Spectroscopic approaches to the study of the interaction of aluminum with humic substances

Aluminum ion (Al³⁺) in the ‘free’ (aquo) state is becoming increasingly prevalent in environmental waters, especially fresh waters, as a consequence of acid rain and other environmental processes. As Al³⁺ ion is known to affect markedly a wide range of biological systems, and since the presence of Al³⁺ in humans has been linked to a number of human diseases, it is important to understand the speciation of Al³⁺ ion in natural waters. Since some of the most important complexation agents for Al³⁺ in both fresh and sea waters are members of the complex humic substances group, it is important to understand the manner in which Al³⁺ interacts with this class of molecules, especially since binding of Al³⁺ to these molecules can effectively increase the bioavailability of this toxic metal ion to biological systems. The objective of this review is to present the current state of our understanding of aqueous aluminum complexation with the most acidic members (and therefore the most likely candidates for serving as Al³⁺ complexing agents) of the humic substances group, the fulvic acids. Much of the current knowledge has been revealed by comprehensive fluorescence titration analyses. Some additional information has come from other experimental approaches, including infrared spectroscopy, nuclear magnetic resonance spectroscopy, and a variety of electrochemical approaches. In this review, we also report on the results of our recent fluorescence and IR spectroscopy survey of the interaction of metals from of all three Nieboer and Richardson categories of environmental metals (Class A, Class B and Intermediate Class) with the fulvic acid sub-group of the humic substances. This has proven helpful in understanding some of the unique spectral behaviors of the Al³⁺-fulvic acid complex vis-a-vis fulvic acid complexes with many other metal ions. The results of our fluorescence and IR experiments with the model compounds, such as salicylic and phthalic acids, have allowed confirmation of the important roles played by both salicylic acid-like sites and phthalic acid-like sites in the unique complexation of Al³⁺ to humic substances, and help to explain some of the observed spectroscopic changes associated with Al³⁺ ion complexation to humic material. From the current work, it seems clear that major sources of the deviation in spectral properties between Al³⁺ and many other metal ions (across all three Nieboer and Richardson categories) are the unusually high value of its charge density and relatively low propensity for involvement in covalent bonding interactions (i.e. a very high ionic index combined with a relatively low covalent index in the Nieboer and Richardson classification of environmental metals), as well as affinity for certain functional groups.     

Development of an automated system for isolation and purification of humic substances

Characterization of humic substances (HS) in environmental samples generally involves labor-intensive and time-consuming isolation and purification procedures. In this paper, the development of an automated system for HS isolation and purification is described. The novelty of the developed system lies in the way the multiple liquids and columns used in the isolation/purification procedure are handled in both forward and back-elution mode by solenoid valves. The automated procedure significantly reduces the total throughput time needed, from 6–7 days to 48 h, and the amount of labor to obtain purified HS for further characterization. Chemical characterization of purified HS showed that results were in good agreement with previously published values for HS from a variety of sources, including the IHSS standard HS collection. It was also shown that the general properties of HS were consistent among the different source materials (soil, waste, aquatic) used in this study. The developed system greatly facilitates isolation and characterization of HS and reduces the risk of potential (time-dependent) alteration of HS properties in the manual procedure. Figure Photograph of the rear (left) and front (right) of the automated system for the isolation and purification of humic substances Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Interaction of atrazine with Laurentian humic acid

Ultrafiltration fractionation and liquid chromatography have been applied to study the binding and hydrolysis of polar herbicide atrazine on a stoichiometrically well characterized Laurentian humic acid. The main advantage of this method over gas chromatography is the simultaneous determination of both free and bound atrazine, hydroxyatrazine and copper(II) ion with satisfactory accuracy and precision. Atrazine binding requires extensive carboxylate site protonation but the binding sites represent only a very small fraction of total carboxylate of humic acid. The results show that binding of atrazine is not competitive with binding of the hydrolysis product hydroxyatrazine, the binding capacity is reduced at higher ionic strength or by cation competition for carboxylate and the atrazine binding constant and free energy of binding can be fitted by a single value at all pH values. The differences between atrazine binding by fulvic acid and humic acid can be ascribed to the structure difference, one being a flexible linear polymer and the other a three-dimensional colloidal gel particle.     

