Research Progress on Fraction and Analysis Methods of Soil Carbon
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摘要: 研究土壤碳的赋存形态不仅利于了解碳的迁移转化规律,而且可以为土壤固碳提供科学依据。目前对各形态碳(尤其是有机碳中的慢性及惰性组分)缺乏系统的分析研究。基于此,本文综述了土壤中碳的主要赋存形态,各形态碳的组成、分布及作用,土壤碳的分析、分离方法。土壤无机碳储量约占全球总碳库的38%,赋存形态以碳酸盐为主。土壤有机碳主要分为活性碳库(周转期0.1~4.5年)、慢性碳库(周转期5~50年)和惰性碳库(周转期50~3000年)。其中可溶性有机碳、易氧化有机碳和微生物量碳属于活性有机碳库这一范畴,可以较为灵敏地反映土壤理化性质的微小变化;轻组有机碳和颗粒有机碳属于慢性有机碳库,可作为土壤有机质周转变化的重要指标;重组有机碳和矿物结合态有机碳属于惰性有机碳库,是土壤有机碳固持的重要机制之一。目前土壤中碳酸盐测定方法主要为气量法和滴定法;有机碳分析方法包括容量法、比色法和重量法。本文提出,今后应加强对无机碳及有机碳中的惰性组分研究,同时对土壤有机碳各组分概念及测定方法进行统一,并开展不同地域、不同土壤类型、不同浓度的土壤碳形态标准物质研制工作。要点
(1) 总结了土壤中无机碳和有机碳的主要赋存形态和各形态碳的组成、分布及作用。
(2) 评述了目前国内外土壤中不同形态碳的分析、分离方法。
(3) 提出应统一土壤中不同形态碳的概念和分析方法,加强相关标准物质研制。
HIGHLIGHTS(1) The main fractions of organic carbon and inorganic carbon in soil and their components, distributions and roles were summarized.
(2) Analysis and separation methods of different soil carbon fractions both at home and abroad were reviewed.
(3) It was proposed that the concepts and analysis methods of different soil carbon fractions should be unified, and it should be strengthened to develop the relevant reference materials.
Abstract:BACKGROUNDResearch on fractions of soil carbon can not only be helpful to realize the migration and transformation of carbon, but also provide the scientific basis for reducing global carbon emissions. There is lack of systematic research on organic carbon fractions, especially the chronic and inert fractions.OBJECTIVESTo lay the foundation for future research through the summary of previous research on composition, distribution and roles of different carbon fractions.METHODSIt proposes development directions for future works through a summary of previous research.RESULTSThe main occurrence form of inorganic carbon was carbonate. The soil organic carbon pool covered the activated carbon pool (turnover