Al Complexes Research Articles

Al(III) and Ga(III) triflate complexes as solvate ionic liquids: speciation and application as soluble and recyclable Lewis acidic catalysts.

This work reports on the first solvate ionic liquids (SILs) based on aluminium(III) and gallium(III) triflates, M(OTf)3, and triglyme (G3). Liquid-phase speciation of these new SILs was studied by multinuclear NMR spectroscopy. Across the compositional range of G3 : M(OTf)3 mixtures, both metals were found to be in a hexacoordinate environment, with both G3 and [OTf]- ligands present in the first coordination sphere, and apparently exchanging through a dynamic equilibrium. The Lewis acidity was quantified by the Gutmann acceptor number (AN) and compared to the performance of SILs as Lewis acidic catalysts in model [3 + 3] cycloadditions. Despite saturated coordination, AN values were relatively high, reaching AN = ca. 71-83 for Al-SILs and ca. 80-93 for Ga-SILs, corroborating the labile nature of the metal-ligand bonding. In a model catalytic reaction, SILs were fully soluble in the reaction mixtures, in contrast to the corresponding triflate salts. The catalytic performance of SILs exceeded that of the corresponding triflate salts, and Ga-SILs were more active than Al-SILs, in agreement with AN measurements. In conclusion, the new Group 13 SILs can be considered as soluble and catalytically active forms of their corresponding metal triflates, with potential uses in catalysis.

Low Valence Triel (I) Systems as Hydrogen Bond Acceptors and their Stability with Respect to Triel (III) Compounds.

A theoretical study of the complexes formed by carbene like Al(I), Ga(I), In(I) and Tl(I) compounds with hydrogen bond donors (HBD), XH (HCCH, HSH,HOH, HCN, HCl, HBr, HF, and HNC) have been carried out at MP2 computational level. The isolated triel(I) compounds show a negative region of the molecular electrostatic potential region associated with the triel atom suitable to interact with electron deficient groups. This region is associated to a lone pair based on the ELF analysis and to the location of the HOMO orbital. The complexes are similar to those found in nitrogen heterocyclic carbenes (NHC) with HBD. In addition, the oxidative addition reactions of those complexes to yield the corresponding valence III compounds have been characterized. The Al(III) compounds are much more stable than the corresponding Al(I) complexes. However, the stability of the triel(III) compounds decreases with the size of the triel atom and for the thallium derivatives, the Tl(I) complexes are more stable than the Tl(III) compounds in accordance with the number of the structures found in the CSD. The barrier of the TS connecting the triel(I) and triel(III) systems increases with the size of the triel atoms.

Exploring the Potential of Al(III) Photosensitizers for Energy Transfer Reactions.

Three homoleptic Al(III) complexes (Al1-Al3) with different degrees of methylation at the 2-pyridylpyrrolide ligand were systematically tested for their function as photosensitizers (PS) in two types of energy transfer reactions. First, in the generation of reactive singlet oxygen (1O2), and second, in the isomerization of (E)- to (Z)-stilbene. 1O2 was directly evidenced by its characteristic NIR emission at around 1276 nm and indirectly by the reaction with an organic substrate [e.g. 2,5-diphenylfuran (DPF)] using in situ UV/vis spectroscopy. In a previous study, the presence of additional methyl groups was found to be beneficial for the photocatalytic reduction of CO2 to CO, but here Al1 without any methyl groups exhibits superior performance. To rationalize this behavior, a combination of photophysical experiments (absorption, emission and excited state lifetimes) together with photostability measurements and scalar-relativistic time-dependent density functional theory calculations was applied. As a result, Al1 exhibited the highest emission quantum yield (64%), the longest emission lifetime (8.7 ns) and the best photostability under the reaction conditions required for the energy transfer reactions (e.g. in aerated chloroform). Moreover, Al1 provided the highest rate constant (0.043 min-1) for the photocatalytic oxygenation of DPF, outperforming even noble metal-based competitors such as [Ru(bpy)3]2+. Finally, its superior photostability enabled a long-term test (7 h), in which Al1 was successfully recycled seven times, underlining the high potential of this new class of earth-abundant PSs.

