Aluminosilicate Clay Research Articles

Mesoporous Metal Catalysts Templated on Clay Nanotubes

Abstract Halloysite is a natural tubular aluminosilicate clay of ca. 50 nm diameter and 0.5–1.5 micrometers in length. The nanoarchitectural modification of halloysite inner/outer surfaces can be achieved through supramolecular and covalent interactions exploiting its different inside/outside chemistry (Al2O3/SiO2). The tubular morphology makes halloysite a prospective nanotemplate for core-shell structured mesoporous catalysts. Catalytic metals can be incorporated on the nanotubes’ outer surface or in the inner lumens with selective metal binding. 2–5 nm diameter Au, Ag, Pt, Pd, Co, Ru, Cu-Ni, Fe2O3, CoxBy, CdS, and CdxZn1−xS particles were templated on halloysite. In this work, CdS and Ru-containing halloysite based nanocatalysts were synthesized via modification with organic ligands and microwave-assisted wetness ion impregnation. The catalytic hydrogenation of benzene and its homologues as well as phenol was performed. The impacts of the core-shell architecture, the metal particle size and seeding density were optimized for high reaction efficiency. An efficient Co-halloysite catalyst was formed using azines as ligands, and it contained 16 wt. % of cobalt with hydrogen evolution rate of 3.0 L/min × g(cat). The mesocatalysts produced are based on a safe and cheap natural clay nanomaterial and may be scaled-up for industrial applications.

Defluoridation of groundwater using mechanochemically-activated clay soil

Abstract High concentrations of fluoride in drinking water are reported to be hazardous to human health. This study was conducted to assess the efficiency of mechanochemically-activated aluminosilicate clay soil in removing fluoride from groundwater. The soil was subjected to physicochemical transformation through mechanochemical activation for different time intervals (5, 10, 15 and 30 min). The highest specific surface obtained was about 50 m2/g, with the sample activated for 30 minutes. Fourier Transform Infrared (FT-IR) analyses of samples showed an increase in absorbance by Si-O-H groups at 510 cm−1 with increasing milling time. X-ray diffraction (XRD) analyses revealed that, at 30 minutes milling time, peak broadening was intensified while reflection peak intensities decreased. X-ray fluorescence (XRF) spectrometry showed that silica and alumina were the major clay soil components. Using a dosage of 0.6 g/100 mL of activated clay, a maximum fluoride removal of 41% was achieved using the batch activated for 30 minutes on water spiked with 9 mg-F/L at pH 2.41. The adsorption data fitted both the Langmuir and Freundlich isotherms, but only the pseudo-second-order kinetic, showing chemisorption fluoride removal.

Synthesis and Characterization of In-Situ-Prepared Nanocomposites Based on Poly(Propylene 2,5-Furan Dicarboxylate) and Aluminosilicate Clays.

Poly(propylene 2,5-furan dicarboxylate) (PPF), or poly(trimethylene 2,5-furan dicarboxylate) (PTF), is a biobased alipharomatic polyester that is expected to replace its fossil-based terephthalate (PPT) and naphthate (PPN) homologues. PPF possesses exceptional gas barrier properties, but its slow crystallization rate might affect its success in specific applications in the future. Therefore, a series of PPF based nanocomposites with the nanoclays Cloisite®-Na (MMT), Cloisite®-20A (MMT 20A), and halloysite nanotubes (HNT) were synthesized via the in situ transterification and polycondensation method. The effect of the nanoclays on the structure, thermal, and crystallization properties of PPF was studied with several methods including infrared spectroscopy (IR), Nuclear Resonance Spectroscopy (1H-NMR), Wide Angle X-ray Diffraction (WAXD), Thermogravimetric Analysis (TGA), and Differential Scanning Calorimetry (DSC). The insertion of the nanofillers in the polymer matrix altered the crystallization rates, and TGA results showed good thermal stability, since no significant mass loss occurred up to 300 °C. Finally, the degradation mechanism was studied in depth with Pyrolysis-Gas Chromatography/Mass Spectroscopy, and it was found that β-scission is the dominant degradation mechanism.