time:0.1-4.5 year), chronic carbon pool (turnover time:5-50 year) and inert carbon pool (turnover time:50-3000 year). Previous studies showed that the activated carbon pool, which contained dissolved organic carbon, labile organic carbon and microbial biomass carbon, could reflect the small change of physical and chemical properties of soil sensitivity. Chronic carbon, as the important indicator of the conversion of organic matters in soil, contained light fraction organic carbon and particulate organic carbon. Heavy fraction organic carbon and mineral-associated organic carbon belonged to the inert carbon pool, which was one of the important mechanisms of organic carbon sequestration in soil. Carbonate in soil was always analyzed by gasometric method or titrimetry, while the organic carbon tended to be measured by the volumetric method, colorimetric method or gravimetric method.CONCLUSIONSStudy on soil inorganic carbon and inert organic carbon should be strengthened. As far as the analysis methods, 'ramped combustion method' should be researched further. The in-situ measurement method of soil carbon (including organic carbon and inorganic carbon) also needs to be developed. Reference materials of soil carbon fractions with different soil type and different concentration are also necessary.-
Keywords:
- soil carbon /
- fractions /
- distribution characteristics /
- analysis method /
- separation method /
- reference material
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重金属的污染已危害到生态环境和人类生命健康[1],水体中重金属离子的检测与污染治理是当今环境科学领域重要的研究课题。重金属离子的常规检测方法主要有原子吸收光谱法、原子发射光谱法、原子荧光光谱法、电感耦合等离子体质谱(ICP-MS)等[2-3]。这些检测方法对于基体复杂、高背景金属离子含量低的样品均存在干扰,测定结果的准确度低、重现性差,甚至对仪器产生损坏而无法检测。因此,在样品进行仪器测试之前进行处理尤为重要。与其他水体中重金属离子测定的样品处理方法相比,固相萃取的操作简单、效率高、重现性好、成本低及环境友好,在分离富集重金属离子方面具有重要作用[4-5]。因此,开发研制具有吸附容量大、萃取效率高和多选择性的固相萃取填料,受到了分析工作者的极大重视。