Natural background levels and driving factors of aluminum in shallow groundwater of an urbanized delta: Insight from eliminating anthropogenic-impacted groundwaters.

Knowledge on natural background levels (NBLs) of aluminum (Al) in groundwater can accurately assess groundwater Al contamination at a regional scale. However, it has received little attention. This study used a combination of preselection and statistic methods consisting of the oxidation capacity and the boxplot iteration methods to evaluate the NBL of shallow groundwater Al in four groundwater units of the Pearl River Delta (PRD) via eliminating anthropogenic-impacted groundwaters and to discuss driving factors controlling high NBLs of Al in groundwater in this area. A total of 280 water samples were collected, and 18 physico-chemical parameters including Redox potential, dissolved oxygen, pH, total dissolved solids, HCO3 -, NH4 +, NO3 -, SO4 2-, Cl-, NO2 -, F-, K+, Na+, Ca2+, Mg2+, Fe, Mn, and Al were analyzed. Results showed that groundwater Al NBLs in groundwater units A-D were 0.11, 0.16, 0.15, and 0.08 mg/L, respectively. The used method in this study is acceptable for the assessment of groundwater Al NBLs in the PRD, because groundwater Al concentrations in various groundwater units in residual datasets were independent of land-use types, but they were opposite in the original datasets. The dissolution of Al-rich minerals in sediments/rocks was the major source for groundwater Al NBLs in the PRD, and the interaction with Al-rich river water was secondary one. The high groundwater Al NBL in groundwater unit B was mainly attributed to the acid precipitation and the organic matter mineralization inducing the release of Al in Quaternary sediments. By contrast, the high groundwater Al NBL in groundwater unit C mainly was ascribed to the release of Al complexes such as fluoroaluminate from rocks/soils into groundwater induced by acid precipitation, but it was limited by the dissolution of Mg minerals (e.g., dolomite) in aquifers. This study provides not only useful groundwater Al NBLs for the evaluation of groundwater Al contamination but also a reference for understanding the natural geochemical factors controlling groundwater Al in urbanized deltas such as the PRD. PRACTITIONER POINTS: The natural background level (NBL) of groundwater aluminum in the Pearl River Delta (PRD) was evaluated. The dissolution of aluminum-rich minerals in sediments/rocks was the major source for groundwater aluminum NBLs in the PRD. The acid precipitation and organic matter mineralization contribute to high groundwater Al NBL in the groundwater unit B. The acid precipitation contributes to high groundwater Al NBL in the groundwater unit C, while dissolution of magnesium minerals limits it.

Investigating the Effect of Cyclodextrin Nanosponges and Cyclodextrin-Based Hydrophilic Polymers on the Chemical Pharmaceutical and Toxicological Profile of Al(III) and Ga(III) Complexes with 5-Hydroxyflavone

In the present study, the complexes of aluminum and gallium with 5-hydroxyflavone were evaluated for their interaction with cyclodextrin polymers, as well as for the pharmacological effect of their inclusion. The cyclodextrin polymers were synthesized using diphenylcarbonate as a crosslinking agent, resulting in a lipophilic nanosponge (DPCNS), and pyromellitic dianhydride, resulting in a hydrophilic polymer (PMDACD). The inclusion complexes were synthesized and characterized via IR spectrometry and thermal analysis. The effect on the solubility of the metal complexes was also studied, where the hydrophobic nanosponge did not lead to an increase in solubility, but on the contrary, in the case of Al, it decreased; meanwhile, in the case of the hydrophilic polymer, the solubility of the metal complexes increased with the amount of polymer added. The cytostatic effect of inclusion complexes was investigated on two cell lines with different localizations, human colon adenocarcinoma (LoVo) and human ovarian adenocarcinoma (SKOV-3). The cytostatic efficacy is increased compared to simple complexes with efficacy on LoVo cells. Compared between the two metals, gallium complexes proved to be more active, with the efficacy of gallium complexes with the PMDACD being approximately the same as that of cisplatin, an antitumor agent used in therapy.