Variations of trace elements under hydrological conditions in the Min River, Eastern Tibetan Plateau

In order to better understand the relative importance of hydrologic variation and anthropogenic disturbance and their complex interactions within the trace elemental geochemical cycle, water samples were collected monthly over 1 year in the Min River, eastern Tibetan Plateau, and analyzed for trace element composition. The dissolved trace elements exhibited different relationships with increasing discharge compared with major elements. The elements analyzed can be divided into three groups according to their behavior in response to changing discharge: (1) elements that showed weak positive correlation with discharge, e.g. Cu, V, and Ba; (2) elements that exhibited weak negative correlation with discharge, including Rb, Sr, Pb, Sb, Zn, Cr, Cd, and U; and (3) elements that displayed no significant correlation with variation in discharge, e.g. Ti, Fe, Co, Ni, and As. Cu was strongly affected by anthropogenic activities and flushed into the river with increasing discharge. Ba has a strong solubility in the terrestrial environment, dissolved quickly, and was released into the river. The positive relationship between V concentration and discharge may be attributed to secondary reactions, such as precipitation and adsorption on oxides and aluminosilicate clays. Conservative behavior had an impact on the geochemical behavior of Sr and Rb across hydrologic variation. Pb, Zn, Sb, Cd, and Cr underwent a mild dilution effect connected with anthropogenic activities. The chemostatic behavior of U was regulated by carbonate dissolution and biological uptake. In addition, higher temperatures enhanced biotic activities, affecting the concentrations of Fe and Ni. The relationship between power law slopes and coefficient of variation for discharge and solute concentration suggests that concentrations of trace elements vary significantly with increasing discharge compared with major elements. Silicate mineral weathering had less effect on the fluvial solutes with increasing discharge. Mining activity may exert an additional control on concentration–discharge dynamics of anthropogenic trace elements.

Formation and Coloring Mechanism of Typical Aluminosilicate Clay Minerals for CoAl2O4 Hybrid Pigment Preparation.

Different kinds of aluminosilicate minerals were employed to fabricate CoAl2O4 hybrid pigment for studying its formation and coloring mechanism. It revealed that the color of the obtained hybrid pigments was determined by the content of Al2O3 and lightness of clay minerals. The higher the Al2O3 content and the lightness of clay minerals, the better the color parameters of hybrid pigments. During the preparation of hybrid pigments, CoAl2O4 nanoparticles were confined to be loaded on the surface of the aluminosilicate minerals, which effectively prevented from the aggregation and the size increase of CoAl2O4 nanoparticles. What's more, aluminosilicate mineral might be an ideal natural aluminum source to compensate the aluminum loss due to the dissolution of Al(OH)3 at alkaline medium during precursor preparation, keeping an optimum molar ratio of Co2+/Al3+ for formation of spinel CoAl2O4 pigments in the process of high-temperature crystallization.

Removal of heavy-metals from wastewater using a hydrous alumino-silicate mineral from Kenya

Heavy metals’ discharge into the environment continues to pose grave concerns around the world. The efficacy of a hydrous alumino-silicate clay (AlSiM) coming obtained from some regions of Kenya to sorb heavy-metal ions from water has been evaluated in batch tests taking into account changes in adsorbent dose, pH, contact time, and temperature. Complete metal removals, from water containing up to 66 mg/L of Pb(II) was achieved using this material at pH value of 5 over a temperature range of 289–333 K. The adsorption data fitted both the Langmuir and the Dubinin-Radushkevich isotherms with R 2 > 0.99. The D-R adsorption energy (−11.7 kJ/mol) indicated that chemisorption was the primary reaction in the adsorption process and the derived ∆ G 0 value (−7.45 kJ/mol) was consistent with the spontaneity of the adsorption process. The kinetic analyses indicated a film-diffusion and surface-chemisorption controlled process. Verification of the initial results on heavy metals-containing wastewaters obtained from a tannery and a leather processing industries revealed excellent adsorption efficacies of AlSiM for Cr 3+ (99−100%), Fe 3+ (96−98%), Mn 2+ (85−97%) and Zn 2+ (78−86%). The use of AlSiM as a plausible low-cost adsorbent for heavy-metal decontamination of industrial effluents has therefore been demonstrated. KEY WORDS : Adsorption, Clays, Dubinin-Radushkevich, Heavy metals, Isotherms, Kinetics, Langmuir, Wastewater Bull. Chem. Soc. Ethiop. 2018 , 32(1), 39-51 DOI: https://dx.doi.org/10.4314/bcse.v32i1.4

Occupational exposure during handling and loading of halloysite nanotubes – A case study of counting nanofibers

