【作者】 曹威；

【导师】 党志；

【作者基本信息】 华南理工大学 ， 环境工程， 2012， 博士

【摘要】 硫酸根离子(SO₄^2-)和六价铬（Cr （VI））等阴离子污染物是水体中常见的一类污染组分，能引起各种各样的环境问题。从它们都呈负电性的特点考虑，基于静电作用的吸附分离有可能成为一种有效的阴离子污染物去除方法。吸附法速度快，吸附剂种类多，有多种选择，还可回收有价值资源，在水处理领域应用前景广阔。特别是，从工农业废弃物中开发低成本吸附剂用于废水处理，可同时实现废物资源化利用和水污染治理的双重目的，这也是国内外研究的热点。本论文以南方常见的稻草秸秆为原料，通过化学改性的方法制备了阴离子吸附剂（RS-AE），对稻草化学改性过程和RS-AE的表面形貌、化学结构进行了分析和表征，并在此基础上研究了RS-AE吸附去除SO₄^2-、Cr （VI）的特性和机理，及RS-AE吸附柱动态分离SO₄^2-的特性，并初步探讨了RS-AE与微生物联合作用去除SO₄^2-的性能。本研究直接以稻草秸秆为原料，通过NaOH碱化、环氧氯丙烷醚化交联、三甲胺季铵化等化学改性步骤，制备出阴离子吸附剂RS-AE。进而探讨了NaOH浓度、碱化产物脱水方式和季铵化反应温度对RS-AE收率和吸附效率的影响。结果发现，增加NaOH浓度和升高季铵化反应温度，可提高RS-AE的吸附能力，但会降低产物收率；碱化产物脱水是关键步骤，相比较不脱水、抽滤、乙醇脱水方式，压榨脱水可得到吸附效率较高的改性稻草。采用10%（W/V）NaOH碱液、压榨脱水方式和80℃季铵化温度时，RS-AE收率约为77%，吸附性能良好，实验条件下对SO₄^2-的吸附效率为75%左右。稻草化学改性机理可概括为稻草纤维结构中的羟基先被NaOH活化形成醇钠，接着与环氧氯丙烷醚化交联，随后与体系中三甲胺发生季铵化反应。利用元素分析、扫描电子显微镜及能谱分析（SEM/EDX）、傅立叶变换红外光谱（FT-IR）、固态核磁共振碳谱(¹³C-NMR)、X-射线衍射（XRD）及自动压汞分析等表征方法，对原稻草和RS-AE的元素组成、表面形貌、主要功能基团、纤维结晶结构以及孔结构进行了系统的对比研究。结果显示，稻草改性后表面暴露出大量纤维丝，表面元素组成中氮和氯含量显著增加，总含氮量为2.75%，理论交换吸附量达1.96mEq·g^-1。RS-AE成分以纤维素为主，主要功能基团是季铵基和羟基，这说明化学改性向稻草结构中引入了大量季铵基。与原稻草相比，RS-AE中纤维素结晶度及结晶度指数都明显降低，而且纤维素101晶面衍射峰消失、002晶面衍射峰强度降低，晶面间距变大，由此推测稻草纤维素聚集结构中101结晶面活性较高，最先被改性试剂侵入反应。稻草原料孔表面积为4.51m²·g^-1，以2～50nm的中孔为主；RS-AE孔表面积为1.19m²·g^-1，且其孔直径较大，以>50nm的大孔居多。采用批吸附实验，从吸附影响因素、吸附平衡、吸附动力学和热力学角度，研究了RS-AE吸附SO₄^2-的特性和机理。结果表明，RS-AE吸附SO₄^2-平衡时间短，约为20min；在pH值为3～8时吸附效果最佳；符合Langmuir吸附等温模型，最大吸附容量为74.76mg·g^-1(远大于原稻草的11.68mg·g^-1)。RS-AE吸附SO₄^2-过程中吉布斯自由能变化ΔG为负值(-2.35到-3.46kJ·mol^-1)，焓变ΔH和熵变ΔS分别为8.49kJ·mol^-1和0.0376kJ·K^-1·mol^-1，吸附活化能Ea为19.3kJ·mol^-1，由此判断此吸附为自发、吸热的化学吸附过程。此吸附过程可由伪二级动力学方程描述，伪二级动力学速率系数kp2随SO₄^2-初始浓度变化不明显，而受温度影响显著，此外整个吸附动力学过程极可能是由液膜扩散过程控制。SO₄^2-的去除机理主要是与RS-AE表面负载的Cl-进行离子交换，实验测定交换系数为1.8，接近离子荷电数之比2。RS-AE对SO₄^2-具有较强的选择吸附能力，对水体中常见阴离子的结合力顺序为SO₄^2-> CrO₄^2-> NO3-> PO43-，并且RS-AE具有良好再生性能和重复利用潜力。在去除Cr （VI）方面，研究发现RS-AE去除Cr （VI）速度快，在pH值为2～6的酸性条件下效果好，中性条件下依然有较高效率；吸附符合Langmuir模型，最大吸附容量在298K时为18.7mg·g^-1。RS-AE吸附Cr （VI）过程ΔG为负值，ΔH和ΔS分别为11.5kJ·mol^-1和0.0664kJ·K^-1·mol^-1，活化能Ea为24.5kJ·mol^-1。这说明RS-AE吸附Cr （VI）也是自发的、吸热的化学吸附过程。吸附符合伪二级动力学方程，而Elovich方程可以解释吸附初始阶段的动力学特性，通常情况下膜扩散过程是吸附动力学控制步骤，而仅在Cr （VI）稀溶液中，颗粒内扩散过程才可能是吸附速率决定步骤。Cr （VI）也是注意以离子交换方式得以去除。此外，在吸附过程中发现了铬价态变化，通过X-射线光电子能谱（XPS）表征证实，部分Cr （VI）在RS-AE表面上被还原为Cr （III），而且酸性溶液条件有利于Cr （VI）还原。实验还制作了RS-AE吸附柱，在固定床和膨胀床操作条件下研究了其分离SO₄^2-的特性，并应用Thomas动态吸附模型对不同条件下的流出曲线进行分析。结果发现，在固定床操作下SO₄^2-穿透时间随高径比的增加而延长，随初始SO₄^2-浓度和流量的增加而显著缩短。RS-AE固定床吸附柱容易再生，采用0.1M NaOH和0.1M HCl作为再生溶液时，SO₄^2-解析效果最佳。RS-AE膨胀床的膨胀率与进水流量呈正相关性；在低膨胀率范围内，SO₄^2-穿透时间随膨胀率的增加逐渐缩短，而耗竭时间略有延长。在膨胀率7%时，从Thomas模型计算得到柱吸附容量最大，为16.69mg/g。对比膨胀床和固定床吸附柱发现，低膨胀率下的膨胀床柱吸附容量较大，床层耗竭时间较长，适合去除SO₄^2-。最后，分别在静态培养和厌氧反应器连续运行条件下，考察了RS-AE联合微生物去除SO₄^2-的效果。RS-AE与硫酸盐还原菌（SRB）静态培养实验发现，与两者单独作用相比，RS-AE联合SRB可以更快、更彻底的去除SO₄^2-，两者具有一定协同作用。SRB附着生长在RS-AE表面形成SO₄^2-吸附-降解体系，使得基质SO₄^2-浓度在60h内从约1000mg·L^-1降至100mg·L^-1以下。在厌氧反应器中，RS-AE联合微生物可以实现SO₄^2-的连续去除，监测到最佳SO₄^2-和总有机碳（TOC）去除率分别为76%和65%；45天左右RS-AE开始产生絮状体，出现降解的现象，降低了TOC去除率。这对反应运行不利，但也启示RS-AE是一种生物可降解吸附材料。更多还原