用于重金属离子分离富集的固相萃取(SPE)填料主要有离子交换型、螯合树脂型、纳米材料型及复合型[6-7]。螯合树脂型填料是一种分子结构中含有可与金属离子的空轨道进行配位的孤对电子原子(如O、N、S、P、As、Se等)的高分子聚合物[8-9]。配位原子的性质和数量决定了其与金属离子形成配合物的稳定性,这种固相萃取填料适用于从多种金属离子共存体系中对特定离子进行选择吸附。但高聚物基质的固相萃取填料存在一些不足:机械强度低,比表面积较小,吸附容量不高,不易合成同时具有多种配位功能原子的螯合吸附材料,选择性不强。因此,对螯合树脂型填料的改进和完善工作很有现实意义。
纳米二氧化钛具有耐高温及强酸强碱、机械强度好等优点,比表面积大,且表面具有不饱和性,对许多金属离子具有吸附能力[10-12]。但纳米二氧化钛颗粒极易团聚,极性强,在有机溶剂中分散稳定性较差。因此,使用纳米二氧化钛作为固相萃取填料时,往往达不到理想的吸附效果。目前对纳米二氧化钛固相萃取填料的改性方式有两种:一是将纳米二氧化钛表面进行修饰,以改善其性能[13-14];二是将含有配位原子的有机螯合基团接枝于其表面,形成一种新型吸附材料。采用第二种方式制得的吸附材料中的有机基团分散更加均匀,同时具有纳米材料的性质和配位原子的螯合功能[15-16],能够获得更高的金属离子吸附容量。本项目组[17]曾采用共混法制备了聚苯乙烯-甲基丙烯醛-氨基硫脲包覆纳米二氧化钛固相萃取填料,考察了其对Cr、As、Cd和Pb金属离子的吸附性能,并应用于猪肉样品消化液中这4种金属离子的萃取分离。但在实验中发现,由于纳米粒子易团聚而从分散介质中沉积出来,粒子微区相尺寸及尺寸分布不易控制,导致制备的复合材料粒子之间的组织结构会有差别,此种填料对金属离子吸附量的重现性不佳。
本次研究通过化学反应在纳米二氧化钛表面键合氨基硫脲,制得一种新型的固相萃取填料。该填料的组成及结构均匀,同时利用配位体中N、S原子的配位作用和纳米材料的化学活性,实现了对不同Lewis酸性的重金属离子均有较强的吸附能力和良好的实验数据重现性。运用Langmuir、Freundlich等温方程和Lagergren准二级吸附方程对实验数据进行拟合分析,准确描述了吸附剂对Sb3+、Cd2+和Ba2+的吸附动力学过程。通过确定最佳的固相萃取小柱对金属离子的吸附与洗脱操作条件,应用SPE-ICP/MS联用技术测定了水样中Sb3+、Cd2+和Ba2+三种金属离子的含量。
1. 实验部分
1.1 仪器和设备
S-3400扫描电子显微镜(日本日立公司);NexION 350X电感耦合等离子体质谱仪(美国PerkinElmer公司);EscaLab 250Xi X射线光电子能谱仪(美国ThermoScientific公司);Spectrun one FT-IR红外光谱仪(美国PekinElmer公司);SPE固相萃取装置(美国Superlco公司);固相萃取柱柱管(管长6.6 cm,内径12.7 mm;筛板直径12.8 mm,厚度2.5 mm,孔径20 μm)。
1.2 标准溶液和主要试剂
Sb、Cd、Ba标准溶液(国家有色金属及电子材料分析测试中心);三氯化锑(分析纯,上海展云化工有限公司);氯化镉(分析纯,天津福晨化学试剂厂);氯化钡(分析纯,天津金汇太亚化学试剂有限公司);硫代氨基脲、甲基丙烯醛、γ-甲基丙烯酰氧基丙基三甲氧基硅烷(均为分析纯,阿拉丁试剂有限公司);聚乙烯吡咯烷酮(分析纯,天津市大茂化学试剂厂);钛酸丁酯(化学纯,天津市光复精细化工研究所);N, N-二甲基甲酰胺(分析纯,天津市凯通化学试剂有限公司)。
实验用水为二次蒸馏水。
1.3 实验方法
1.3.1 纳米二氧化钛的制备
取4 mL乙酸、10 mL蒸馏水和35 mL无水乙醇于200 mL烧杯中,磁力搅拌均匀。取35 mL无水乙醇、10 mL钛酸丁酯于100 mL烧杯中,滴加盐酸,使pH≤3,磁力搅拌10 min。将钛酸丁酯-乙醇溶液转移至漏斗中,缓慢滴入上述200 mL烧杯中,继续磁力搅拌30 min,将烧杯置于80℃水浴锅中,反应1 h。将反应物转移至布氏漏斗中,抽滤,将滤饼置于蒸发皿中,在100℃电热炉上烘干,转移至瓷坩埚中,在800℃高温炉中灼烧3 h,得到纳米二氧化钛。
1.3.2 二氧化钛-甲基丙烯醛-氨基硫脲的合成
