Preparation of functional dendritic composite adsorbents and their adsorption properties for Cr(Ⅵ)

SHI Hang; LI Bing; GUO Jianzhong

doi:10.11833/j.issn.2095-0756.20200119

Volume 39 Issue 2

Mar. 2022

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SHI Hang, LI Bing, GUO Jianzhong. Preparation of functional dendritic composite adsorbents and their adsorption properties for Cr(Ⅵ)[J]. Journal of Zhejiang A&F University, 2022, 39(2): 396-404. doi: 10.11833/j.issn.2095-0756.20200119

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SHI Hang, LI Bing, GUO Jianzhong. Preparation of functional dendritic composite adsorbents and their adsorption properties for Cr(Ⅵ)[J]. Journal of Zhejiang A&F University, 2022, 39(2): 396-404. doi: 10.11833/j.issn.2095-0756.20200119

Preparation of functional dendritic composite adsorbents and their adsorption properties for Cr(Ⅵ)

doi: 10.11833/j.issn.2095-0756.20200119

SHI Hang^{1, 2
,},
LI Bing^{1, 2},
GUO Jianzhong^{1, 2
,}

1.
College of Chemistry and Materials Engineering, Zhejiang A&F University, Hangzhou 311300, Zhejiang, China
2.
Zhejiang Provincial Key Laboratory of Chemical Utilization of Forestry Biomass, Zhejiang A&F University, Hangzhou 311300, Zhejiang, China

Received Date: 2021-01-07
Accepted Date: 2021-12-22
Rev Recd Date: 2021-12-12

Available Online: 2022-03-25

Publish Date: 2022-03-25

Abstract

Objective The objective is to study the adsorption property of dendritic composite adsorbent synthesized by chemical modification of mesoporous molecular sieve for the treatment of heavy metal wastewater. Method The composite adsorbent SBA-15-G₃ of dendritic compounds was synthesized by chemical modification with mesoporous material SBA-15 as silicon source, and the functional adsorbent SBA-15-G₃-SH was successfully synthesized by grafting sulfhydryl onto SBA-15-G₃ with thioglycolic acid as coupling agent. The structure and physicochemical properties of the materials were characterized by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), Zeta potential analysis, and nitrogen adsorption-desorption isotherms (BET). The effects of pH, time, initial Cr(Ⅵ) concentration, and temperature on the adsorption properties were studied through the adsorption of Cr(Ⅵ) in aqueous solution by SBA-15-G₃-SH. The adsorption isotherms, kinetics and thermodynamic characteristics of SBA-15-G₃-SH adsorbent were analyzed. Langmuir and Freundlich adsorption isotherm models were used to analyze the experimental data. Result The Langmuir adsorption isotherm model fitted the adsorption process well, and the adsorption kinetic model accorded with the pseudo second-order kinetic equation. Conclusion The adsorption mainly results in the formation of stable compounds through the surface complexation of —SH group on the surface of SBA-15-G₃-SH and the electrostatic attraction of heavy metal ions. The thermodynamic results show that Cr(Ⅵ) adsorption onto SBA-15-G₃-SH is a spontaneous exothermic process. The high adsorption performance indicates that the SBA-15-G₃-SH is an efficient adsorbent to eliminate Cr(Ⅵ) from wastewater. [Ch, 10 fig. 3 tab. 21 ref. ]
- dendritic adsorbent,
- sulfhydryl,
- chromium ions,
- adsorption,
- heavymetal