【Abstract】 Anions pollutants such as sulphate (SO₄^2-), hexavalent chromium （Cr （VI）） are commoncomposition in water, and can cause various environmental problems. Electrostaticforce-based adsorption technique may be effective to eliminate negative charged anionpollutants from aqueous solution. Absorption is rapid, and has the potential to recover someelements. Many kinds of materials can be used as adsorbents in wastewater treatment.Especially, low-cost adsorbents originated from agricultural/industrial wastes has becameattractive in recent years because of its significance both in environmental remediation and inreuse of wastes resource. In this study, rice straw that is a common agricultural residues insouthern China was modified into anionic adsorbent （RS-AE） to remove SO₄^2-and Cr （VI）from water. The surface morphology and chemical structure of raw straw and RS-AE werecharacterized and analyzed. Then, the removal characteristics and mechanism of sulphate andCr （VI） by RS-AE were investigated. The RS-AE adsorption column was made and used toseparate sulphate from water. In addition, removal of SO₄^2-by the method of RS-AEcombined with microorganism （sulphate reducing bacteria） also was carried out.To prepare RS-AE, raw rice straw was firstly treated by NaOH alkaline treatment, thenreacted with epichlorohydrin and finally quaternized with trimethylamine solution. The effectof NaOH concentration, dehydration of alkaline treated products and the temperature ofquaternization on the yield and adsorption efficiency of RS-AE was discussed. The resultsshowed that the yield was enhanced with increase of NaOH concentration and quaternizationtemperature, but the adsorption efficiency was decreased. Dehydration of alkaline treatedstraw was important for preparation of RS-AE. Compared with non-dehydration, filtrationand ethanol treatment, compression was more suitable for preparation of RS-AE. The yield ofRS-AE could reach77%under the condition of10%（W/V） NaOH solution, compressiondehydration and80℃quaternization temperature, and RS-AE exhibited a higher adsorptionefficiency75%for sulphate. The modification mechanism included three steps: the–OHcontained in straw was activited by NaOH; the activated–OH was etherized and crosslinked by epichlorohydrin; the etherized product was quaternized with trimethylamine.Characterization methods, such as scanning electron microscope （SEM）, Fouriertransform infrared spectrum （FT-IR）, solid state¹³C nuclear magnetic resonance (¹³C-NMR),X-ray diffraction （XRD） and pore autoanalyzer （Hg compression method） and so on, wereemployed to determine the element content, surface morphology, functional groups, crystaland pore structure of raw rice straw and prepared RS-AE. The results showed that manycellulosic fibers were exposed on the surface of straw after modification, and the content