Extraction behavior of Cr(VI) and coexistence with interfering ions by trialkylamine

Cr(VI) is toxic to organisms and difficult to biodegrade. Besides, it can accumulate inside the bodies of humans, animals and plants and spread through the food chain, existing as a threat to ecosystems (Garg et al. 2012). Heavy metal toxicity can be described as ‘murder by slow knife’ and ‘biological time bomb’. As a typical heavy metal element, Cr(VI) is widely used in the fields of leather, electroplating, metallurgy, printing, and the coating industry, and a large amount of wastewater is mostly discharged directly without appropriate recovery and treatment, which can bring about serious pollution to the environment. Thus, it is urgent to eliminate the hazards and realize the reuse of metal ions (Zhitkovich 2011).

Generally, effluent containing Cr(VI) has such characteristics as complicated element, intricate species of metal ions, organics and inorganics, where inorganic anions such as and metal ions are common species. Sulfate ions are common anions in the inorganic chemical industry, metallurgy, and acid mine drainage. The main sources of sulfate ions are chemical weathering of sulfur-bearing ore and the oxidation of sulfides and sulfur (Silva et al. 2010). Besides, tailings from coal and some metal-bearing ores are readily oxidized, resulting in acid drainage. Such acid mine drainages contain heavy metal ions, sulfate ions and acidity, and constitute one of the main challenges in the mining industry (Dong et al. 2011). Nitrogen and phosphate ions discharged into the aquatic ecosystem come from sources including agricultural fertilizers, septic tank systems, animal waste disposal, domestic sewages and industrial effluents. The inorganic anions exist as a main threat to aquifers of agricultural and urban areas (Pintar et al. 2001; Huang et al. 2008). In addition, industries of dyeing, electroplating, leather, metallurgy and mining will result in the loss of metal ions into the aquatic environment. Thus, in the fields of ion separation and recycling, it is imperative to investigate the influence of coexisting inorganic anions and metal ions.

Solvent extraction is a promising technology for the separation of metal ions (Wang et al. 2012; Elizalde et al. 2013; Hu et al. 2013; Nikam et al. 2013; Okewole et al. 2013; Senol 2013). In the field of separation of heavy metal ions, long-chain amine extractants such as alkaline extractant-tertiary amine (Alamine 336, TOA) (Sadoun & Hassaine-Sadi 2004; El-Hussaini et al. 2012) and quaternary ammonium salt (Aliquat 336) (Galan et al. 2006; El-Hefny 2009) are considered as universal extractants in Cr(VI) ion separation. Furthermore, the organic phosphorus extractants (tributyl phosphate (TBP), Cyanex 921, Cyanex 923) (Alguacil et al. 2004; Zhang et al. 2007) and ionic liquid CYPHOS IL101 are also used to separate Cr(VI) ions (Alguacil et al. 2010b). However, currently, though a variety of studies has mainly focused on the transport property of metal ions with different extractants, the influence of coexisting contaminants on extraction also cannot be neglected. In addition, membrane-extraction processes derived from solvent extraction have attracted wide attention because of their versatility and the characteristics that they overcome problems encountered in other established separation technologies, i.e. liquid–liquid extraction. The great potential for energy conservation, low capital and operating cost, and easy scale-up makes supported liquid membranes an area deserving attention. In addition, theories of membrane materials and manufacturing methods have advanced dramatically. Thereinto, hollow fiber has been proven to be economical to construct. It has characteristics such as unique separation capabilities and ability to adapt in diversified conditions. Systems based on hollow fiber membrane contactors can make chemical plants more compact, more energy saving, cleaner, and safer by providing a lower equipment size-to-production capacity ratio and decreasing waste generation, with the correct choice of membrane material (Sirkar 1997; Kocherginsky et al. 2007; Parhi 2013). Owing to the large surface area per volume, which is typically 100 times bigger than in conventional equipment, the mass transfer rate of hollow fibers is quick and the liquid extraction in fibers is 600 times faster than in mixer settlers. Extensive studies on hollow fiber membrane based separation technology have been carried out for removal of toxic heavy metals because the rapid mass transfer rate, enhances ease of operation and scaling up the devices (Aguilar & Cortina 2007; Kislik 2010). However, the obstacle of the technology application is the stability. Recently, a new liquid membrane technique, named pseudo-emulsion based hollow fiber strip dispersion (PEHFSD) was proposed for the treatment of liquid effluent due to its stability (Alguacil et al. 2010a, b). It combines the advantages of emulsion liquid membrane and non-dispersion solvent extraction. It is noteworthy that this technique can not only separate and recycle the metal ions, but can also realize the reuse of resources (Aguilar & Cortina 2007; Kislik 2010).