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沈阳化工大学材料科学与工程学院沈阳 110142

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随着电镀、制革、冶金、染料等工业的不断发展，重金属对水体污染已严重破坏了生态平衡^[1]。中国长江三峡水库区段沉积物中重金属元素含量受上游泥沙及沿江城市和工厂废水、废气和固体废弃物排放影响，已严重超标^[2]。因此，如何将排放废水中重金属含量有效降低至排放标准以下，是当前水处理领域需要迫切解决的问题之一。吸附法作为一种有效去除废水中污染物的方法，因其低成本、实用、高效和环境友好性，得到了广泛应用。介孔二氧化硅(如：MCM-41、SBA-15和MCM-48)等材料因其具有均匀的孔结构、孔容、热稳定性等优良特性，常被选作吸附材料而被广泛应用^[3-4]，但因其表面吸附官能团的数量及类型单一，限制了其在低浓度污水处理中的应用。树枝状大分子是一类具有高度支化结构的分子，具有大量的末端基团，且可形成笼状结构，可与重金属离子配位形成稳定的螯合物，有利于重金属的吸附，且毒性低，安全可靠^[5]，所以，在介孔二氧化硅上合成树枝状化合物成为一种有效的研究方向。DIALLO等^[6]研究了在有序介孔分子筛(SBA)上合成PAMAM树枝状化合物，并应用于重金属离子处理，取得较好吸附效果，因其特殊结构被广泛应用于生物医药、电子传感器、废水处理等领域^[7-9]。SUN等^[10]报道了在硅胶表面合成低代的枝状化合物(G₁)，并研究了其对二价汞离子[Hg(Ⅱ)]的吸附性能，吸附容量达260 mg·g⁻¹，表现出良好的吸附性能。BARAKAT等^[11]研究了在二氧化钛表面合成枝状化合物并应用于水溶液中Pb²⁺的吸附及动力学，认为PAMAM枝状化合物是一种非常有前景的吸附剂。本研究将SBA-15接枝改性合成树枝状大分子，引入氨基，并用巯基乙酸改性嫁接巯基，成功合成了功能化枝状复合吸附材料(SBA-15-G₃-SH)，并通过傅里叶变换红外光谱(FTIR)、X射线衍射(XRD)、Zeta电位分析和氮气吸附-脱附等温线(BET)等方法对合成材料的结构和物理化学性质进行了表征；研究了SBA-15-G₃-SH吸附剂对铬离子[Cr(Ⅵ)]的吸附及环境因素(如pH、吸附时间、浓度等)对吸附性能的影响，对吸附等温线、动力学、热力学等进行了研究，以期为有效去除水中重金属污染物提供理论依据。

1. 材料与方法

1.1. 实验材料和仪器

SBA-15介孔分子筛(购于上海卓悦化工科技有限公司，孔容1.178 cm³·g⁻¹，BET比表面积≥600 m²·g⁻¹，孔径7~9 nm)；甲苯、氨丙基三乙氧基硅烷、甲醇、丙烯酸甲酯、巯基乙酸、重铬酸钾、乙二胺四乙酸二钠盐等均为分析纯。722型紫外可见分光光度计(上海精密科学仪器有限公司)，ZJSHA-BA水浴恒温振荡器(常州中捷试验仪器制造有限公司)，PHS-2F型酸度计(上海仪电科学仪器股份有限公司)。

1.2. 枝状复合功能吸附剂(SBA-15-G₃-SH)的制备

将10 g SBA-15介孔分子筛和300 mL甲苯溶剂加入烧杯，水浴搅拌30 min后放入500 mL圆底烧瓶中；将溶有3 g氨丙基三乙氧基硅烷的甲醇溶液20 mL缓慢滴入上述溶液中，80 ℃反应；6 h后，将经过改性后的SBA-15-G₀加入到60 mL丙烯酸甲酯和300 mL甲醇溶剂中，于25 ℃反应24 h得到产物SBA-15-G_0.5，经抽滤，用甲醇溶液洗涤，干燥过夜。再将SBA-15-G_0.5与150 mL乙二胺，25 ℃在200 mL甲醇溶剂中反应24 h，抽滤，用甲醇溶液洗涤，干燥过夜，产物记为SBA-15-G₁(1代)。重复进行上述步骤可得SBA-15-G₂(2代)、SBA-15-G₃(3代)。将50 mL巯基乙酸改性剂与5 g的SBA-15-G₃放入300 mL乙醇水溶液(体积比为1∶1)中，室温磁力搅拌48 h。反应后的混合物经过滤后，分别用乙醇和去离子水洗涤3次，在室温下自然干燥后备用，产品记为SBA-15-G₃-SH。