向三口瓶中加入2.0 g二氧化钛和30 mL丙酮,超声分散10 min。将0.2 g的γ-甲基丙烯酰氧基丙基三甲氧基硅烷、5 mL蒸馏水加入三口瓶中,于60℃恒温水浴中机械搅拌反应4 h,得白色悬浊液,离心分离。将白色产物烘干。取1.0 g上述产物于三口瓶中。取0.2 g聚乙烯吡咯烷酮和0.1 g偶氮二异丁腈溶于20 mL无水乙醇后,移入三口瓶,在氮气保护下,于80℃恒温水浴中机械搅拌反应8 h。将得到的产物离心分离,烘干,备用。
取0.5 g氨基硫脲和10 mL N, N-二甲基甲酰胺于三口瓶中,搅拌溶解,加入0.5 g上述备用产物及0.8 mL冰乙酸,在85℃恒温水浴中磁力搅拌反应4 h。将反应物中的溶剂蒸干,用无水乙醇洗涤3次,烘干,得到纳米TiO2/TSC复合固相萃取填料,备用。
1.3.3 固相萃取小柱的制备
将固相萃取小柱管和筛板用甲醇净洗,晾干后用推杆将筛板置于小柱底端,通过长颈漏斗将0.1 g固相萃取填料填于柱管中,并轻敲漏斗使填料上表面平齐,用推杆将上端筛板装入小柱中,并用力压实,制得固相萃取小柱,备用。
1.3.4 溶液配制
标准溶液的配制:分别取5、10、20、40、80、120、160 μL质量浓度为100 mg/L的Sb3+、Cd2+和Ba2+标准溶液于100 mL容量瓶中,加水定容,得到Sb3+、Cd2+和Ba2+浓度为5、10、20、40、80、120、160 μg/L的混合系列标准溶液,备用于制作分析标准曲线。
吸附实验溶液的配制:分别取18.7 mg氯化锑、20.3 mg氯化镉和17.8 mg氯化钡于烧杯中,用水溶解后转移至1000 mL容量瓶中,加水定容,得到Sb3+、Cd2+、Ba2+浓度均为10 mg/L的混合溶液,备用于吸附容量试验。
取10 mg/L混合溶液10 mL于1000 mL容量瓶中,加水定容,得到Sb3+、Cd2+、Ba2+浓度均为100 μg/L的混合溶液,备用于吸附性能条件实验。
洗脱液配制:取35 mL 65%硝酸于100 mL容量瓶中,加2.5 mL三乙醇胺,用水定容。
1.3.5 计算公式与方程
固相萃取小柱吸附金属离子的回收率计算公式:
$ S = 100 \times \frac{{{C_{\rm{x}}}}}{{{C_0}}} $
(1) 式中:S—重金属离子的回收率(%);Cx—洗脱液中金属离子的浓度(μg/L);C0—原始溶液中金属离子的浓度(μg/L)。
洗脱率计算公式:
$ m = 100 \times \frac{{{C_{\rm{x}}}}}{{{C_0} - {C_{\rm{L}}}}} $
(2) 式中:m—重金属离子的洗脱率(%);Cx—洗脱液中金属离子的浓度(μg/L);C0—原始溶液中金属离子的浓度(μg/L);CL—通过固相萃取小柱溶液中金属离子的浓度(μg/L)。
吸附量计算公式:
$ q = \left( {{C_0} - C} \right) \times V/W $
(3) 式中:q—吸附剂对金属离子的吸附量(mg/g);C0—重金属离子的初始浓度(μg/L);C—吸附后流出液中重金属离子的浓度(μg/L);V—溶液的体积(mL);W—吸附剂的质量(g)。
Langmuir吸附等温方程:
$ \frac{{{C_{\rm e}}}}{{{q_{\rm e}}}} = \frac{{{C_{\rm e}}}}{{{Q_{\max}}}} + \frac{1}{{K{Q_{\max}}}} $
(4) Freundlich吸附等温方程:
$ \lg{q_{\rm e}} = \lg{k_{\rm F}} + \frac{1}{n}\lg{C_{\rm e}} $
(5) Lagergren准二级动力学方程:
$ \frac{t}{{{q_{ \rm t}}}} = \frac{1}{{{K_2}{q_{\rm e}}^2}} + \frac{t}{q_{\rm e}} $
(6) 式中:Ce为平衡浓度(mg/L);qe为平衡吸附量(mg/g);K为Langmuir吸附常数;Qmax为饱和吸附量(mg/g);kF和n为Freundlich特征参数;qt为吸附平衡时间t(min)的吸附剂吸附金属离子的容量(mg/g);K2是二级动力学方程的速率常数。
2. 结果与讨论
2.1 固相萃取填料的表征与分析
2.1.1 填料结构及成分
