Mg-doped CaCO3 nanoarchitectures assembled by (441¯) high-index facets for efficient trace removal of Pb(II)

Removal of trace heavy metal ions puts high demands on designing adsorbents with favorable surfaces. Crystal-plane engineering can provide controllable adsorption energy between surficial planes and adsorbents. Herein, we have creatively synthesized Mg-doped CaCO 3 nanoarchitectures assembled by layered sheets (Mg-CaCO 3 LSs) with high-index facets of ( 4 41 ¯ ) through a facile wet chemical process. Adsorption tests reveal that the layer-by-layer assembled sample exhibits a maximum Pb(II) adsorption capacity of 1961.9 mg·g −1 , agreeing with the monolayer-adsorption Langmuir model. At an initial Pb(II) ion concentration of 20 mg·L −1 , the adsorption can achieve a high removal rate near 99.0% within 1 min, and the adsorption kinetics follows a chemisorption pseudo-second-order model. Interestingly, the Mg-CaCO 3 LSs show much-improved adsorption properties towards low-concentration Pb(II) ions, which could reduce the concentration from 1 mg·L −1 to ~ 2.9 μg·L −1 in 3 h (within 30 min decrease to less than 10 μg·L −1 , meeting drinking water standard from WHO). For comparison, the commercial CaCO 3 and collected CaCO 3 scale show much lower adsorption values with Pb(II) ion residual concentration of ~ 935.0 and ~ 944.9 μg·L −1 in 3 h, respectively. X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and inductively coupled plasma (ICP) characterizations on the Mg-CaCO 3 LSs before and after adsorbing Pb(II) confirm that the high removal performance could be ascribed to fast metal ion exchange and excellent physical adsorption contributed by high-index planes. The density functional theory (DFT) calculations also confirm that the much-enhanced adsorption kinetics benefits from the optimal adsorption of the ( 4 41 ¯ ) planes. This work will provide a feasible route to design high-efficient low-cost adsorbents through crystal-plane engineering. Graphical abstract 摘要 有效去除水中痕量重金属离子对吸附剂的结构设计提出了很高的要求。利用晶面构建工程对催 化剂结构进行设计是一个有效地解决方法。在本工作中，我们通过湿化学合成法，制备了由( 4 41 ¯ ) 高指数面组装的 Mg 掺杂 CaCO 3 纳米结构(Mg-CaCO 3 LSs)。实验结果证明，层状组装结构 的Mg-CaCO 3 LSs对Pb(II)的最大吸附容量为1961.9 mg·g -1 , 并符合单层Langmuir 吸附模型。当 Pb(II)的初始浓度为20 mg·L -1 时, Mg-CaCO 3 LSs表现出接近于99%的去除效率(吸附时间为1 min)。该吸附动力学过程遵循化学吸附准二级动力学模型方程。同时，在极低Pb(II)浓度下, Mg-CaCO 3 LSs还表现出优异的吸附去除性能。当Pb(II) 的浓度为1 mg·L -1 时, Mg-CaCO 3 LSs能够在3 小时内将浓度降至~2.9 μg·L -1 。其中，在30 分钟内就能将Pb(II)浓度降至10 μg·L -1 以下, 这是符合WHO 所颁布的饮用水标准的。作为对比，商业CaCO 3 和收集的CaCO3 水垢在相 同的测试条件下却分别只能将浓度降低至~935 μg·L -1 和~944.9 μg·L -1 。通过对吸附前后Mg-CaCO 3 LSs样品进行XRD, EDS 和ICP 表征，我们发