ofnitrogen and chlorine increased. The total nitrogen content of RS-AE is2.75%, from whichthe total exchange capacity was calculated as1.96mEq·g^-1. The main functional groups ofRS-AE were quaternary amino and hydroxyl groups. The crystalinity degreee and crystalinityindex of straw was decreased after modification, especially,101-crystal plane of cellulose wasdisappeared and diffraction intensity at002-crystal plane was impaired. The pore area of rawstraw was4.51m²·g^-1and pores maily were mesopores （2⁵0nm）; while pore area of RS-AEwas1.19m²·g^-1and macropores （>50nm） account for about94%.Batch adsorption experiments showed that adsorption of sulphate on RS-AE was rapid,and the equilibrium time was about20min. The best sulphate removal was observed in pHrange of3⁸. The adsorption followed Langmuir adsorption model, and the maximumsulphate adsorption capacity of RS-AE was evaluated as74.76mg·g^-1, which was higher thanthat of raw rice straw (11.68mg·g^-1). Gibbs free energy ΔG of the adsorption were negative(-2.35^-3.46kJ·mol^-1), and ΔH, ΔS and adsorption activated energy Eawere8.49kJ·mol^-1,0.0376kJ·K^-1·mol^-1and19.3kJ·mol^-1, respectively. These adsorption thermodynamicparameters suggested that the adsorption of sulphate by RS-AE was a spontaneous,endothermic chemical adsorption. Pseudo-second order rate equation can be used to describethe adsorption process. The pseudo-second order rate constant kp2increased with the rise oftemperature, and fluctuated with sulphate initial concentration. In addition, the adsorptionkinetics was possibly controlled by liquid membrane diffusion. Sulphate anions were mainlyremoved through the way of ion exchange with chlorine ions on RS-AE. The measuredexchange coefficient was1.8that was close to the theoretical value2. RS-AE exhibited an selectivity for common anions in the order SO₄^2-> CrO₄^2-> NO3-> PO43-. Regenerationexperiments suggested that RS-AE has the potential for repeatedly use in removal of sulphate.Batch adsorption experiments for Cr （VI） removal by RS-AE indicated that theadsorption was apparently affected by solution pH conditions. For instance, best removal wasobtained in pH range of2⁶, then with the increase of pH, adsorption efficiency wasdecreased, and when the pH>9, the adsorption was inhibited. The adsorption equilibriumtime for Cr （VI） was short. Langmuir adsorption model fitted the adsorption equilibrium databetter, and the calculated maximum adsorption capacity was18.7mg·g^-1at298K. Theadsorption thermodynamic parameters （ΔG, ΔH, ΔS, and Ea） were evaluated. It found that ΔGwas negative, and ΔH, ΔS and Eawere11.5kJ·mol^-1,0.0664kJ·K^-1·mol^-1and24.5kJ·mol^-1,repectively. Tthe results clearly demonstrated that the adsorption of Cr （VI） on RS-AE was aspontaneous, endothermic chemical adsorption. Adsorption kinetics studies showed that thecomplete process of