1.3. 吸附实验

用去离子水配置质量浓度为1 000 mg·L⁻¹的Cr(Ⅵ)溶液，实验时再稀释至所需质量浓度。典型的SBA-15-G₃-SH对Cr(Ⅵ)溶液吸附实验：取50 mL质量浓度为200 mg·L⁻¹的Cr(Ⅵ)溶液于250 mL的锥形瓶中，用5 mol·L⁻¹的硫酸调节Cr(Ⅵ)溶液至一定的pH，加入100 mg制备的SBA-15-G₃-SH吸附剂，在水浴振荡器中振荡吸附反应一定时间，然后用0.22 μm的滤膜过滤，滤液用紫外可见分光光度计在540 nm处测定Cr(Ⅵ)的吸光度，计算出其质量浓度、吸附量和去除率。

SBA-5-G₃-SH吸附剂对Cr(Ⅵ)的吸附效果用吸附量(q_e)和去除率(E)来表示。其计算公式分别如式(1)和式(2)所示^[12]。

式(1)和式(2)中：q_e为在平衡时的吸附量(mg·g⁻¹)；c₀和c_e分别为初始和平衡时的Cr(Ⅵ)质量浓度(mg·L⁻¹)；c_t为在吸附时间t时Cr(Ⅵ)的质量浓度(mg·L⁻¹)；V为Cr(Ⅵ)溶液的体积(L)；m为吸附剂的质量(g)；E为Cr(Ⅵ)的去除效率(%)。

离子强度的影响实验在Cr(Ⅵ)离子质量浓度为200 mg·L⁻¹，温度为298 K，硫酸钠浓度为 0.02~0.10 mol·L⁻¹的条件下进行。

1.4. 吸附动力学和热力学实验

吸附动力学实验：取100 mL初始质量浓度为200 mg·L⁻¹的重铬酸钾溶液，加入100 mg的SBA-15-G₃-SH吸附剂，设置一系列反应时间梯度(5~1 800 min)，测量吸附后的Cr(Ⅵ)质量浓度。分别用准一级和准二级吸附动力学模型分析吸附过程并计算吸附速率。准一级和准二级吸附动力学模型如式(3)和式(4)所示。

式(3)和式(4)中：q_t表示t时刻吸附量(mg·g⁻¹)；t表示反应时间(min)；q_e表示平衡吸附量(mg·g⁻¹)；k₁和k₂分别表示准一级和准二级动力学方程反应速率常数，单位分别为min⁻¹和g·mg⁻¹·min⁻¹。吸附等温线实验：分别取一系列质量浓度梯度(250~500 mg·L⁻¹)的Cr(Ⅵ)溶液50 mL，加入SBA-15-G₃-SH吸附剂100 mg，吸附时间为1 800 min。采用Langmuir和Freundlich方程分别对吸附等温线进行拟合。Langmuir和Freundlich方程分别见式(5)和式(6)所示。

式(5)和式(6)中：c_e表示吸附平衡时Cr(Ⅵ)的质量浓度(mg·L⁻¹)；q_m表示Langmuir最大吸附量(mg·g⁻¹)；K_L为Langmuir吸附常数(L·mg⁻¹)；K_F为Freundlich吸附常数，n是与温度、体系有关的常数。

吸附热力学：为了研究温度对SBA-15-G₃-SH上吸附Cr(Ⅵ)的影响，通过式(7)和式(8)计算不同温度下的焓变(ΔH^θ)、熵变(ΔS^θ) 和吉布斯自由能变(ΔG^θ)值。