Spectral and temporal luminescent properties of Eu(III) in humic substance solutions from different origins

Although a high heterogeneity of composition is awaited for humic substances, their complexation properties do not seem to greatly depend on their origins. The information on the difference in the structure of these complexes is scarce. To participate in the filling of this lack, a study of the spectral and temporal evolution of the Eu(III) luminescence implied in humic substance (HS) complexes is presented. Seven different extracts, namely Suwannee River fulvic acid (SRFA) and humic acid (SRHA), and Leonardite HA (LHA) from the International Humic Substances Society (USA), humic acid from Gorleben (GohyHA), and from the Kleiner Kranichsee bog (KFA, KHA) from Germany, and purified commercial Aldrich HA (PAHA), were made to contact with Eu(III). Eu(III)-HS time-resolved luminescence properties were compared with aqueous Eu³⁺ at pH 5. Using an excitation wavelength of 394 nm, the typical bi-exponential luminescence decay for Eu(III)-HS complexes is common to all the samples. The components τ₁ and τ₂ are in the same order of magnitude for all the samples, i.e., 40 ≤ τ₁ (μs) ≤ 60, and 145 ≤ τ₂ (μs) ≤ 190, but significantly different. It is shown that different spectra are obtained from the different groups of samples. Terrestrial extract on the one hand, i.e. LHA/GohyHA, plus PAHA, and purely aquatic extracts on the other hand, i.e., SRFA/SRHA/KFA/KHA, induce inner coherent luminescent properties of Eu(III) within each group. The ⁵D₀ → ⁷F₂ transition exhibits the most striking differences. A slight blue shift is observed compared to aqueous Eu³⁺ (λ_max = 615.4 nm), and the humic samples share almost the same λ_max ≈ 614.5 nm. The main differences between the samples reside in a shoulder around λ ≈ 612.5 nm, modelled by a mixed Gaussian–Lorentzian band around λ ≈ 612 nm. SRFA shows the most intense shoulder with an intensity ratio of I_612.5/I_614.7 = 1.1, KFA/KHA/SRHA share almost the same ratio I_612.5/I_614.7 = 1.2–1.3, whilst the LHA/GohyHA/PAHA group has a I_612.5/I_614.5 = 1.5–1.6. This shows that for the two groups of complexes, despite comparable complexing properties, slightly different symmetries are awaited.     

HPLC of humic substances fractionated by Flow FFF

This communication reports a study of the effect of ionic strength and electrolyte composition on fractions, separately collected by flow FFF, of a mixture of humic substances. Reverse phase HPLC analysis of three early eluting fractions suggests that the components released by the column behave as organic acids. The baseline‐resolved peaks of the first two fractions, subject to higher retention in solutions of lower pH and/or higher polarity, substantiate this suggestion. The fraction with larger components, as measured by flow FFF, also appears to contain acidic species. Their retention level, however, may not be accurately modulated by varying the mobile phase properties as these species are either totally retained in acidic phases or released before the void peak at pH ≥ 4.2. Besides showing the effective separation achieved in the flow FFF channel, this study reveals the pronounced difference in the physicochemical properties of some components of a humic mixture even with very close particle size.     

Multivariate curve resolution analysis excitation-emission matrices of fluorescence of humic substances

Excitation-emission matrices (EEM) of fluorescence of aqueous solutions of humic substances (HS), and sets of EEM acquired as function of the HS concentration, were analysed by multivariate curve resolution alternating least squares (MCR-ALS). Three types of HS samples were studied: one commercial humic acid; two samples of fulvic acid (FA) extracted from a pinewood soil; two samples of FA extracted from recycled wastes. The fluorescence measurements were carried out at HS concentration between 5 and 100 mg/L and at pH 6. The application of MCR-ALS algorithm on each individual EEM, as well as on column-wise augmented matrices, allows the identification of three major fluorophores in all HS samples analysed. The emission and excitation spectra of these fluorophores were recovered and are characteristic of each sample. Moreover, the variation of the fluorescence intensities of each fluorophore with HS concentration shows deviations from linearity at HS concentration higher than 30 mg/L, depending on the fluorophore and/or sample. This behaviour reveals the existence of inner filter effects that affect the proportionally between the fluorescent signal and concentration but do not provoke measurable distortions on the fluorescence spectra of the detected fluorophores.     