adsorption could be described by pseudo-second order adsorption kineticmodel, while the Elovich’s equation only can be used to describe the initial adsorption stage.Diffusion model analysis suggested that Cr （VI） removal rate was controlled by liquidmemerbrane diffusion process, however, in diluted Cr （VI） solution, intraparticle diffusionmight be the determined step. The removal mechanism of Cr （VI） was ion exchange coupledwith Cr （VI） reduction to Cr （III）. X-Ray photoelectron spectrum （XPS） characterizationshowed that Cr （III） was formed in the surface of RS-AE, moreover, adsorption of Cr （VI）under acidic conditions can favor the reduction of Cr （VI） to Cr （III）.RS-AE adsorption columns were prepared to investigated the dynamic adsorption ofsulphate under fixed-and expanded-bed operation conditions. The breakthrough curves wereanalyzed with Thomas dynamic adsorption model. The experimental results showed that, forRS-AE fixed-bed columns, sulphate breakthrough time increased with the rise of H/D（height/diameter） ratio; while with the increase of sulphate initial concentration, thebreakthrough time was shorten. And RS-AE fixed-bed columns were easy to be regeneratedby0.1M NaOH and0.1M HCl solution. For RS-AE expanded-bed columns, a positiverelationship between influent flow rate and the expansion ratio was found. With the increase of expansion ratio, the breakthrough time reduced, however, the exhaustion time wasextended. At expansion ratio7%, the maximum column adsorption capacity from Thomasmodel was16.69mg·g^-1. From the comparison between fixed-and expanded-bed operations,it found RS-AE expanded-bed column with low expansion ratio was more suitable to removesulphate because of its larger column adsorption capacity and longer exhaustion time.The method of RS-AE combined with sulphate reducing bacteria （SRB） was tested forsulphate removal by static cultivation experiments. A better removal of sulphate can beobtained with the combined method in contrast to separately use of RS-AE or SRB. FromSEM images, it could be seen that many spherical bacteria were attached on the surface ofRS-AE forming adsorption-degradation system of sulphate. The experiments results showedthat sulphate concentration of culture medium can be reduced from1000mg·L^-1to100mg·L^-1in60hours by RS-AE combined with SRB. Furthermore, a anaerobic bioreactor was designedto evaluate the continuously removal of sulphate from synthesized wastewater by RS-AEcombined with microorganism. The best removal percentage of sulphate and total organiccarbon （TOC） were observed as76%and65%in two months running experiments. It isnecessary to point out that RS-AE started to be degraded by bacteria in45days, whichreduced the TOC removal percentage of effluent. However, this phenomenon suggested thatRS-AE was a biodegradable adsorbent that deserved further research.更多还原