Halloysite nanotubes (HNTs) are abundant naturally-occurring hollow aluminosilicate clay mineral fibers with a typical diameter < 100 nm and an aspect ratio of up to 200. Here we assessed the potential inhalation exposure to HNTs in an industrial research laboratory. Inside a fume hood, ten times 100 g of HNTs were poured at rate of 0.5 kg min−1, which increased concentrations from the background level up to 2900 cm−3 and 6.4 μm2 cm−3. Inside the fume hood, the respirable mass concentration was 143 μg m−3 including background particles. Outside the fume hood we did not measure elevated concentrations. We classified 1895 particles according to their length and aspect ratio. Five particles were in aspect ratio > 3 and in length > 2 μm. These particles were agglomerated and/or aggregated particles where the longest individual fiber was 2 μm in length. The occupational exposure limits for refractory mineral fibers vary from 0.1 to 2 fibers cm−3. Following standard protocols for fiber analysis, detection of 0.1 fibers cm−3 would require analysis on 4 × 104 images when the filter loading is good. Thus, the fiber sampling and quantification procedures needs to be improved significantly if nanofibers <100 nm in diameter are included in regulatory exposure assessment. Due to very limited toxicological information of HNTs we recommend avoiding inhalation exposure.

Geopolymer assembly by emulsion templating: Emulsion stability and hardening mechanisms

This work investigates emulsion templating to synthesize hexadecane oil/geopolymer composites. In a system with hexadecane as the internal (dispersed) phase and an alkali activated continuous phase without added surfactant, adding aluminosilicate clay particles does not increase resistance against creaming or coalescence, while adding a surfactant (L35 or CTAB) stabilizes the solid-liquid interface. Infrared studies and rheological studies of the associated geopolymerization determined that the presence of the organic phase or surfactant has no significant effect on the geopolymerization kinetics, as determined by the change in time of the Si-O-T IR stretching frequency and the rheological moduli involved during the process. The stabilization of the organic template is reminiscent of Pickering emulsion even though we employ a much greater amount of inorganic material for geopolymer formation. Although the addition of surfactant has a significant effect on the behavior of the paste, the percolation of the network remains unmodified, highlighting the fact that the phenomenon is not dependent on viscosity. Finally, rheological measurements were used to obtain the mass fractal dimension of the as-made gel network, which is able to differentiate the interfacial effect between surfactant molecules with a slightly denser interphase when a cationic surfactant is used.

Synthesis and physicochemical characterization of beta zeolite–bentonite composite materials for shaped catalysts

Composite materials for shaped catalysts were prepared from three commercial beta zeolites using bentonite as an aluminosilicate clay binder.

Retention of contaminants Cd and Hg adsorbed and intercalated in aluminosilicate clays: A first principles study.

Layered clay materials have been used to incorporate transition metal (TM) contaminants. Based on first-principles calculations, we have examined the energetic stability and the electronic properties due to the incorporation of Cd and Hg in layered clay materials, kaolinite (KAO) and pyrophyllite (PYR). The TM can be (i) adsorbed on the clay surface as well as (ii) intercalated between the clay layers. For the intercalated case, the contaminant incorporation rate can be optimized by controlling the interlayer spacing of the clay, namely, pillared clays. Our total energy results reveal that the incorporation of the TMs can be maximized through a suitable tuning of vertical distance between the clay layers. Based on the calculated TM/clay binding energies and the Langmuir absorption model, we estimate the concentrations of the TMs. Further kinetic properties have been examined by calculating the activation energies, where we found energy barriers of ∼20 and ∼130 meV for adsorbed and intercalated cases, respectively. The adsorption and intercalation of ionized TM adatoms were also considered within the deprotonated KAO surface. This also leads to an optimal interlayer distance which maximizes the TM incorporation rate. By mapping the total charge transfers at the TM/clay interface, we identify a net electronic charge transfer from the TM adatoms to the topmost clay surface layer. The effect of such a charge transfer on the electronic structure of the clay (host) has been examined through a set of X-ray absorption near edge structure (XANES) simulations, characterizing the changes of the XANES spectra upon the presence of the contaminants. Finally, for the pillared clays, we quantify the Cd and Hg K-edge energy shifts of the TMs as a function of the interlayer distance between the clay layers and the Al K-edge spectra for the pristine and pillared clays.

