Phosphate removal by a La(OH)3 loaded magnetic MAPTAC-based cationic hydrogel: Enhanced surface charge density and Donnan membrane effect
Graphical Abstract
Introduction
Phosphorus (P) is an essential macronutrient for organisms and a key ingredient in crop fertilizers and industrial products (Afridi et al., 2020; Cordell et al., 2011). As the main resource of phosphorus, phosphate rock is non-renewable and is likely to run out within a century or even in the next decades (Wilfert et al., 2018; Zhang et al., 2016a). On average, 35.2 kg phosphate rock is consumed to support one person per year, which becomes increasingly precious and scarce with the rapidly world's population growth (Macintosh et al., 2018; Zhang et al., 2021b). However, a large amount of phosphorus is continuously discharged into surface water by domestic, mining, industrial, and agricultural activities (Hu et al., 2020; Ulrich et al., 2016; Zhao et al., 2020). This not only wastes phosphorus resources but also accelerates eutrophication of water body once the concentration exceeds 0.02 mg P/L, leading to algae bloom, water quality deterioration and ecological balance destruction as well as the crisis of human and animal health (Lin and Chen, 2021). Therefore, the recovery and reuse of phosphorus from wastewater is an effective method to treat phosphate pollution as well as an attractive option for alleviating the shortage of phosphorus resources in the world (Zhao et al., 2020).
Adsorption is considered as a promising technology for its high removal effectivity and the potential for recovery (Lin and Chen, 2021). A variety of functionalized adsorbents have been utilized for the elimination of phosphate nowadays including layered double hydroxides (Yu et al., 2019), metal organic frameworks (MOF) (Wang et al., 2021), porous biochar (Zhang et al., 2021c), polymer membrane (Xia et al., 2021) and hydrogel (Dong et al., 2017). Transition metal (hydr)oxides are regarded as effective adsorbents by forming strong inner-sphere complexes with phosphate (Qiu et al., 2017), including alumina (Sun et al., 2020), hydrated ferric oxides (HFOs) (Pan et al., 2009), hydrous zirconium oxide (Pan et al., 2013), lanthanum (hydr)oxides (Dong et al., 2017) and so on. Among these transitional metals, lanthanum (La) is an environmentally friendly rare earth element with high abundance in the Earth's crust, which is more inexpensive in comparison with other metals (Blaney et al., 2007; Liu et al., 2018; Wu et al., 2020). In particular, lanthanum (hydr)oxide has a strong affinity and provides a great number of coordination sites towards phosphate even at low concentration. Nevertheless, lanthanum (hydr)oxide nanoparticles tend to aggregate in water due to their high surface energy and the existence of van der Waals force, which is not conducive to their separation and recovery after adsorption (Razanajatovo et al., 2021). In order to overcome this issue, a growing attention has been paid to La-based composites which combined the specific affinity of lanthanum (hydr)oxide for phosphate and the excellent mechanical properties of host materials (Qiu et al., 2017).
Actually, loading La on the porous and cross-linked host materials can improve the accessibility of La to phosphate as well as increase the phosphate adsorption capacity (Zhi et al., 2020). However, most host materials only play a supporting role which don't provide adsorption sites for phosphate (Zhang et al., 2016b). Hydrogel is a kind of hydrophilic polymer with three-dimensional network structure. In particular, hydrogels directly polymerized by monomers with quaternary ammonium group (R4N+) are favorable for phosphate adsorption through electrostatic attraction, therefore, they are considered as ideal host materials. The swollen polymeric network of hydrogels in water and the abundant functional groups attached to the polymer chains result in their high adsorption capacity and fast adsorption kinetics (Zheng and Wang, 2015). Dong et al. (2017) prepared a type of magnetic cationic hydrogel (MCH) with (3-acrylamidopropyl) trimethylammonium chloride (APTMACl) as monomer, and its saturated phosphate capacity was 26.1 mg/g. According to the study of Aydinoglu. (2021), a type of p(MAPTAC) hydrogel with 3-(methacryloylamino) propyl-trimethylammonium chloride (MAPTAC) as monomer could adsorb 41.5 mg/g phosphate from 50 ppm raw solution. Among these two types of hydrogel, their monomers of APTMACl and MAPTAC are highly hydrophilic cationic organics, while the latter monomer has one more methyl group (-CH3) on the vinyl substituent with the increased steric effect and electronic effect, which is beneficial to phosphate adsorption. However, reports on phosphate adsorption by MAPTAC-based hydrogel were still limited. Only the study on the hydrogel polymerized by MAPTAC monomer treating anionic dyes indicated that p(MAPTAC-co-VI)-q hydrogel could absorb 1818.2 mg/g eriochrome black T (EBT) and 1449.3 mg/g methyl orange (MO), respectively (Kivanc et al., 2020). These results implied that MAPTAC-based hydrogels had promising potential for phosphate adsorption.