Factors Governing Site and Charge Density of Dissolved Natural Organic Matter

Rising organic charge in northern freshwaters is attributed to increasing levels of dissolved natural organic matter (DNOM) and changes in water chemistry. Organic charge concentration may be determined through charge balance calculations (Org.−) or modelled (OAN−) using the Oliver and Hruška conceptual models, which are based on the density of weak acid functional sites (SD) present in DNOM. The charge density (CD) is governed by SD as well as protonation and complexation reactions on the functional groups. These models use SD as a key parameter to empirically fit the model to Org.−. Utilizing extensive water chemistry datasets, this study shows that spatial and temporal differences in SD and CD are influenced by variations in the humic-to-fulvic ratio of DNOM, organic aluminum (Al) complexation, and the mole fraction of CD to SD, which is governed by acidity. The median SD values obtained for 44 long-term monitored acid-sensitive lakes were 11.1 and 13.9 µEq/mg C for the Oliver and Hruška models, respectively. Over 34 years of monitoring, the CD increased by 70%, likely due to rising pH and declining Al complexation with DNOM. Present-day median SD values for the Oliver and Hruška models in 16 low-order streams are 13.8 and 15.8 µEq/mg C, respectively, and 10.8 and 12.5 µEq/mg C, respectively, in 10 high-order rivers.

On the identification of favourable factors for corrosion inhibition of aluminium by 8-hydroxyquinoline and its derivatives: DFT and electrochemical studies

Computational and electrochemical experimental approaches were combined to identify factors which would be favourable or detrimental to corrosion inhibition of aluminium by 8-hydroxyquinoline (8Hq) and its sulfonic derivative (8HqSH). 8HqSH exhibited a strong adsorption on Al(111) but a greater solubility in water than 8Hq, including Al complexes, which might increase Al corrosion. This was confirmed by the electrochemical measurements. In the presence of 8HqSH, an acceleration of the aluminium dissolution was observed, whereas, in the presence of 8Hq, a protective film was formed on the Al surface.

Expanding the Scope of Aluminum Chemistry with Noninnocent Ligands.

ConspectusAluminum is the most abundant metal in the earth's crust at 8%, and it is also widely available domestically in many countries worldwide, which ensures a stable supply chain. To further the applications of aluminum (Al), such as in catalysis and electronic and energy storage materials, there has been significant interest in the synthesis and characterization of new Al coordination compounds that can support electron transfer (ET) and proton transfer (PT) chemistry. This has been achieved using redox and chemically noninnocent ligands (NILs) combined with the highly stable M(III) oxidation state of Al and in some cases the heavier group 13 ions, Ga and In.When ligands participate in redox chemistry or facilitate the breaking or making of new bonds, they are often termed redox or chemically noninnocent, respectively. Al(III) in particular supports rich ligand-based redox chemistry because it is so redox inert and will support the ligand across many charge and protonation states without entering into the reaction chemistry. To a lesser extent, we have reported on the heavier group 13 elements Ga and In, and this chemistry will also be included in this Account, where available.This Account is arranged into two technical sections, which are (1) Structures of Al-NIL complexes and (2) Reactivity of Al-NIL complexes. Highlights of the research work include reversible redox chemistry that has been enabled by ligand design to shut down radical coupling pathways and to prevent loss of H2 from unsaturated ligand sites. These reversible redox properties have in turn enabled the characterization of Class III electron delocalization through Al when two NIL are bound to the Al(III) in different charge states. Characterization of the metalloaromatic character of square planar Al and Ga complexes has been achieved, and characterization of the delocalized electronic structures has provided a model within which to understand and predict the ET and PT chemistry of the NIL group 13 compounds. The capacity of Al-NIL complexes to perform ET and PT has been employed in reactions that use ET or PT reactivity only or in reactions where coupled ET/PT affords hydride transfer chemistry. As an example, ligand-based PT reactions initiate metal-ligand cooperative bond activation pathways for catalysis: this includes acceptorless dehydrogenation of formic acid and anilines and transfer hydrogenation chemistry. In a complementary approach, ligand based ET/PT chemistry has been used in the study of dihydropyridinate (DHP-) chemistry where it was shown that N-coordination of group 13 ions lowers kinetic barriers to DHP- formation. Taken together, the discussion presented herein illustrates that the NIL chemistry of Al(III), and also of Ga(III) and In(III) holds promise for further developments in catalysis and energy storage.

