Metal Particle Size Distribution Research Articles

Thermal treatment effect on the particle size distribution of alkaline earth metals hydroxyapatite

Hydroxyapatites of certain alkaline earth metals were synthesised, and their phase composition was determined using X-ray phase analysis. Thermal modification of the studied compounds was performed at temperatures not exceeding 800°C. The laser diffraction method determined the size distribution of the samples subjected to thermal treatment. It was found that the mean particle size ranged from 5,48±1,28 to 126,71±3,68 μm. It has been demonstrated that particle aggregation and fragmentation processes are possible depending on the synthesised compounds' qualitative and quantitative phase composition and the modification temperature.

Particle Size Distribution and Enrichment of Alkali and Heavy Metals in Fly Ash on Air and Oxy-Fuel Conditions from Sludge Combustion

Comparative tests in air and oxy-fuel combustion were conducted in a 30 kWth circulating fluidized bed (CFB) pilot plant for waste sludge combustion. General combustion characteristics of the CFB, such as pressure profiles, temperatures along the bed, and flue gas composition, were different under the air and oxy-fuel conditions. At the bottom and in the fly ash, alkali and heavy metals had different distributions under the air and oxy-fuel combustion conditions. The particle size distribution in fly ash from air combustion was dominated by coarse particles, over 2.5 μm in size, whereas with oxy-fuel combustion, most particles were submicron in size, approximately 0.1 μm, and a smaller quantity of coarse particles, over 2.5 μm in size, formed than with air combustion. Mass fractions of Al, Ca, and K, below 2.5 μm in size, were found in the ashes from oxy-fuel combustion and in higher quantity than those found in air combustion. Submicron particle formation from Cr, Ni, Cu, and Zn in the fly ash occurred more during oxy-fuel combustion than it did in air combustion.

Hypergolicity improvement by activated carbon-supported catalysts for hydrogen peroxide oxidizer

In recent years, hydrogen peroxide (H2O2) has been attracting attention as an alternative oxidizer to replace the traditional toxic hypergolic oxidizers. However, hypergolic solid fuel (HSF) candidates for H2O2 are few in number and offer limited applications for space propulsions. In this work, various activated carbon (AC)-supported catalysts for a H2O2 hybrid rocket were studied to improve the hypergolicity of HSF, thereby widening the selection range for hypergolic fuels. For this purpose, eleven H2O2 catalyst candidates (Cr, Mn, Fe, Co, Ni, Cu, Ru, Pd, Pt, Pt–Ru, and Pb) were deposited onto AC supports at contents of 20 wt%. The nanostructure and metal particle size distribution of these prepared catalysts were investigated via transmission electron microscope imagery. The oxidation states of the supported catalysts were determined by X-ray diffraction, while the ignition characteristics of the pure catalysts and HSFs with catalyst additives were examined by conducting hypergolic drop tests using a 95 wt% H2O2. The hypergolicity of the prepared catalysts was evaluated by measuring their reaction delay and ignition delay times. Among the studied catalysts, Pt/AC, Ru/AC, and Pt–Ru/AC exhibited the shortest reaction delay time of 0.3 ms. Moreover, the dependences of the reaction delay time on the presence of Na+ ions in Mn/AC and Mn concentration were established. Finally, the ignition delay time of an HSF containing 25 wt% ammonia borane and 75 wt% polyethylene decreased from 20.3 to 9.8 ms after the addition of 1 wt% Pd/AC. These results can provide a variety of additive options for improving hypergolicity of solid fuels with H2O2.

Multiple Evaluation of Typical Heavy Metals Pollution in Surface Soil and Road Dust from Beijing and Hebei Province, China.

Soil and road dust are important receptors of heavy metals in the environment. Meanwhile, heavy metal could transfer to the atmosphere through resuspension. Due to the serious consequences and atmospheric haze in Jing-Jin-Ji area, it's important to evaluate the pollution level, particle size distribution and sources of heavy metals. For heavy metals in soil samples, similar concentrations to the background values and no obvious pollution or low-level pollution was presented. Higher concentration of Cu (78.9mg/kg) and Zn (261mg/kg) were found in road dust. The source appointment results showed that Mn, Co, Cr, Ni, Zn and Pb in soils and Cr, Co and Mn in road dust were mainly from the natural sources, while traffic source contributed to most of Cu, Zn and Pb in road dust. Different particle size distribution patterns were found in soils and road dusts, and the finest particles presented the highest heavy metal concentrations.

Antimicrobial and Aging Properties of Ag-, Ag/Cu-, and Ag Cluster-Doped Amorphous Carbon Coatings Produced by Magnetron Sputtering for Space Applications.