Environmental Remediation of Chlorinated Hydrocarbons Using Biopolymer Stabilized Iron Loaded Halloysite Nanotubes

Dense nonaqueous phase chlorinated compounds such as trichloroethylene (TCE) and tetrachloroethylene (PCE) are widespread groundwater and soil contaminants which cause long-term environmental pollution. Extensive efforts have been carried out to develop materials for in situ remediation particularly using nanoscale zerovalent iron (NZVI) to reduce TCE to relatively innocuous products such as ethane and ethylene. A novel technology is described here that uses earth-abundant natural tubular aluminosilicate clays known as halloysite (HNT) to support NZVI. These systems are efficient at the reductive dechlorination of such chlorinated hydrocarbons indicating a pseudo-first order rate constant of 0.1 L g–1 h–1 with NZVI particle size between 5 and 10 nm. The adsorption of the naturally derived polyelectrolytes chitosan and carboxymethyl cellulose on the surface of HNT provides easy dispersibility in aqueous solutions and colloidal stability to the NZVI supported on HNT, with chitosan adsorption leading to stab...

Life cycle assessment of emergent masonry blocks

This life cycle assessment evaluates the environmental impacts of two emergent masonry blocks designed to serve as sustainable replacements for conventional masonry blocks. The emergent blocks offer the same strength, durability, and form as conventional structural concrete blocks, and include hollow cores to enable placement of reinforcement in accordance with standard construction practices. The first emergent block consists of stabilized engineered soil that is compacted to reduce the use of ordinary Portland cement and eliminate curing. The second block, which is also compacted, contains alkali activators that promote a geopolymerization reaction among naturally occurring aluminosilicate clays, eliminating the need for ordinary Portland cement. QuantisSuite 2.0 software is used to perform a cradle to gate life cycle assessment comparing the environmental impacts of the two blocks to conventional concrete blocks across a range of indicators. The compacted stabilized soil block offers 46% less embodied carbon than conventional concrete block, which it outperforms in all categories except water consumption. The compacted alkali-activated block reduces embodied carbon by 42% when compared to concrete block, but has a higher impact on ecosystem, water withdrawal and resources. In terms of ecosystem and health indicators, the compacted stabilized soil block is the preferred building block, while conventional concrete block architectural blocks are the most water efficient. As understanding of alkali activation improves and manufacturing of compacted block is optimized, further environmental benefits are possible in both stabilized soil block and the alkali-activated block.

Selective modification of inner surface of halloysite nanotubes: a review

AbstractIn this paper, we review the chemical strategies used for the modification of the inner surface of halloysite nanotubes (HNTs). The HNTs are nanotubular materials formed by rolling up the 1:1 aluminosilicate clays, where the composition is similar with kaolin. Owing to many virtues, including the high ratio of length to diameter, large cavity volume, desirable biocompatibility, and low cost, the HNTs have been applied to numerous promising domains. The modification of the outer surface is usually intended to decrease the HNT dispersal in aqueous media. Considering that the selective modification for the inner surface gives excellent prospects for hybrid HNT-based materials, herein, we explore the advances in the selective modification of the inner surface that expanded the applications of the HNTs.

Structure and Dynamics of Nonionic Surfactant Aggregates in Layered Materials.

The aggregation of surfactants on solid surfaces as they are adsorbed from solution is the basis of numerous technological applications such as colloidal stabilization, ore flotation, and floor cleaning. The understanding of both the structure and the dynamics of surfactant aggregates applies to the development of alternative ways of preparing hybrid layered materials. For this purpose, we study the adsorption of the triethylene glycol mono n-decyl ether (C10E3) nonionic surfactant onto a synthetic montmorillonite (Mt), an aluminosilicate clay mineral for organoclay preparation with important applications in materials sciences, catalysis, wastewater treatment, or as drug delivery. The aggregation mechanisms follow those observed in an analogous natural Mt, with the condensation of C10E3 in a bilayer arrangement once the surfactant self-assembles in a lamellar phase beyond the critical micelle concentration, underlining the importance of the surfactant state in solution. Solid-state 1H nuclear magnetic resonance (NMR) at fast magic-angle spinning (MAS) and high magnetic field combined with1H-13C correlation experiments and different types of 13C NMR experiments selectively probes mobile or rigid moieties of C10E3 in three different aggregate organizations: (i) a lateral monolayer, (ii) a lateral bilayer, and (iii) a normal bilayer. High-resolution 1H{27Al} CP-1H-1H spin diffusion experiments shed light on the proximities and dynamics of the different fragments and fractions of the intercalated surfactant molecules with respect to the Mt surface. 23Na and 1H NMR measurements combined with complementary NMR data, at both molecular and nanometer scales, precisely pointed out the location of the C10E3 ethylene oxide hydrophilic group in close contact with the Mt surface interacting through ion-dipole or van der Waals interactions.

