RSC Advances PAPER View Article Online View Journal | View Issue Waste to resource: preparation of an efficient adsorbent and its sustainable utilization in flame Cite this: RSC Adv.,2021,11, 9942 retardant polyurethane composites Bei Tu, Keqing Zhou, Qianqian Zhou, Kaili Gong and Dongtao Hu* In order to realize the comprehensive utilization of industrial solid waste and the treatment of water eutrophication, the flower-like magnesium hydroxide (MH) was synthesized from phosphorus tailings by sulfuric acid hydrolysis and a hydrothermal method and then was modified with a metal organic framework (MOF) to remove the phosphates enriched in water through adsorption. Both MH and MOF- modified MH (MH@MOF) presented good removal performance of phosphates. The phosphate-adsorbed composites (MH–P and MH@MOF–P) were sustainably used as effective flame retardants for thermoplastic polyurethane (TPU) at low loadings by a solution blending method. The cone calorimetry test results showed that MH@MOF–P can significantly reduce the heat release rate (HRR), smoke Received 27th December 2020 Creative Commons Attribution-NonCommercial 3.0 Unported Licence. production rate (SPR), total smoke release (TSR), CO release rate and CO release rate of TPU Accepted 3rd March 2021 2 composites, compared with those of neat TPU. The novel strategy proposed in this work is of great DOI: 10.1039/d0ra10873a significance for resource recycling, environmental governance and improving fire safety of polymer rsc.li/rsc-advances materials. 1. Introduction fact that magnesium hydroxide, as an adsorbent, can effectively remove acids and direct dyes from water. Jiang et al.9 synthe- Phosphorus tailings, as a remaining solid waste of phosphorus sized ower globular magnesium hydroxide and then modied 1 This article is licensed under a ore aer otation, is usually deposited in the environment it with trisodium citrate to remove Ni(II) ions in water with an À forming tailing ponds in the past, which have caused a serious adsorption capacity up to 287.11 mg g 1. Through a series of 13 waste of resources and environmental pollution problems.2 adsorption experiments, Guo et al. had proved that magne- Therefore, recycling phosphorus tailings for secondary utiliza- sium hydroxide powders could effectively remove Co(II) from Open Access Article. Published on 09 March 2021. Downloaded 9/28/2021 8:20:58 AM. tion gradually attracted the attention of a wide range of water and had a higher adsorption capacity than other adsor- 14 researchers. Up to now, the utilization of phosphorus tailings bents. Vafaeifard et al. had prepared a novel nanostructured 3 mainly includes lling material, producing agricultural three-dimensional ower-like Mg(OH)2-incorporated granular 4,5 1 chemical fertilizer, construction materials and chemical polyurethane which showed a high removal capacity of Cu(II) À products.6 The main components of phosphorus tailings are (184 mg g 1). According to different preparation methods, dolomite, and contain a large amount of calcium and magne- magnesium hydroxide has different morphologies, such as 15 16 sium.6,7 Recycling calcium and magnesium from phosphorus needle-, lamellar-, rod and ower-like, etc. Compared with tailings to prepare chemical products with high added value has other forms of magnesium hydroxide, ower-like magnesium great application prospects. hydroxide is considered to have more excellent adsorption Among the massive amount of chemical products made performance due to its advantages such as large specic surface 9 from phosphorous tailings, magnesium hydroxide is a kind of area and large adsorption capacity. In addition, as a kind of 17 inorganic material which has attracted intensive attention in clean and efficient inorganic ame retardant, magnesium recent years. Its application generally focuses on adsorption hydroxide is widely applied to prepare a variety of ame retar- 18–20 and ame retardant.8 In terms of adsorption, magnesium dant polymer composites, which can decompose and absorb hydroxide is considered to be highly reactive to the separation heat during combustion, generate MgO and release water vapor 21 and removal of traditional pollutants in wastewater treatment9 and non-ammable gas, with good ame retardant effect. and mainly focuses on the removal of reactive dyes10 and heavy In recent years, with the use of various agricultural fertil- metal ions.11,12 Soldatkina et al.10 calculated various thermody- izers, eutrophication of water due to excessive phosphates in namic parameters in the adsorption process, supporting the water has gradually become one of the main problems of water pollution.22 It can lead to the growth of algae and other aquatic Faculty of Engineering, China University of Geosciences (Wuhan), Wuhan, Hubei organisms and the collapse of aquatic ecosystems. Up to now, 430074, China. E-mail: [email protected] eutrophication of water has caused great damage to water 9942 | RSC Adv.,2021,11, 9942–9954 © 2021 The Author(s). Published by the Royal Society of Chemistry View Article Online