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氟是自然界中广泛存在的元素之一,也是人体生长必须的微量元素[1]。我国在《生活饮用水卫生标准》(GB 5749-2022)中规定了氟离子的最大允许浓度为1.0 mg·L−1,超过这一范围可能会导致骨骼变形,甚至引起机体慢性中毒[2-3]。由于饮用高氟地下水而导致的氟中毒在我国十分普遍[4]。我国的高氟地下水主要分布于北方的干旱、半干旱地区[5]。其中,关中盆地地热水中氟含量可达1.2~6.9 mg·L−1[6]。因此,开发高效的除氟方法具有重要意义。
氟化物的去除方法主要有沉淀法、离子交换法、膜分离法和吸附法等[7-8]。吸附法因具有成本效益高、操作简单、去除率高、吸附剂可重复使用等优点,在除氟方面具有很大潜力[9-10]。目前,常用的吸附剂主要有活性氧化铝、骨炭、羟基磷灰石等[11],但常规吸附剂普遍存在吸附容量低的缺点[12]。
粉煤灰是燃煤电厂排放的最主要固体废弃物,也是当前我国排放量较大的工业固体废渣之一。粉煤灰与沸石分子筛(zeolite molecular sieve, ZMS)组成相似[13],可用于合成沸石分子筛,在某些情况下可以降低40%左右的成本,是一种有前途的固体废物再利用技术[14]。废弃贝壳是贝类养殖的副产物,由于难自然分解,已成为世界性的环境问题。而贝壳中碳酸钙含量高达95%,因此,可作为很好的绿色钙源用于制备羟基磷灰石[15]。羟基磷灰石(hydroxyapatite, HAP)是钙磷灰石(Ca10(PO4)6(OH)2)的自然矿化物,由于有较强的离子交换和生物相容性,被认为是一种潜在的吸附剂[16]。但常规HAP比表面积和活性位点少,导致其除氟容量较低[17]。表面改性技术已被证明可以有效提高HAP的吸附能力[18],而表面活性剂因具有可以降低溶液表面张力的特性,使粒子更加具有亲水性,成为羟基磷灰石表面改性的绝佳选择[19]。PRABHU等[20]采用十六烷基三甲基溴化铵(cetyltrimethylammonium bromide, CTAB)等阳离子表面活性剂改性HAP,使改性后HAP的除氟容量由2.6 mg·g−1提高到9.4 mg·g−1。但HAP存在着合成成本高、自身力学性能较差、抗剪切、弯曲强度低等问题,极大地限制了其实际应用潜力,而合成HAP复合材料可以改善力学性能上的缺陷,提高其性能和实用性[21]。沸石由于其独特的特点,能够和HAP合成性能更好的吸附材料。目前国内外将复合材料应用于含F−地下水的处理已成研究热点,但依然不完全具备经济、环保又适合大规模推广应用的条件[22]。所以寻求低成本的除氟吸附剂显得尤为重要。
综上所述,本研究利用废弃贝壳和粉煤灰为原料合成CTAB-HAP@ZMS复合吸附剂,探究了其对水溶液中氟的去除行为,并用于解决关中盆地地热水氟污染的问题。该研究不仅可以解决废弃物资源浪费的难题,还能降低复合材料的制备成本,可为低成本的复合吸附剂除氟提供一种新的思路。
CTAB-HAP@ZMS复合材料的制备及其对氟离子的吸附性能
Preparation of CTAB-HAP@ZMS composites and their adsorption properties towards fluoride ions
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摘要: 使用贝壳和粉煤灰等固体废弃物为原材料,采用水热合成法将十六烷基三甲基溴化铵(CTAB)改性后的羟基磷灰石(HAP)负载到粉煤灰基沸石分子筛(ZMS)上,得到一种高效的吸附剂(CTAB-HAP@ZMS)用于去除氟离子。当投加量为8.0 g·L−1、pH为3、温度为55 ℃时,CTAB-HAP@ZMS复合材料的最高吸附容量达到了10.4 mg·g−1,对10.0 mg·L−1氟化钠溶液中F−的去除率可以达到95%。动力学和热力学拟合参数表明吸附过程主要为多分子层式、自发、吸热的化学吸附。将此吸附剂用于处理模拟地热水(F−初始浓度为8.0 mg·L−1),去除率达到89%,残留浓度低于我国饮用水质量标准中F−的浓度限值(1.0 mg·L−1)。此外,经4次循环再生吸附后,CTAB-HAP@ZMS复合材料仍然表现出较高的氟离子去除效率。该复合材料的制备不仅能使固体废物资源化,还在氟离子去除方面有广阔应用前景。Abstract: In this study, solid wastes such as shell and fly ash were used as raw materials. Hydroxyapatite (HAP) modified by cetyltrimethyl ammonium bromide (CTAB) was loaded onto flyash-based zeolite molecular sieve (ZMS) by hydrothermal synthesis method. A highly efficient adsorbent (CTAB-HAP@ZMS) was obtained for fluoride removal. At the dosage