Heterogeneous Distribution of Adsorbed Bitumen on Fine Solids from Solvent-Based Extraction of Oil Sands Probed by AFM

Mineral particles encountered in oil production and many chemical processes are generally adsorbed with organics (e.g., bitumen), playing an important role in determining their wettability and interaction behaviors. In this work, the surface properties of fine solids, collected from the solvent-based extraction of Athabasca oil sands using cyclohexane as the extraction solvent, have been systematically characterized by several complementary techniques. The fine solids were shown to be mainly composed of silica and aluminosilicate clays, analyzed using X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy. The particle wettability was determined using the Washburn method. The mineral particle surfaces were further characterized by atomic force microscope (AFM) techniques. Various regimes on the particle surfaces with and without adsorbed bitumen were identified and distinguished using PeakForce quantitative nanomechanics AFM imaging of surface topography, adhesion, modulus, and deformat...

Effect of addition of silicate and aluminosilicate fluxes on structure and phase composition of chromium sinter

The microstructure of the fluxes (aluminosilicate clays) tested during pilot sintering of the chromium ore fines of the Don ore-dressing and processing enterprise is studied. The composition of the liquid phase that forms upon softening of materials and bonds chromospinelide grains is analyzed by chemical, electron- probe, and X-ray diffraction analyses. The results demonstrate that the clays have a similar morphology and differ substantially in the phase composition of both the initial fluxes and the binder in the composition of the chromium ore sinter with clay additions. The phase composition of the binder formed upon addition of aluminosilicate clays is compared with the application of an anthropogenic silica-containing flux (microsilica). A relation between the phase composition of the initial fluxes and the aluminosilicate binder in the composition of chromium sinter is found, and the advantages of aluminosilicate clays are demonstrated.

Engineering a tubular mesoporous silica nanocontainer with well-preserved clay shell from natural halloysite

The in situ synthesis of mesoporous nanotubes from natural minerals remains a great challenge. Herein, we report the successful synthesis of mesoporous silica nanotubes (MNTs) with a varying inner-shell thickness and a preserved clay outer shell from natural-halloysite nanotubes (HNTs). After the enlargement of the lumen diameter of the tubular aluminosilicate clay by acid leaching, uniform mesopores were introduced by a modified pseudomorphic transformation approach, while the clay outer shell was well-preserved. Using density functional theory calculations, the atomic structure evolution and the energetics during Al leaching and Si–OH condensation were studied in detail. After the leaching of Al ions from the HNTs, local structural changes from Al(Oh) to Al(V) at a medium leaching level and to Al(Td) at a high leaching level were confirmed. The calculated hydroxylation energy of two kinds of silica components in the acid-leached HNTs (the distorted two-dimensional silica source in the inner shell and the intact aluminosilicate structure in the outer shell) was 0.5 eV lower or 1.0 eV higher than that of bulk silica, which clarifies the different behavior of the silica components in the hydrothermal process. The successful synthesis of reactive MNTs from HNTs introduces a new strategy for the synthesis of mesoporous nanocontainers with a special morphology using natural minerals. In particular, MNT samples with numerous reactive Al(V) species and a specific surface area up to 583 m2/g (increased by a factor of 10) are promising drug-loading nanocontainers and nanoreactors.

Catalytically active silver nanoparticles loaded in the lumen of halloysite nanotubes via electrostatic interactions

Halloysites are a kind of aluminosilicate clay with a morphology of a nanotube. The inner wall of halloysite is positively charged, and the external surface is negatively charged. In this work, we propose a simple and facile method to prepare Ag NPs loaded in the lumen of halloysite nanotubes (HNTs). Herein, N-acetyl-l-cysteine modified silver nanoparticles (Ag NPs) with negative charges spontaneously and stably resided in the lumen of HNTs via electrostatic interactions. The images of transmission electron microscopy and scanning transmission electron microscopy showed that Ag NPs with a size of ~2.6 nm were uniformly distributed in the lumen of HNTs. The catalytic activity of the obtained Ag NPs/HNTs composites was evaluated by the reduction reaction of 4-nitrophenol (4-NP) as a model reaction. When the molar ratio of Ag and 4-NP was set at 0.008, the rate constant of the reaction was found to be 0.91 min−1, two times higher than that of Ag NPs adsorbed on the external surface of HNTs. Additionally, no Ag NPs were found in the supernatant after the Ag NPs/HNTs suspension was stirred for 30 min. Such structural stability implies good reusability as a catalyst.

