pressure and lowering the pH, and increasing temperature and Mg²⁺/Li⁺ ratio was unfavorable for the separation of Mg²⁺/Li⁺. However, few studies regarding the separation of rare earth ions with different valences by NF membranes have been reported. Léniz-Pizarro et al. [25] explored the rejection mechanism of multivalent ions in positively and negatively charged nanofiltration membranes. Positively charged NF membranes performed high and stable lanthanides rejections (99.3%) and low Na⁺ rejection (0%) operating at low permeate fluxes. For negatively charged NF membrane, they found the high rejections of lanthanides cations were caused by charge repulsion, which resulted from the strong adsorption affinity of La³⁺ on the membrane surface. Because the membrane charge greatly varied with pH, Murthy et al. [26] explored the effects of pH on REE rejection, they found the rejection rate of Nd(III) could be up to 90% in alkaline solution. But rare earth solutions tend to be acidic and contain high concentrations of transition elements (Fe, Al, Cu and Zn), López et al. [27] used NF membranes (NF270, Desal DL and HydraCoRe 70pHT) to recover sulfuric acid in the permeate and, at the same time, to enrich the metal ions in the retentate. The results showed that NF270 had higher metal (Zn, Cu and REE) rejections (greater than98%) and greater acid recovery (80%). In order to improve the separation performance in fly ash leachate stream, Mutlu et al. [28] combined microfiltration (MF) pre-treatment and NF process to concentrate REEs and separate major elements (Al, Si, Fe), the results showed that the process can remove approximately 98% Fe, 41% Si and 50% Al and reach the maximum recovery of REEs at 12 bar and pH 3.5. According to the above analysis, the current interests in NF membrane technique for REEs separation are mainly in the separation of monovalent metal ions and divalent metal ions, or the enrichment of rare earth ions, but the separation between high-valence metal ions ( ≥ +2) are relatively scarce.

In this study, NF membranes were used for the separation of metal ions with different valences from rare earth ore leaching solutions, including Na⁺, Ca²⁺, RE³⁺ and RE⁴⁺. The properties of NF membranes were assessed by measurements of zeta potential and contact angle. The separation of metal ions in unitary salt solution was studied by changing the pH of the solution. Then, the influences of operational factors such as solution pH, operating pressure, salt concentration ratio and cross-flow velocity on the separation behavior were investigated using binary salt solution. Finally, the separation of metal ions in quaternary salt solution was also evaluated.