非水滴定测定烟叶中烟碱含量实验的微型化

研制了一种微型滴定装置,利用该装置(WD-COⅡ型微型滴定装置)对烟叶中烟碱含量进行非水滴定,通过数理统计方法将微型滴定管与常规滴定管的平行测定结果进行了比较,得到相同样品的一对非常接近的测定结果。证明新型微量滴定管具有较好的操作性能和精密度。     

电感耦合等离子体原子发射光谱(ICP-AES)法与EDTA滴定法联用测定含白云岩矿石中氟化钙

氟化钙的测定目前较多采用的是GB/T5195.1-2017和DZG93-05两种方法,前者测定范围≥60%,后者测定范围>3%。两种方法各有优缺点,然而却都无法满足矿石中含有白云岩时的测定。近年来,青海省地勘局大力探寻萤石矿,各矿区矿石组成、矿床类型各不相同,且氟化钙品位高低不一,尤其是青海省海西州喀雅克登矿区中常见萤石矿与白云岩共存,直接采用国家标准方法,测定结果偏高,行业标准方法的精密度和准确度也很难达到要求。本文对方法进行优化,准确称量样品0.2000g至50mL的锥形瓶中,加入10mL含钙冰乙酸(1+9),瓶口放置φ40mm的短颈漏斗,在控温电热板上100℃加热40min,取下过滤,残渣放回原锥形瓶,加入20mL混合酸,瓶口放置漏斗,于控温电热板上150℃微沸30min,将溶液转移至100mL比色管中,摇匀,澄清。在Ca 317.933 nm处,使用电感耦合等离子体原子发射光谱法(ICP-AES)测定氟化钙。根据ICP-AES的测定结果,将氟化钙质量分数大于15%的样品用EDTA滴定法再次测定。按照实验方法测定喀雅克登矿区实际样品,两种方法测定结果的相对标准偏差(RSD,n=11)分别为1.86%和0.77%,加标回收率为97.9%～102.2%,可满足含白云岩矿石中氟化钙的准确测定。     

非水介质毛细管电泳电导检测罗红霉素及其制剂

采用甲醇为分离介质，三(羟甲基)氨基甲烷-硼酸(Tris—H3BO3)为支持电解质，采用负高压，使用电导检测，对罗红霉素及其制剂进行了毛细管电泳分离检测，对电泳介质的种类、浓度、表观pH、以及操作电压和进样时问对分离的影响进行了研讨，在选定的条件下，罗红霉素的线性范围为19．0—142．0mg/L，检出限为0．8mg/L(S／N≥3)，峰面积的相对标准偏差RSD(n=6)为4．3％。3种供试品中罗红霉素的平均加标回收率分别为97．7％、94．8％、93．6％。     

Characterisation of humic substances by acid catalysed transesterification

The chemical composition of low molecular weight moieties linked to the core structures of humic substances (HS) are of substantial importance for the understanding of the chemical structures and mode of interactions of HS with other substances in the environment. In this study a novel approach to characterise certain low molecular weight compounds bound to HS is suggested. The method includes transesterification (TE) of ester and amide bound structures, and esterification (E) of free carboxylic groups using acid catalysed methanolysis followed by gas chromatography (GC)-mass spectrometry (MS) and GC-FID analysis. Methanolysis of five HS of different origin, demonstrated the presence of multifunctional hydroxy-substituted benzoic acids, hexoses, and long chain fatty acids. Based on GC-FID and addition of the internal standard before methanolysis, the total amounts of low molecular weight material could be estimated. In case of HS from the aqueous sources the yields were below 1%, whereas in case of the HS derived from lignite the yield was significantly higher. The hydrophobic long chain fatty acids constituted about one-third of this material. Principal component analysis (PCA), which was used for extended data evaluation, differentiated between the samples in terms of chemical composition.     