图 1a为表面键合配位体二氧化钛固相萃取填料的红外光谱图。3372.69 cm-1处为N—H伸缩振动吸收峰。3073.18 cm-1处为不饱和C—H伸缩振动吸收峰,2929.88 cm-1和2856.08 cm-1处为饱和C—H伸缩振动吸收峰。2054.09 cm-1处为材料表面吸附CO2吸收,1651.67 cm-1处为C=N伸缩振动吸收峰,1201.95 cm-1处为C=S伸缩振动吸收峰。
图 1b为纳米二氧化钛和固相萃取填料的X射线衍射图谱。锐钛型二氧化钛在低温下稳定,温度达到610℃时则开始缓慢转化为金红石,915℃时可完全转化为金红石型。衍射峰表明,纳米二氧化钛的晶型为锐钛矿与金红石相共存,表面接枝改性后没有改变二氧化钛的晶体结构。
图 2为表面键合配位体二氧化钛固相萃取填料表面N、S元素的X射线光电子能谱图谱。由图 2可知,N 1s拟合后得到三个峰,结合能位置在398.75 eV处的拟合峰对应于N=S基团,结合能位置在399.55 eV处的拟合峰对应于N—C基团,结合能位置在400.43 eV处的拟合峰对应于N—H基团。S 2p拟合后得到两个峰,结合能位置在161.52 eV与162.81 eV处的拟合峰均对应于C=S基团。表明材料表面共存氨基硫脲纳米二氧化钛活性位点。
2.1.2 填料的形貌
通过扫描电镜分别对本文制备的表面键合氨基硫脲配位体的二氧化钛及按文献[17]采用共混法制备的聚苯乙烯-甲基丙烯醛-氨基硫脲包覆纳米二氧化钛的形貌进行观察(图 3)。由图 3a可见,表面键合氨基硫脲配位体二氧化钛固相萃取填料粒子直径约为200~300 nm,分布较为均匀;由图 3b可见,聚苯乙烯-甲基丙烯醛-氨基硫脲包覆纳米二氧化钛粒子直径约为100~800 nm,分布不均匀。
2.2 标准曲线
在ICP-MS工作条件下,分别测定不同浓度标准溶液中Sb3+、Cd2+、Ba2+的计数值,以浓度为横坐标,计数率为纵坐标,绘制标准曲线。Sb3+的标准曲线的线性回归方程为:Y=676.32C-4704.8,R2=0.9958;Cd2+的标准曲线的线性回归方程为:Y=42173C-46758,R2=0.9985;Ba2+的标准曲线的线性回归方程为:Y=10201C-102857,R2=0.9959。用于测定各项实验中三种离子浓度。
2.3 固相萃取条件的确定
2.3.1 过柱流速
将100 mL 100 μg/L混合金属离子溶液用氨水调节至pH=7,以不同速度通过固相萃取小柱,待小柱内溶液抽至近干,用10 mL 5 mol/L硝酸+0.25 mL三乙醇胺洗脱液以0.5 mL/min流速过萃取小柱,洗脱重金属离子,用水定容至100 mL。用ICP-MS测定洗脱溶液中Sb3+、Cd2+和Ba2+的浓度,根据公式(1)计算各金属离子回收率。
流速对回收率的影响见表 1。表明随着流速增加,金属离子的回收率逐渐降低。当流速为0.5 mL/min时,Sb3+、Cd2+、Ba2+回收率分别为97.94%、95.65%、94.04%。
表 1 流速对金属离子回收率的影响Table 1. The influence of velocity on recovery rate of metal ions金属离子 不同流速下金属离子的回收率(%) 0.5 mL/min 1.0 mL/min 1.5 mL/min 2.0 mL/min Sb3+ 97.94 96.03 92.18 85.43 Cd2+ 95.65 94.44 90.57 86.74 Ba2+ 94.04 93.41 90.56 85.05 2.3.2 溶液的pH
金属离子与配位体所形成的配合物的稳定性与溶液的酸度有关;纳米二氧化钛的等电点是6.2,当溶液的pH值高于金属阳离子的等电点时,氧化物表面被羟基覆盖而显负电性,金属阳离子才能被吸附。取100 mL 100 μg/L的Sb3+、Cd2+、Ba2+混合溶液,用盐酸和氨水调节pH值,在最佳流速下经过固相萃取小柱萃取,洗脱后用ICP-MS测定浓度,计算回收率。结果显示,随着pH的增加,固相萃取填料对三种金属离子的吸附率均逐渐升高,当pH=7时,回收率达到最大值。
2.3.3 洗脱液的选择
取100 mL 100 μg/L混合金属离子通过固相萃取小柱,使用不同类型洗脱液对金属离子进行洗脱,洗脱液用水定容到100 mL,用ICP-MS检测重金属离子浓度,根据公式(2)计算洗脱率,实验数据见表 2。可见,以10 mL、5 mol/L硝酸和0.25 mL三乙醇胺混合作为洗脱剂时,Sb3+、Cd2+、Ba2+的洗脱率分别为98.43%、98.28%、99.07%。