式(7)和式(8)中：R表示热力学常数(8.314 J·mol⁻¹·K⁻¹)；T表示热力学温度(K)；ΔS^θ(J·mol⁻¹·K⁻¹)和ΔH^θ(kJ·mol⁻¹)分别从lnK_L的截距和斜率计算得到；ΔG^θ(kJ·mol⁻¹)根据式(7)计算得到。

1.5. 性能表征

场发射扫描电子显微镜(FESEM)和能量色散X-射线光谱(EDS)表征：采用德国Carl Zeiss公司生产的GeminiSEM 300型场发射扫描电子显微镜进行。样品经喷金处理。红外表征：取1~2 mg样品在玛瑙研钵中研磨成细粉末与干燥的溴化钾混合均匀，装入模具内，在压片机上压制成片测试。采用Nicolet 6700傅里叶变换红外光谱仪对催化剂样品进行表征，扫描范围为 400~4 000 cm⁻¹，扫描100次。粉末X射线衍射仪分析：实验在XRD-6000型粉末X射线衍射仪上进行，扫描电压为 40.0 kV，扫描电流为30.0 mA，铜靶，2θ为10°~50°，扫描速率为2°·min⁻¹。BET表征：采用美国康塔仪器公司生产的Novae孔径测试仪测定催化剂的比表面积、孔径分布等，以氩气(Ar)为工作气体，高纯氦(He)为吸附气体。催化剂质量为0.1~0.2 g，从室温以10 ℃·min⁻¹升温到200 ℃，脱气180 min，在液氮(−196 ℃)温度下进行比表面积及孔结构等测试。

3. 结论

本研究通过对介孔材料SBA-15的改性，成功地合成了具有树状大分子结构和巯基基团的复合吸附剂SBA-15-G₃-SH。结果表明：合成的SBA-15-G₃-SH中—NH₂和—SH官能团被成功地嫁接在SBA-15中，且吸附剂对Cr(Ⅵ)具有较强的吸附能力，最大吸附量达192.31 mg·g⁻¹。吸附效果随着温度升高而降低，吸附等温线和吸附动力学分别符合Langmuir方程和准二级动力学方程。热力学参数表明：SBA-15-G₃-SH对Cr(Ⅵ)的吸附过程是自发的和吸热的。FESEM、EDS、FTIR、XRD的表征结果表明：SBA-15-G₃-SH上成功地嫁接了—NH₂和—SH官能团，并且这些基团的存在有利于吸附重金属Cr(Ⅵ)。吸附的主要作用为SBA-15-G₃-SH表面的—SH基团的表面络合作用以及金属离子的静电吸引，形成稳定的化合物。循环重复性实验结果表明：枝状复合吸附剂SBA-15-G₃-SH能重复利用且具有较好的吸附性能，具有高效、经济、环保等优良特性，是处理含重金属废水很有前景的一种吸附剂。

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T/K	准一级吸附			准二级吸附
T/K	q_e/ (mg·g⁻¹)	k₁/min⁻¹	R²	q_e/(mg·g⁻¹)	k₂/(g·mg⁻¹·min⁻¹)	R²
298	93.615 9	0.000 6	0.760 8	164.745	0.000 18	0.999 2
308	95.985 8	0.000 6	0.767 2	163.399	0.000 18	0.999 2
318	100.133 1	0.000 6	0.801 5	162.075	0.000 16	0.998 9

Langmuir 等温模型			Freundlich 等温模型
q_m/(mg·g⁻¹)	K_L/(L·mg⁻¹)	R²	K_F	n	R²
192.31	0.294	0.993 8	117.69	10.07	0.960 5

T/K	ΔG^θ /(kJ·mol⁻¹)	ΔH^θ/(kJ·mol⁻¹)	ΔS^θ/(J·mol⁻¹·K⁻¹)	R²
298	−12.99	−1.334	39.10	0.998 7
308	−13.40
318	−13.77