Furthermore, dispersing metal (hydr)oxide nanoparticles within polymers could greatly increase the adsorption capacity of ligands (Cumbal and SenGupta, 2005). Earlier studies by Lehigh University proposed that the fixed positive charge of R4N+ covalently bonded to the ion exchanger matrix made it function like a virtual semi-permeable membrane, which restricted the movement of cations across the phase boundary to maintain the electroneutrality and led to the development of Donnan potential. Under the influence of Donnan potential, cations were excluded from the exchanger phase, while anions tend to be enriched in the exchanger phase. In other words, the metal (hydr)oxides loaded on the ion exchanger were exposed to higher concentrations of anions, thereby increasing their adsorption capacity (Sarkar et al., 2012; Sarkar et al., 2010). Qiu et al. (2017) prepared a La(OH)3-loaded R4N+ wheat straw (Ws-N-La) for phosphate adsorption, and indicated that the Donnan membrane effect produced by the immobilized R4N+ functional groups on Ws-N-La could enhance the diffusion and enrichment of phosphate. In fact, R4N+-based hydrogels could attract phosphate through electrostatic attraction, and under the potential Donnan membrane effect produced by R4N+, La(OH)3 loaded on magnetic cationic hydrogels showed high usage efficiency and great ligand exchange with phosphate (Dong et al., 2017). However, limited studies have focused on the influence of Donnan membrane effect on phosphate adsorption process by La-based hydrogels.
The objective of this study was to develop an adsorbent named La(OH)3@MMCH by doping lanthanum (hydr)oxides on the surface of a magnetic MAPTAC-based cationic hydrogel for phosphate removal. The phosphate adsorption performance by La(OH)3@MMCH was investigated as a function of initial concentration, reaction time, pH value and coexisting ions. The feasibility and stability of La(OH)3@MMCH in practical application were tested by magnetic separation system. Furthermore, the potential Donnan membrane effect provided by MMCH was intensively discussed by adsorption isotherm, swelling kinetics and surface charge density measurement to illustrate the underlying adsorption process. The adsorption mechanism was further studied by a combination of FTIR, XRD and XPS.
Section snippets
Materials
All chemicals used in the experiment were of analytical grade. The mass specification of 3-(methacryloylamino) propyl-trimethylammonium chloride (MAPTAC) was 50 wt.% in water, (3-acrylamidopropyl) trimethylammonium chloride (APTMACL) was 75 wt.% in water. N, N’-methylenebis (acrylamide) (MBA), N, N, N’, N’-tetramethylethylenediamine (TEMED), potassium persulfate (KPS), iron oxide (II, III) (Fe3O4) (200 nm) and lanthanum (III) chloride trihydrate (99.99% metals basis) were purchased from
Characterization
Fig. 1 showed the surface morphology and crystalline structure of MMCH and La(OH)3@MMCH. Fe3O4 particles formed regularly spherical state (Fig. 1a), and the raw MAPTAC-based cationic hydrogel (MCH) had a three-dimensional (3D) entangled state with many open pore-like structures (Fig. 1b) (Pirgalioglu et al., 2015), which resulted from the crosslinking of MAPTAC and MBA. Meanwhile, the magnetic MAPTAC-based cationic hydrogel (MMCH) (Fig. 1c) exhibited a similar 3D hierarchical structure and
Conclusions
In this study, a new La(OH)3@MMCH composite showed high surface charge density of 34.38–59.38 meq/kg and great swelling ratio of 3014.57%. Its maximum adsorption capacity was 105.72±5.99 mg P/g at pH of 7.0±0.2, which exceeded many reported adsorbents. The adsorption kinetics results indicated that 95.28% of the maximum adsorption capacity could reach within 2 hr. The adsorption performance of La(OH)3@MMCH exhibited high stability over the pH range from 3.0 to 9.0. Coexisting anions didn't show
Acknowledgments
This work was supported by the Beijing Municipal Science and Technology Project (No. Z181100005518007), the National Key Research and Development Program of China (No. 2017YFC0505303) and the National Natural Science Foundation of China (Nos. 51978054 and 51678053).
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