表 2 洗脱剂对金属离子洗脱率的影响Table 2. The influence of the elution liquid on recovery rate of metal ions洗脱剂 金属离子回收率(%) Sb3+ Cd2+ Ba2+ 10 mL 1 mol/L硝酸 88.45 89.92 90.94 10 mL 3 mol/L硝酸 92.46 91.54 93.26 10 mL 5 mol/L硝酸 95.03 96.05 96.72 10 mL 1 mol/L硝酸+0.25 mL三乙醇胺 91.78 92.65 92.82 10 mL 3 mol/L硝酸+0.25 mL三乙醇胺 94.43 96.91 96.09 10 mL 5 mol/L硝酸+0.25 mL三乙醇胺 98.43 98.28 99.07 2.4 固相萃取填料的吸附机理及吸附性能
2.4.1 吸附热力学与动力学
在30℃下,以表面键合配位体二氧化钛固相萃取填料对初始浓度分别为5、10、15、20、30、40 mg/L的Sb3+、Cd2+、Ba2+进行吸附,分别用Langmuir方程和Freundlich方程进行拟合。在三种离子浓度均为10 mg/L时,在10~90 min内制作吸附量-时间关系曲线,并用Lagergren二级吸附方程进行拟合。拟合结果表明,Langmuir方程拟合效果优于Freundlich方程。由Langmuir方程计算的饱和吸附量分别为13.9 mg/g、12.9 mg/g和11.2 mg/g,与实验结果基本符合,也说明二氧化钛表面键合配位体固相萃取填料对Sb3+、Cd2+、Ba2+的吸附属于单分子层吸附[18]。由Langmuir方程计算得到的吸附常数K值说明二氧化钛表面键合配位体固相萃取填料易于吸附Sb3+、Cd2+和Ba2+。由Freundlich方程得到的n值也可证明二氧化钛表面键合配位体固相萃取填料易于吸附三种离子。用Lagergren准二级动力学方程进行拟合,线性相关系数均高于0.99,吸附符合准二级动力学反应,说明吸附过程为化学吸附[19]。
2.4.2 固相萃取小柱的穿透体积
取0.5 mL的10 mg/L混合离子储备液,加入到盛有20、40、60、100、120、140、160 mL去离子水的烧杯中。在pH=7,流速为0.5 mL/min条件下经固相萃取小柱萃取,用10 mL 5 mol/L硝酸和0.25 mL三乙醇胺的混合溶液洗脱,将洗脱液定容到100 mL,计算金属离子回收率。以金属离子回收率为纵坐标,流过小柱的体积为横坐标绘制穿透曲线。实验结果表明试样过柱体积应小于100 mL。
2.4.3 填料的吸附机理分析与性能对比
纳米粒子表面原子能够与金属离子以静电作用等方式相结合,对一些金属离子具有很强的吸附能力。二氧化钛的等电点为6.2,当pH>6.2时,表面被羟基覆盖带负电,也可以吸附带正电的重金属离子。纳米粒子表面原子与处于晶体内部的原子所受力场有很大的不同。内部原子所受作用力受力对称,其价键是饱和的;而表面原子受力为与其邻近的内部原子的非对称价键力和其他原子的远程范德华力,其裸露在外的部分没有力的作用,受到的作用力不对称,其价键是不饱和的,存在与外界原子键合的倾向,而使得纳米粒子发生团聚,由于颗粒团聚又影响了其活性,导致对重金属离子的吸附容量不高。
在纳米二氧化钛表面接枝含N、S配位原子的有机官能团,一方面对粒子之间的团聚起到阻碍作用;另一方面增加了对金属离子的配位吸附功能。根据软硬酸碱理论[20],硬酸与硬碱结合,软酸与软碱结合,生成的酸碱配合物稳定性高。硬酸与硬碱生成的化合物主要具有离子键的特征,软酸与软碱生成的化合物主要具有共价健的特征。氮原子属于中间配体,硫原子属于软配体,可以与Sb3+(Lewis交界酸)、Cd2+(Lewis软酸)形成稳定配合物,Ba2+(Lewis硬酸)的吸附是基于纳米二氧化钛的静电作用及氧原子的配位作用。二氧化钛表面键合配位体固相萃取填料充分利用了配位体中配位原子的配合作用和纳米材料的化学活性,从而提高了吸附容量。