Paper RSC Advances supply, shery, transportation and even tourism, causing huge obtain ame retardant TPU composites with high added value economic losses.23 At present, the common treatment methods simultaneously. of water eutrophication mainly include adsorption, chemical In this paper, MH was rstly prepared by hydrothermal precipitation, biological decomposition, membrane separa- method and sulfuric acid hydrolysis of phosphorus tailings and tion.24–28 Among them, adsorption is considered to be an then was coated with MOF. Next, the adsorption effect of MH economical, clean and efficient method of phosphates and MOF@MH on phosphates in water was investigated, and removal.29 Inspired by this, it is of great signicance to produce the adsorbed products MH–P and MOF@MH–P were collected. ower-like magnesium hydroxide from phosphorus tailings and The morphology and structure of MH, MOF@MH, MH–P and then use it as adsorbents for the removal of phosphates in MOF@MH–P were characterized by X-ray power diffraction water. At the same time, phosphorus-based ame retardants (XRD), Fourier infrared spectrum (FTIR), scanning electron have also been shown to endow excellent ame retardancy to microscope (SEM), and transmission electron microscopy polymer materials.30,31 It can promote the formation of a dense (TEM). The recovered products were added into TPU matrix to carbon layer during the combustion of polymers,32 and form fabricate composites, and their dispersion in the matrix was glass like molten substances covering the polymer surface.33 observed by SEM. Meanwhile, the combustion properties of Due to their different main ame retarding mechanisms, TPU composites were studied by cone calorimeter test (CCT), magnesium hydroxide and phosphorus may have synergistic and the char residues were analyzed. effect on boosting the ame retardancy of polymer materials,34 which provides a new strategy for the sustainable utilization of magnesium hydroxide aer phosphates adsorption. 2. Experimental Considering the surface polarity of magnesium hydroxide, it 2.1 Materials is necessary to modify magnesium hydroxide to improve its 8 35 $ $ compatibility with polymer. Noh et al. selected hexyl phos- H2SO4,H2O2,NH3 H2O, ethanol, methanol, Zn(NO3)2 6H2O, 2- methylimidazole, NaH2PO4$2H2O, NaOH and N,N-dime- Creative Commons Attribution-NonCommercial 3.0 Unported Licence. phoric acid as modi er to prepare surface hydrophobic modi- ed magnesium hydroxide and successfully prepared ame thylformamide (DMF) were provided by Sinopharm Chemical retardant polyethylene composites at low lling amount. Zhu Reagent Co., Ltd. (Shanghai, China). Thermoplastic poly- et al.36 modied magnesium hydroxide whisker with vinyl- urethane (TPU) pellets (1180A) were purchased from BASF triethoxysilane, the surface of whisker changed from hydro- Company (Germany). The phosphate tailings were obtained philic to hydrophobic, and the dispersion and compatibility of from Yichang Tiaoshuihe Phosphate Mine. XRF was carried out the modied whisker in the organic phase were signicantly to analyze its chemical components. Table 1 shows that phos- improved. Among a large number of modifying agent, metal phorus tailings are mainly composed of CaO (62.05%) and MgO organic framework materials (MOF), composed of ligands and (22.92%). The high content of magnesium oxide indicates that This article is licensed under a metal ions, is considered to be a good modier37 and ame the phosphorus tailings are suitable for preparation of retardancy material.38,39 First of all, MOF has the characteristics magnesium hydroxide as magnesium sources. of good crystallinity and large specic surface area, and there is Open Access Article. Published on 09 March 2021. Downloaded 9/28/2021 8:20:58 AM. a strong interaction between its organic skeleton and polymer 2.2 Preparation of ower-like MH molecular chain, so it has good dispersion in the polymer matrix.40 Secondly, in the process of combustion, MOF can not 50.0 g phosphorus tailings were added into a three-necked ask only absorb smoke, but also produce metal oxides to catalyze with 250 mL distilled water under condition of stirring for 41–47 the carbonization of polymers. Therefore, it is a promising 5 min. Diluted solutions of H2SO4 were slowly dripped into the strategy to modify magnesium hydroxide with MOF. above suspension and stirring for 3 h at 50 C, then the ob- Based on the abovementioned analysis, we designed tained products were centrifuged and the supernatant was a simple and effective method to synthesize ower-like MH and transferred to a three-mouth ask. 5 mL of H2O2 were dripped $ MOF modied MH (MH@MOF). The obtained MH and into the supernatant and then NH3 H2O were introduced into MH@MOF were used as adsorbents to remove phosphates from the above suspension to adjusted the pH value for 6–7. The water to deal with the eutrophication of water. Finally, the mixture was stirred continuously for 1 h and then the super- sustainable application of the phosphates adsorbed MH natant was separated by centrifugation.