of 8.0 g·L−1, pH 3, and 55 ℃, the highest adsorption capacity of CTAB-HAP@ZMS composite could reach 10.4 mg·g−1, and the removal rate of F− in 10.0 mg·L−1 sodium fluoride solution could reach 95%. The kinetic and thermodynamic fitting parameters showed that the adsorption process was mainly a type of multi-molecular layer, spontaneous and endothermic chemisorption. When the adsorbent was used to treat the simulated geothermal water with F− initial concentration of 8.0 mg·L−1, the removal rate reached 89%, and the F−residual content was lower than the concentration limit of F− in China's drinking water quality standard (1.0 mg·L−1). In addition, CTAB-HAP@ZMS composite still showed a high fluoride removal efficiency after 4 cycles of regeneration and adsorption. The preparation of the composite material can not only make solid waste resources, but also have broad application prospects in fluoride ion removal.
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Key words:
- modified hydroxyapatite /
- zeolite molecular sieve /
- fluoride removal /
- adsorption
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表 1 CTAB-HAP@ZMS 复合材料的准一级动力学和准二级动力学参数
Table 1. Quasi-first-order and quasi-second-order dynamics parameters of CTAB-HAP@ZMS composites
复合材料 准一级动力学 准二级动力学 k1/min−1 Q1/(mg·g−1) R2 k2/(g·(mg·min)−1) Q1/(mg·g−1) R2 CTAB-HAP@ZMS 0.676 2 1.059 7 0.833 0.612 2 1.288 0.941 2 表 2 CTAB-HAP@ZMS复合材料吸附等温模型参数
Table 2. Parameters of adsorption isothermal model for CTAB-HAP@ZMS composite materials
复合材料 T/K Langmuir Freundlich Qm/(mg·g−1) KL/(L·mg−1) R2 1/n KF/[mg·g−1(L·mg−1)1/n ] R2 CTAB-HAP@ZMS 298 26.881 7 0.003 1 0.690 2 0.858 9 0.143 1 0.996 4 308 34.722 2 0.003 9 0.682 1 0.885 4 0.149 1 0.997 8 318 27.027 0 0.004 3 0.744 2 0.859 0 0.163 8 0.994 9 表 3 热力学参数
Table 3. Thermodynamic parameters
T/K ∆H/(kJ·moL−1) ∆S/(kJ·(moL·K)−1) ∆G/(kJ·moL−1) $ \mathrm{l}\mathrm{n}\dfrac{{q}_{\mathrm{e}}}{{C}_{\mathrm{e}}} $ 298 21.906 0.065 6 −2.246 7 0.353 3 308 21.906 0.065 6 −1.893 9 0.739 6 318 21.906 0.065 6 −0.934 1 0.906 8 表 4 本研究和其他吸附剂的除氟性能和成本的比较
Table 4. Comparison of the fluoride removal capacity and cost of adsorbents in this study and other studies
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