Specific Discrimination Polymerization for Highly Isotactic Polyesters Synthesis.

Isotactic polymers have emerged with unique and excellent properties in material sciences. Specific discrimination polymerization provides an ideal pathway to achieve highly isotactic polymers from their racemic monomers, which is of great significance and a challenge in polymeric chemistry. Although an enantioselective catalyst-mediated asymmetric kinetic resolution polymerization (AKRP) process makes it possible, a general and well-defined strategy for catalyst design is still rarely reported. Here, based on a novel dual-ligand strategy, a new type of chiral (BisSalen)Al complex with high enantioselectivity has been described, in which perfect AKRP of racemic phenethylglycolide (Pegl) is achieved for the first time. The more confined asymmetric microenvironment formed by a dual ligand is the key to improve the enantioselectivity of the original catalyst. To illustrate the generality of this strategy, a series of (BisSalen)Al complexes with homo- or heterodual ligands were designed for the AKRP of Pegl.

A simple vitamin B6-based fluorescent chemosensor for selective and sensitive Al3+ recognition in water: A spectral and DFT study

This paper describes the development and characterization of a hydrazone-type chemosensor for the detection of Al3+ ions. Hydrazone is based on pyridoxal 5′-phosphate and pyrazine-2-carbohydrazide and exhibits a blue emissive turn-on response upon binding to Al3+. The aggregation-induced emission enhancement phenomenon of the sensor and its Al3+ complex was investigated in mixed DMSO/H2O solvents. The sensor is capable of detecting extremely low levels of Al3+ ions (8 nM) in almost pure water, making it suitable for analyzing water with good ion recovery. Quantum chemical calculations were used to optimize the structure and electron absorption spectra of the sensor and its Al complex. The chemosensor has low toxicity levels (up to 50 μM) on tumor and non-tumor cell lines, which makes it a potential candidate for bioimaging use. This study provides valuable insights into the development of fluorescence sensors for precise recognition of metal ions.

Membrane fouling in engineering nanofiltration process for drinking water treatment: The spatial and chemical aspects

Nanofiltration (NF) has become an established process for advanced treatment of drinking water, while membrane fouling remains as the main issue slowing the widespread application of the process. In this study, the key foulants were identified, and their spatial distribution and potential interactions were investigated based on a two-stage NF engineering project for municipal drinking water treatment. Over half a year of operation showed that the membrane fouling was mostly chemically reversible. Characterizations of the cleaning waste revealed that inorganic fouling dominated at both stages, which was mainly caused by the deposition of Al, Ca, and Si. While the overall fouling was much more severe at Stage 2, the organic fouling was the reverse. Autopsy of one full-size membrane element from each stage indicated that in addition to Al, P and humic acid-like compounds were abundant in the fouling layer for both stages despite the spatial differences. The spatial distribution of foulants was influenced by both concentration effects and presence of feed channel spacers. For the free membrane area, inorganic foulants and humic acid-like compounds had higher contents at Stage 2 than Stage 1. While for the membrane area contacting with spacers, Al and P at Stage 1 were 0.74 and 1.51 times higher than those at Stage 2, respectively. Moreover, statistical analysis, complexation experiments and engineering operation strongly supported that the residual Al and natural organic matter (NOM) were key foulants for the NF process, mainly by forming Al-NOM complexes. Besides the Al-NOM complexes, condensed phosphate, primarily sourced from the dosed phosphate-containing antiscalants, might also co-precipitate with Al or Al-NOM complexes.