Inside a spacecraft, the temperature and humidity, suitable for the human crew onboard, also creates an ideal breeding environment for the proliferation of bacteria and fungi; this can present a hazard to human health and create issues for the safe running of equipment. To address this issue, wear-resistant antimicrobial thin films prepared by magnetron sputtering were developed, with the aim to coat key internal components within spacecrafts. Silver and copper are among the most studied active bactericidal materials, thus this work investigated the antibacterial properties of amorphous carbon coatings, doped with either silver, silver and copper, or with silver clusters. The longevity of these antimicrobial coatings, which is heavily influenced by metal diffusion within the coating, was also investigated. With a conventional approach, amorphous carbon coatings were prepared by cosputtering, to generate coatings that contained a range of silver and copper concentrations. In addition, coatings containing silver clusters were prepared using a separate cluster source to better control the metal particle size distribution in the amorphous carbon matrix. The particle size distributions were characterized by grazing-incidence small-angle X-ray scattering (GISAXS). Antibacterial tests were performed under both terrestrial gravity and microgravity conditions, to simulate the condition in space. Results show that although silver-doped coatings possess extremely high levels of antimicrobial activity, silver cluster-doped coatings are equally effective, while being more long-lived, despite containing a lower absolute silver concentration.

Catalyst Ni-Mo/Al2O3 promoted with infrared heating calcination for hydrodesulfurization of shale oil

Ni-Mo/Al2O3 catalyst calcinated using infrared radiation heating was applied for the first time to hydrodesulfurization (HDS) of shale oil in a fixed bed reactor. Its catalytic performance was measured under varied operating conditions and compared to that obtained by using a catalyst with the same Ni-Mo composition but calcinated in a conventional electric furnace. The operation parameters of HDS were varied among temperature (320–400 °C), pressure (1.0–4.0 Mpa), H2/oil (v/v) (200:1–600:1) and LHSV (1–4 h−1). Under all tested conditions, the catalyst using the infrared heating method always yielded a product oil having the lower sulfur content than the catalyst heated by the conventional furnace did. At the optimal condition of T = 380 °C, P = 4 MPa, H2/oil (v/v) = 600:1 and LHSV = 4 h−1, the infrared-heating catalyst enabled the oil to decrease its sulfur content to 2600 ppm, which thus met the national Marine fuel oil standard (S < 0.5 wt%, GB1711-2015, China). Characterization of the fresh, sulfided and spent catalysts shown that the catalyst calcinated with the infrared heating had the higher BET surface area, smaller and narrower size distribution of metal particles, higher binding energy between Mo and S, and more oxygen-deficient sites in comparison with the catalyst calcined using conventional electric heating.

Stable Palladium Oxide Clusters Encapsulated in Silicalite-1 for Complete Methane Oxidation

Zeolite-supported metal catalysts are widely employed in a number of chemical processes, and the stability of the catalytically active species is one of the most critical factors determining the reaction performance. A good example is the Pd/zeolite catalyst, which provides high activity for methane oxidation but deactivates rapidly under the reaction conditions due to palladium nanoparticle sintering. Although coating the metals with thin shells of porous materials is a promising strategy to address the sintering of metals, it is still challenging to fix small metal particles completely inside zeolite crystals. Here, using an amine-based ligand to stabilize palladium during the zeolite synthesis, we realize the exclusive encapsulation of highly dispersed palladium oxide clusters (1.8–2.8 nm) in the microporous channels and voids of the nanosized silicalite-1 crystals. The synthesis conditions of the zeolite-supported catalyst influence the encapsulation degree and the size distribution of metal particles. Thanks to the encapsulation effect of small palladium oxide clusters, together with the inherent properties of silicalite-1 such as low acidity, high hydrophobicity, and high hydrothermal stability, the optimized Pd@silicalite-1 catalyst outperforms the traditional Pd-based catalysts prepared by wetness impregnation, exhibiting both high activity and better stability in the lean methane oxidation reaction.

Measurement and Characterisation of Metallic Iron Particles on Coal-Based Reduction and Magnetic Separation of Refractory Titanomagnetite

Coal-based reduction followed by magnetic separation is a promising technology for processing refractory iron ores. Analysis of the size distribution of metallic iron particles is an effective approach for exploring the mechanisms of coal-based reduction. As metallic iron particles exhibit very complex shapes compared with nature ores, approaches for measuring and characterising metallic iron particles still remain unclear. In this study, the size of metallic iron particles was measured and characterised using manual point-counting by optical microscopy technics and image analysis. The effects of CaF2 as an additive on behaviour of the particle size distribution (i.e. mean size and cumulative frequency distribution) were also determined. Results indicate that CaF2 had a significant effect on the growth of metallic iron particles. When the CaF2 dosage increased from 0 to 8%, the mean size according to the area method increased from 9.03 to 16.28 μm, while the promotion effect gradually weakened and generated many fine metallic iron particles when the CaF2 dosage exceeded 8%. An increase in CaF2 dosage also led to cumulative distribution curves gradually shifting to the right mainly because F− can break the crystal structure of each phase during the carbothermic reduction of titanomagnetite and reduce the stability of the lattice structure, thus reducing the melting point and accelerating the growth of metallic iron grains. Compared to different measuring methods, the area method is a more suitable approach for measuring and characterising the size of metallic iron particles.