为了评价表面键合配位体纳米二氧化钛的吸附性能,将之与聚苯乙烯-甲基丙烯醛-氨基硫脲包覆纳米二氧化钛及纳米二氧化钛进行实验对比。这三种材料制成固相萃取小柱,在相同条件下,分别将含有Sb3+、Cd2+、Ba2+的混合溶液进行过柱、洗脱和测试,重复6次,计算平均回收率和重现性,结果见表 3。由表 3中的数据可见,表面键合配位体纳米二氧化钛的吸附回收率和重现性均最佳(RSD<5.5%);聚苯乙烯-甲基丙烯醛-氨基硫脲包覆纳米二氧化钛的吸附回收率略低于前者,但重现性较差(RSD为9.9%~11.6%);纳米二氧化钛的吸附回收率最低,重现性也较差(RSD为8.8%~11.5%)。聚合物包覆纳米二氧化钛过程中,由于粒子分散不均匀而导致包覆后的颗粒之间的组织结构有差别;纳米二氧化钛填料的吸附方式单一,颗粒易团聚;而有机物在纳米二氧化钛表面进行化学反应,对于每个粒子的机会都是等同的,随着反应的进行,纳米粒子间的分散度也不断增大,最终形成颗粒结构相同、分散均匀的表面键合配位体纳米二氧化钛复合材料。
表 3 固相萃取填料的吸附性能对比Table 3. A comparison of adsorption performance of the SPE packings填料 回收率(%) RSD(%) Sb3+ Cd2+ Ba2+ Sb3+ Cd2+ Ba2+ 表面键合配位体二氧化钛 97.94 95.65 94.04 5.4 4.7 5.1 聚合物包覆纳米二氧化钛 96.87 94.23 93.67 10.2 11.6 9.9 纳米二氧化钛 88.33 85.26 86.84 11.5 9.3 8.8 2.5 实际水样测定结果与方法检出限
取江水、湖水和地下水各1 L,用中速滤纸过滤,加硝酸至pH为1~2,在室温下超声振荡30 min,取100 mL水样,调节pH=7,以0.5 mL/min流速通过纳米TiO2/TSC复合固相萃取小柱,待小柱内溶液抽至近干,用10 mL 5 mol/L硝酸+0.25 mL三乙醇胺洗脱液以0.5 mL/min流速洗脱,用水定容至10 mL,经0.45 μm滤膜过滤后用ICP-MS测定溶液中Sb3+、Cd2+和Ba2+的浓度。
以纯水做空白溶液重复测定20次,计算标准偏差,以标准偏差的3倍除以Sb3+、Cd2+、Ba2+标准曲线的斜率计算仪器检出限,再除以浓缩倍数和回收率即得到方法的检出限。各类水样中Sb3+、Cd2+、Ba2+的含量测定值及方法检出限见表 4。Sb3+、Cd2+和Ba2+的方法检出限分别为0.061 μg/L、0.013 μg/L和0.075 μg/L。
表 4 样品测定结果(n=6)及检出限(n=20)Table 4. Analytical results (n=6) and detection limits (n=20) of the sample样品 Sb3+测定值(μg/L) Cd2+测定值(μg/L) Ba2+测定值(μg/L) 江水 0.42 0.23 2.56 湖水 1.53 3.01 10.6 地下水 0.11 0.14 0.87 加标回收率(%) 97.6~106.0 98.8~103.0 99.2~101.0 检出限(μg/L) 0.061 0.013 0.075 3. 结论
本研究将纳米二氧化钛经γ-甲基丙烯酰氧基丙基三甲氧基硅烷表面修饰后,键合醛基,接枝氨基硫脲,制得的纳米TiO2/TSC复合固相萃取填料的粒子尺寸为200~300 nm,对Sb3+、Cd2+、Ba2+的吸附容量分别为13.9 mg/g、12.9 mg/g和11.2 mg/g,吸附回收率均大于94.0%,性能优于文献[17]采用共混法制备的固相萃取填料。该固相萃取填料静态吸附Sb3+、Cd2+和Ba2+的反应符合Langmuir等温模型和Lagergren准二级动力学方程,吸附过程为化学吸附。
该项研究提出了制备有机/无机复合型固相萃取填料的新途径,优化了固相萃取操作条件,确定了固相萃取填料静态吸附离子的机理及等温模型和动力学模型。采用SPE-ICP-MS测定Sb3+、Cd2+、Ba2+的检出限分别为0.061 μg/L、0.013 μg/L和0.075 μg/L,灵敏度高、精密度高,具有较强的理论和实际应用价值。
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表 1 土壤中不同形态有机碳分离/分析方法
Table 1 Methods of separation and/or analysis of organic carbon fractions in soil
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