Undescribed matrine-type alkaloids from Sophora alopecuroides with anti-inflammatory activity

A phytochemical investigation on the alkaloid fractions of Sophora alopecuroides L. led to the production of 11 undescribed matrine-type alkaloids, sophaloseedlines I–S (1−11), 12 known analogs (12−23), and an unexpected artificial matrine-derived Al(III) complex (24). The corresponding structures were elucidated by the interpretation of spectroscopic analyses, quantum chemical calculation, and six instances (1−4, 18, and 24), verified by X-ray crystallography. The biological activities screening demonstrated that none of the isolates exhibited cytotoxicity against four human cancer cell lines (HepG2, A549, THP-1, and MCF-7) and respiratory syncytial virus (RSV) at 50 μM, while moderate anti-inflammatory activity with IC50 value from 15.6 to 47.8 μM was observed. The key structure-activity relationships of those matrine-type alkaloids for anti-inflammatory effects have been summarized. In addition, the most potent 7-epi-sophoramine (19) and aluminum sophaloseedline T (24) could effectively inhibit the release of pro-inflammatory factors (TNF-α, IL-6, and IL-1β), as well as the expression of iNOS and COX-2 proteins.

Synthesis of Mono- and Dinuclear Aluminum Complexes Bearing Aromatic Amino-Phenolato Ligands: A Comparative Study in the Ring-Opening Polymerization of Cyclohexene Oxide.

Dinuclear aluminum methyl complexes bearing aromatic diamine-bridged tetra(phenolato) ligands and the mononuclear aluminum methyl complex with the phenylamine-bridged bis(phenolato) ligand have been synthesized and characterized. Structure determination revealed that the Al-Al distances in these dinuclear aluminum complexes are tunable by the choice of the suitable aromatic backbone of the diamine-bridged tetra(phenolato) ligands. The catalytic behaviors of these mono- and dinuclear aluminum complexes for cyclohexene oxide (CHO) polymerization were investigated. The activities of these dinuclear Al complexes were observed to increase with the decrease of Al-Al distances, and the dinuclear Al complexes appeared to have better catalytic activity than the mononuclear Al complex, even if the Al-Al distance is as long as 9.401 Å. Dinuclear aluminum complex 2, with the shortest Al-Al distance (7.236 Å), showed the highest activity toward CHO polymerization with TOFs up to 6460 h-1 in neat CHO at 30 °C. Furthermore, comparative kinetic studies revealed that the polymerization is first-order for CHO concentration, and the reaction orders for initiator concentration are different for the mono- and dinuclear Al complexes. The polymerization mechanism study revealed that both the methyl and phenolate groups were involved in the initiation process.

Metal cation concentrations improve understanding of controls on soil organic carbon across a precipitation by vegetation gradient in the Patagonian Andes

Tephra-derived soils retain more organic carbon (C) than soils formed from any other parent material, but this C may be sensitive to changes in climate and land use. Here we evaluate the effects of precipitation, temperature, and afforestation on extractable metals and organic C storage in young tephra-derived soils in a temperate climate. We conducted our investigation across five sites in the Patagonian Andes that vary from 250 mm to 2200 mm mean annual precipitation, and 12 to 9.7 ℃ mean annual temperature from east to west. At each of the five sites are paired plots of natural vegetation, varying from grasses and shrubs at the dry sites to closed-canopy forest at the wet, and stands of Pinus ponderosa planted in monocultures 35 years prior to sampling. Previous research at these sites showed that aboveground net primary production and soil organic C increased with rainfall, but total soil organic C content was lower in pine plantations than natural vegetation. Here we assess whether variation in precipitation and vegetation type also affect soil mineral properties that promote soil C stabilization. Soils were collected to the depth of auger refusal and extracted with 0.5 M HCl for 24 h to target the combined exchangeable and adsorbed metals, including secondary short-range-ordered mineral phases and the plant available pools of Mg, Ca, and K. Pine afforestation lowered concentrations of HCl-extractable K (p < 0.1) and Ca (p < 0.01) within the top 0 – 30 cm. Other elements, while not affected by vegetation type, did respond to the rainfall gradient. Al, Si, P, and Mn all increased in the surface soils with increasing rainfall (p < 0.01), suggesting the development of short-range-order volcanic mineral phases that retain nutrients such as P and Mn. The addition of Al and Ca in the linear model to describe soil organic C explained more of the total variance than rainfall and vegetation type alone, indicating the importance of Al complexes and cation (Ca) bridging with secondary minerals to soil C retention. Importantly, the lower concentration of Ca in planted pine soils may signal a long-term decrease in the potential soil C stored in afforested soils due to a lower capacity for cation bridging. Our results show that the chemistry of these young tephra soils is dynamic, responding to both precipitation and afforestation in distinct ways with potential long-term impacts on nutrient cycling and C storage.