Within the idea of green chemistry, the expensive and toxic extraction reagent becomes a bottleneck to utilization in the fields of separation. Therefore, the design of an efficient and economical extraction system is imperative. In addition, whether the coexisting contaminants could interfere with the extraction reaction and the action principles is the cornerstone in the exploration of technology applications. Furthermore, it is also imperative to explore whether the contaminants could react with extractants and thus disturb the transport of Cr(VI). The objective of this work is to investigate the characteristic of tertiary amine N235 in kerosene/n-heptane, and TBP as modifier on the extraction of Cr(VI). In addition, reaction mechanisms of organophosphorus TBP and inorganic phosphorus on extraction have been discussed. Besides, the influence of inorganic anions and metal ions on extraction and reaction mechanisms has been evaluated. Based on the analysis of the basic condition in the solvent extraction reaction, the derived liquid membrane technology used for separation and recovery of Cr(VI) ions has been verified, and the extraction behaviors of Cr(VI) with coexisting metal ions by the PEHFSD technique have been discussed.

Trialkylamine (N235), Shaoyang Institute of Chemical Industry (Hunan, China); TBP, Tianjin Hengxing Chemical Preparation Co., Ltd; Kerosene, n-heptane, analytical reagent, Tianjin Hengxing Chemical Preparation Co., Ltd. All other chemicals used were of analytical reagent grade.

H2S-H Water Bath Oscillator (Harbin Dong Lian Electronic Technology Development Co., Ltd); PHS-3C PH Meter (Shanghai INESA Corporation); 722 UV-vis Spectrophotometer (Shanghai Spectrum Instruments Co., Ltd); Optima 5300DV Plasma Atomic Emission Spectrometry (Perkin-Elmer, USA); ADVANCE III 400 MHz Nuclear Magnetic Resonance Spectrometer (Bruker Corporation, Germany).

The hollow fiber device designed with polyvinylidenefluoride material was obtained from Tianjin Polytechnic University. The module inner and outer fiber diameters are 0.80 mm and 0.15 mm, respectively. The bore diameter of the hollow fiber is 0.16 μm and the porosity is 82–85%. The fiber length is 22 cm and the number of fibers in the module is 100. The thickness and active interfacial area of the membrane are 0.18 mm and 0.05 m², respectively.

The simulated wastewater is circulated in the tube side with the peristaltic pump and the pseudo-emulsion solution is circulated in the shell side with the gear pump in counter current mode to provide organic solution to the membrane pore, and the organic solution in pseudo-emulsion was loaded on the pore of hollow fiber to form a stable membrane. The metal ions selectively transport into the liquid membrane and move into sodium hydroxide solution in pseudo-emulsion to realize ions separation. The differential pressure was always kept below the breakthrough pressure and transmembrane pressure was maintained at 0.2 × 10⁵ Pa (0.2 bar) between aqueous solution and pseudo-emulsion solution. The stirring rate of the pseudo-emulsion solution using the mechanical stirrer was kept at 120 rpm.

Optima·5300DV Plasma Atomic Emission Spectrometry was used to analyze the metal ions concentration; Diphenylcarbazide hydrazine method was used to analyze the Cr(VI) ions concentration by 722 UV-vis Spectrophotometer; ADVANCE III 400 MHz NMR Nuclear Magnetic Resonance Spectrometer was used to characterize the structure of trialkylamine.

Extraction behavior of Cr(VI) and coexistence with contaminants by trialkylamine using kerosene/n-heptane as the diluent and TBP as the modifier was investigated. According to the results, 0.145 mol L⁻¹ TBP can not only improve extraction rate, but can also not disturb the homogenization rate of the organic solution. The competition coordination of inorganic anions such as ⁠, and with Cr(VI) ions resulted in the decrease of the extraction rate. Besides, N235 has specificity for Cr(VI) extraction in acid solution, and is able to achieve Cr(VI) separation with its coexisting metal cations (Cd²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Fe³⁺, Mn²⁺, Pb²⁺) except for Fe²⁺ ions. In addition, it is proved that the PEHFSD can realize the separation of Cr(VI) by trialkylamine using n-heptane as the diluent. Extraction behavior of Cr(VI) and coexistence with interfering ions was basically identical to the data obtained from solvent extraction. This technology is a promising alternative to conventional methods.

This study was funded by the Commonweal Project of the Environmental Protection Agency through grant number 201209048.