Development of antibacterial activity and corrosion resistance properties of electrodeposition of mineralized hydroxyapatite coated on titanium alloy for biomedical applications

Titanium and its alloy are potential biocompatible implant materials, due to their imposing biological properties. Hydroxyapatite [Ca10(PO4)6(OH)2, HAP], is a principal inorganic constituent of bone, teeth and it is the most suitable nontoxic biocompatible material. Halloysite nanotubes (HNTs) are a type of natural alumino-silicate clay mineral. To improve the anticorrosion protection properties of HNT, it is deposited on Titanium alloy. The Halloysite Nanotube-Cerium substituted Hydroxyapatite (HNT-Ce-HAP) composite was electrodeposited on Ti alloy using the electro deposition process as a bioactive coating with improved antibacterial activity and anti-corrosion stability in simulated body fluid. The formation of HNT-mHAP composite coating was characterization using FT-IR, X-ray diffraction (XRD), Contact angle, (SEM-EDX) scanning electron microscopy. The antibacterial activity of the as-coated HNT-Ce-HAP particles was evaluated against two pathogen bacterial strains. The HNT-mHAP composite coated Ti6Al4V was analyzed towards human osteoblast (HOS MC3T3-E1 cells) cells using MTT assay. Thus, the as- coated HNT-Ce-HAP composite serves as a better candidate for biomedical applications with good anticorrosion, antibacterial properties and with bone bonding ability.

Bias in determining aluminum concentrations: Comparison of digestion methods and implications on Al management

Aluminum is an important aquatic contaminant due to its ubiquity, toxicity and low regulatory discharge limits. Aluminum is mobilized in mining related, acidic drainage and is commonly a regulated pollutant. However, while aquatic toxicity studies and toxicity criteria are based on dissolved aluminum(Ald), discharge levels are, for statutory reasons, based on total recoverable aluminum (Alt). The rationale for using total recoverable aluminum recognizes the potential for the release of exchangeable, toxic cations or dissolution of metastable metal flocs in the event the discharge enters an acidic receiving stream. The digestion methods used in determining total recoverable metals are not meant to dissolve aluminosilicate clay particles but we found that they do, resulting in positively biased total recoverable aluminum values. This study explored the interaction between total suspended solids (TSS) and total recoverable aluminum using three digestion methods to evaluate which method introduced the least bias.Using field collected water and sediment samples from two coal mine drainage sites in Central West Virginia, three total recoverable digestion methods (USEPA Method 200.7, M1; USGS In-Bottle method, M2; and a Modified In-Bottle method, M3) were used to determine total recoverable aluminum across a range of total suspended solids concentrations. Baseline simulation experiments were conducted at pH 2.5, 3.5, 4.5 and 5.5 at different total suspended solids concentrations. Results indicated that dissolved aluminum did not respond to increasing total suspended solids concentrations while determined total recoverable aluminum increased with total suspended solids, indicating varying degrees of clay dissolution and, thus bias in the total recoverable aluminum concentration. While all three digestion methods overestimated total recoverable aluminum, at the same total suspended solids concentration, total recoverable aluminum extracted by USEPA Method 200.7 (M1) was much higher than the other two digestion methods (M2 and M3). Total recoverable aluminum from different digestion methods indicated that amount of aluminosilicate clay is digested in decreasing order: USEPA 200.7 (M1) > USGS in-bottle (M2) > modified in-bottle (M3). At pH 2.5, positive bias using methods M1, M2 and M3 was 153–287%, 53% and 40% respectively. Positive bias was greatest at pH greater than 4.5. Methods M1, M2 and M3 yielded positive biases of 660–1060%, 120–360% and 200-320% respectively. The results suggest that USEPA method 200.7 (M1) resulted in the greatest bias. Given its application in determining regulatory compliance, this is an important issue requiring further study.