Smelting of Fe–Si–Mn–Al Complex Alloy Using High-Ash Coal

Smelting of Fe–Si–Mn–Al Complex Alloy Using High-Ash Coal

Mechanistic Studies on Aluminum-Catalyzed Ring-Opening Alternating Copolymerization of Maleic Anhydride with Epoxides: Ligand Effects and Quantitative Structure-Activity Relationship Model.

Previous work has indicated that aluminum (Al) complexes supported by a bipyridine bisphenolate (BpyBph) ligand exhibit higher activity in the ring-opening copolymerization (ROCOP) of maleic anhydride (MAH) and propylene oxide (PO) than their salen counterparts. Such a ligand effect in Al-catalyzed MAH-PO copolymerization reactions has yet to be clarified. Herein, the origin and applicability of the ligand effect have been explored by density functional theory, based on the mechanistic analysis for chain initiation and propagation. We found that the lower LUMO energy of the (BpyBph)AlCl complex accounts for its higher activity than the (salen)AlCl counterpart in MAH/epoxide copolymerizations. Inspired by the ligand effect, a structure-energy model was further established for catalytic activity (TOF value) predictions. It is found that the LUMO energies of aluminum chloride complexes and their average NBO charges of coordinating oxygen atoms correlate with the catalytic activity (TOF value) of Al complexes (R2 value of 0.98 and '3-fold' cross-validation Q2 value of 0.88). This verified that such a ligand effect is generally applicable in anhydride/epoxide ROCOP catalyzed by aluminum complex and provides hints for future catalyst design.

A fluorescent and colorimetric probe for sensing trace water in ethanol and methanol based on an Al (III) complex of rhodamine-B Schiff base

A novel fluorescent and colorimetric probe was synthesized by the complexation of rhodamine-B Schiff base (L) and Al3+, which could be used to quickly detect trace water in ethanol and methanol. L showed a remarkable fluorescence enhancement and colorimetric responses upon reaction with Al3+. However, the intensity was significantly quenched upon the addition of trace water to the L-Al3+ solution, and the degree of quenching (1-I/I0) reached 94 % in ethanol and 88 % in methanol. Meanwhile, a significant change in color was also observed by the naked-eye from pink to colorless in the L-Al3+ solution. The fluorescent and colorimetric changes indicated that water could induce the dissociation of the L-Al3+ complex. Moreover, the L-Al3+ complex exhibited a good anti-interference fluorescence response to trace water in ethanol and methanol compared with other solvents. Also, the limit of detection (LOD) for ethanol and methanol were calculated as 0.0053 % v/v and 0.0080 % v/v respectively based on 3.3δ/k. Furthermore, the sensing mechanism of the L-Al3+ complex toward trace water was investigated by 1H NMR, HR-MS, FTIR, pH and AIE. Additionally, the L-Al3+ complex showed potential application in the detection of trace water in real ethanol methanol.

Synthesis and Characterization of Phosphate‐Catecholate Chelated Nd(III), Zr(IV), and Al(III) Complexes

AbstractMetal phosphates are important catalysts and materials in synthesis chemistry. Herein, we describe the synthesis and characterization of phosphate‐catecholate chelated Nd(III), Zr(IV) and Al(III) chlorides (2–5). These species are achieved via ethyl chloride elimination reaction of oxophosphoranes with corresponding metal chlorides. The product 2–5 represent a new serial of monometallic and bimetallic phosphate‐catecholate chelated metal complexes stabilized by both P−O and catecholate‐O donors. These findings pave the way for future explorations of such species in catalysis.