非水介质毛细管电泳电导法检测盐酸胺碘酮

采用非水介质毛细管电泳电导检测法对盐酸胺碘酮进行检测。探讨了缓冲溶液的种类、pH和浓度、分离电压、进样时间、进样高度等因素对检测效果的影响,建立了测定盐酸胺碘酮的新方法。用乙醇作为非水介质,在30mmol L三羟甲基氨基甲烷 15mmol L柠檬酸(pH6.90)运行缓冲溶液中,盐酸胺碘酮在5～200mg L范围内的线性回归方程为y=74.94x-7.83,r=0 999。检出限(S N=3)为0.5mg L,样品回收率为98.9%。适用于含盐酸胺碘酮的药剂的分析。     

Determination of critical micelle concentration of bile acid salts by micro-calorimetric titration

Critical micelle concentrations (CMC) of sodium salts of cholic, deoxycholic and chenodeoxycholic acids in phosphate buffer (pH 7.92) have been determined from microcalorimetric titration curves. The obtained values of 18.4±0.6, 5.3±0.2 and 7.0±0.2 mM, respectively, for Na cholate, Na deoxycholate and Na chenodeoxycholate are close to literature values obtained by other methods. CMC values for secondary micelles were also obtained. This microcalorimetric titration method gives highly reproducible results and rapid determination of CMC values of bile acid salts.     

Determination of dissolved organic matter based on UV-light induced reduction of ionic silver to metallic nanoparticles by humic and fulvic acids

We describe a novel solution-based method for the determination of dissolved organic matter (DOM) relying on the formation of silver nanoparticles (AgNPs) via photo-stimulated reduction of silver ions by humic and fulvic acids. The method is based on natural driven formation of nanoscale materials yielding a direct relationship between DOM concentration and AgNPs formation. The aqueous dispersion of the formed AgNPs show strong and uniform UV–Vis absorption bands between 450 and 550 nm irrespectively of DOM nature and properties (i.e. humic or fulvic acids). The ensuing chromatic shift accompanying the appearance of the new absorption bands is easily conceivable by a simple spectrophotometer and the bare eye, holding great promise for the on-site, instrumental-free screening of DOM levels. Under the optimum experimental conditions the determination of DOM was successfully demonstrated to various water samples with high sensitivity (<1.0 mg L⁻¹), satisfactory recoveries (87.5–123.5%) and reproducibility (5.87–6.73%).     

Ultraviolet absorbance titration for determining stability constants of humic substances with Cu(II) and Hg(II)

We describe an ultraviolet (UV) absorbance titration method that can be used to determine complexing capacities (C_L) and conditional stability constants (log K) of humic substances (HSs) with metal ions such as Cu(II) and Hg(II). Two fulvic acids (FA) and one humic acid (HA) were used for this study. UV absorbance of HSs gradually increased with the addition of Cu(II) or Hg(II) after blank correction, and these increases followed the theoretical 1:1 (ligand:metal ion) binding model. The results from the absorbance titration calculation for HSs with Cu(II) and Hg(II) compared well with those from fluorescence quenching titration. The titration of the model compound l-tyrosine with Cu(II) proved the validity of this method, and the K and C_L were within 2.3% and 7.4% of the fluorescence quenching titration. The results suggest that the UV absorbance titration can be used to study the binding capacities of HSs and/or dissolved organic matter (DOM) with trace metals. The advantages and disadvantages of the absorbance titration method were also discussed.     