Silicon Incorporation Reduces the Reactivity of Short-range Ordered Aluminosilicates Toward Organic Acids

The structure and composition of short-range ordered aluminosilicates (SROAS) may control their affinity for organic acids with potential effects on soil organic matter stabilization. Adsorption mechanisms of model organic acids were studied to resolve the effect of Si incorporation. Adsorption of oxalic, salicylic, and octanoic acid on Al-rich (Al:Si = 3.7) and Si-rich (Al:Si = 1.4) SROAS was quantified by analyses of dissolved organic carbon using catalytic high-temperature combustion. The initial pH of 5 and 6.5 increased to 6.3–8.2 during adsorption of oxalic and salicylic acid, demonstrating hydroxyl release by ligand exchange. Minor changes in pH indicated weak interactions of octanoic acid with both SROAS. Adsorbates were characterized by Fourier-transform infrared spectroscopy. Asymmetric stretching of carboxylate groups at 1720 and 1700 cm–1, and symmetric stretching at 1430 cm–1 evinced the formation of chelate complexes for oxalic acid. An absorption band centered at 1545 cm–1 indicated partial inner-sphere binding of salicylic acid on both SROAS. Silicon-rich SROAS adsorbed 80–90% less than Al-rich SROAS, suggesting that adsorption of oxalic and salicylic acid was controlled by surface aluminol groups. Fast kinetics of oxalate adsorption on Al sites was studied by a conductivity-based stopped-flow technique. Ligand exchange proceeded at a rate constant of 3.5 s–1 (25°C), similar to solute Al complexation, with an activation energy of up to 34.1 kJ mol–1. A slow process with a rate constant of 0.13 s–1 (25°C) was attributed to diffusion of oxalate at the surface or into SROAS particles. As supported by structural characterization of Si-rich SROAS, the much lower susceptibility of Si-rich SROAS to ligand exchange relates to Al speciation. The formation of tetrahedral Al precludes its complexation by carboxyl groups.

Phytotoxic Effects of Al on Root Growth Are Confounded in the Presence of Fulvic and Humic Acids

Background and Aims: In acidic soils, aluminum (Al) toxicity remains a critical crop limitation that can be ameliorated by organic amendments through Al complexation with high-molecular-weight carbon compounds, particularly fulvic and humic acids (FA and HA) However, no research discriminates between the direct effects of FA and HA on plant growth and the indirect effect that occurs through ameliorating Al toxicity. This study delineates the direct and indirect effects of FA and HA on plant growth. Methods: Eucalyptus and Hay FA and HA, and Al effects on maize (Zea mays) root growth were investigated using dilute nutrient solution. Five Al concentrations (0–270 µM) were combined with four organic acid (OA) treatments, including Nil-OA, FA40, and HA40 (each at 40 mg C L−1) and a combined treatment FA40HA40 (80 mg C L−1). Results: Eucalyptus FA and HA stimulated root growth by ~20% compared with root growth in the Nil-OA (17.4 cm). In the absence of Al, Hay FA and HA inhibited root growth (by ~20%) compared with the Nil-OA but the addition of Al resulted in stimulation of root growth. In the presence of FA and HA, root growth was not inhibited by nominally toxic monomeric Al (Al3+) concentrations (~20 µM Al). However, when expressed on a relative basis to remove the direct effect of the ligand, the response was consistent with Al toxicity. Conclusions: The effects of FA and HA were either inhibitory or stimulatory depending on the source while both sources of FA and HA mitigated Al toxicity through complexation. The study provides mechanistic data that highlights limitations of soil bioassays where the direct effects of organic ligands on root growth are confounded with the indirect effect of their reduction of Al toxicity. These two independent processes must be considered in evaluating the amelioration of Al by organic amendments.