Fouling of reverse osmosis and nanofiltration membranes by humic acid—Effects of solution composition and hydrodynamic conditions

Fouling of reverse osmosis (RO) and nanofiltration (NF) membranes by humic acid, a recalcitrant natural organic matter (NOM), was systematically investigated. The membrane flux performance depended on both hydrodynamic conditions (flux and cross-flow velocity) and solution composition (humic acid concentration, pH, ionic strength, and calcium concentration), and was largely independent of virgin membrane properties. While increasing humic acid concentration and ionic strength, and lowering cross-flow velocity affected flux performance moderately, severe flux reduction occurred at high initial flux, low pH, and high calcium concentration. At a calcium concentration of 1 mM, all the membranes exhibited an identical stable flux, independent of their respective intrinsic membrane permeabilities. The effect of solution composition was more significant at higher fluxes. Improved salt rejection was observed as a result of humic acid fouling, which was likely due to Donnan exclusion by humic material close to membrane surfaces. Greater rejection improvement was observed for membranes with rougher surfaces.     

Effect of humic substances on the electrochemical reduction of p -nitrophenol

p-Nitrophenol (PNP) is the main hydrolysis product of methylparathion (MP), one of the most commonly used organophosphate insecticides in the world. Such a product is very toxic for human and animals. Humic substances (HS) are natural recalcitrant organic matter found in soil and waters that have an ability to interact, immobilize, and degrade pesticides. This article presents electroanalytical and UV-Visible studies, conducted to understand the effect of the HS on the PNP reduction process and therefore to understand how the HS can influence PNP degradation in the environment. Electroanalytical results showed that the HS benefit the reduction of the nitro-group of PNP by electrocatalysis. UV-Visible spectra showed that the catalytic effect of HS occurs due to the interection between the PNP and the HS followed by a proton donor mechanism.     

Gran电位滴定法测定乳酸环丙沙星

介绍了乳酸环丙沙星测定的Gran电位滴定法。采用Gran线性函数进行电位滴定,经图解外推或线性回归处理求出计量点,可直接用于测定乳酸环丙沙星原料药的含量。该法用于多批原料药的测定,结果与药典法基本一致。     

Characterization of humic acids in sediments from dam reservoirs by pyrolysis-gas chromatography/mass spectrometry using tetramethylammonium hydroxide: Influence of the structural features of humic acids on iron(II) binding capacity

The structural features of humic acids (HAs) isolated from sediments on the bottom of dam reservoirs that can affect their binding capacities for Fe(II) were investigated by pyrolysis-gas chromatography/mass spectroscopy using tetramethylammonium hydroxide (TMAH-py-GC/MS). The binding capacities for Fe(II) increased with increasing O/C molar ratio, suggesting that the oxygen-containing functional group content plays a role in the binding of Fe(II). However, it was not possible to identify specific binding-sites for Fe(II) by TMAH-py-GC/MS analysis. Although C_16:1ω7, iso-C_15:0 and anteiso-C_17:0 fatty acids, which serve as molecular markers of anaerobic microbial activity, were detected in all of the HA samples, the contents of these acids were not correlated with binding capacities for Fe(II). However, the ratio of C_16:0 to C_16:1ω7 fatty acids, which is used as an index of anaerobic bacterial activity, increased with increasing Fe(II) binding capacities of the HAs. It thus appears likely that the activities of anaerobic bacteria on the bottom of dam reservoirs contribute to alterations in the structural features for HAs, and that this process results in increased binding capacities for Fe(II).     

Determination of isoquinoline alkaloids inThalictrum herbal drugs by non-aqueous capillary electrophoresis

Summary A rapid non-aqueous capillary electrophoresis method has been developed for the separation and determination, within 14 min, of eight isoquinoline alkaloids (berberine, palmatine, jatrorrhizine, (+)-tetrandrine, berbamine, thalifaricine, northalfine, and thalistine) in seventeen samples of the herbal drug thalictrum. A methanolic solution of sodium acetate (75mm) and acetic acid (1m) was found to be the optimum running buffer for the separation. Thalictrum Atriplex Finet et Gagep (T.AFG) was selected for further study, including investigation of recovery and precision, because this preparation contained all the isoquinoline alkaloids tested. Calibration curves were highly linear over a 20-fold concentration range and detection limits for all eight alkaloids were in the range 0.42–3.04 μg mL⁻¹.