Insights into the active sites of chlorine-resistant Pt-based bimetallic catalysts for benzene oxidation

Since the Pt-based catalysts often suffer severe deactivation by chlorine, development of a catalyst with good activity and chlorine-resistant ability is of importance in effectively controlling emissions of (chlorinated) volatile organic compounds ((C)VOCs). In this work, we prepared the Al2O3-supported bimetallic catalysts with narrow metal particle size distributions and their application in benzene oxidation. It is found that PtW/Al2O3 showed the highest catalytic activity and the lowest activation energy. The characterization results were well correlated with the catalytic activities, and a Pt–MOx (M = W, Mo) bimetallic synergistic effect was proposed. In situ FTIR studies reveal that the transformation from phenol to benzoquinone was the key step in benzene oxidation process. The influence of 1,2-dichloroethane (DCE) on catalytic activity for benzene oxidation of the as-obtained samples was also examined. It is demonstrated that there was a competitive adsorption between benzene and DCE molecules, and the DCE exerted an inhibitive effect on benzene oxidation. We conclude that excellent catalytic performance of the bimetallic sample was associated with its good abilities to adsorb and activate benzene on small ensembles of the MOx in proximity contact with Pt as well as the strong interaction between the highly dispersed metallic Pt species and the MOx clusters (which led to enhancement in anti-deactivation of the Pt species under the oxygen-rich and Cl-containing reaction conditions). We are sure that the present work can provide an idea for understanding the nature of bimetallic active sites and rationally designing the efficient bimetallic catalysts for the oxidative removal of VOCs.

برآورد احتمالاتی توزیع بار در واحدهای فرآوری مواد معدنی با روش زنجیره مارکف

در روش‌های مرسوم موازنه جرم، برای تعیین عیار فلز، درصد جامد و توزیع دانه‌بندی نمونه‌برداری در مسیرها (شاخه‌ها) انجام می‌شود و در نهایت خطاهای مربوط به آنها سرشکن می‌شود. در این روش‌ها، برای هر واحد فرآوری (تجهیزات کارخانه)، در خصوص مقدار بار و عیار یا محتوای فلز اطلاعات لحظه‌ای وجود ندارد. در این مقاله با بهره‌گیری از نظریه فرآیند‌های تصادفی، مقادیر کمی و کیفی بار ورودی به هر یک از واحدهای عملیاتی در مسیر فرآوری به شکل احتمالات در نظر گرفته و توزیع بار در واحدهای مذکور برآورد شده است. به این منظور، ابتدا کارخانه فرآوری به صورت یک گراف جهت‌دار و موزون مدل‌سازی شده است. در این گراف، هرگره بیانگر یک واحد عملیاتی و هر یال، نشان‌دهنده مسیر واقع بین دو واحد است. وزن هر یال معرف وزن مواد عبوری در مسیر از گره‌ای به گره دیگر بوده و در یک بازه زمانی مشخص به‌عنوان یک متغیر تصادفی منظور شده است. پس از مدل‌سازی فرآیند، مدل گرافی به‌دست آمده که در برگیرنده شاخص‌های تصادفی است با استفاده از نظریهزنجیرهای مارکف تحلیل شده است. با روش ارایه شده در این مقاله نه تنها توزیع بار، بلکه احتمال حضور هر ذره یا کانی، در هر یک از واحدهای عملیاتی کارخانه فرآوری نیز قابل پیش‌بینی است. همچنین در صورت توقف کارخانه، می‌توان تناژ موجود در هر یک از واحدها را تخمین زد و اقدامات لازم برای تخلیه آنها را فراهم کرد. در این تحقیق، خط فرآوری سنگ آهن چادرملو با روش بالا، مدل‌سازی و نتایج حاصل تحلیل و سپس با استفاده از شاخص‌های احتمالات گزارش شده است.

Screening of untreated municipal solid waste incineration fly ash for use in cement-based materials: chemical and physical properties

The environmental impact during concrete manufacturing can be reduced by using less cement or using types of cement with high amounts of secondary cementitious materials (SCMs) and fillers. Fly ash from municipal solid waste incineration (MSWI) is an unused material, which could potentially be used as an SCM or filler. The applicability of MSWI fly ash samples in cement-based materials was investigated through an indirect determination based on the chemical composition, toxic metal content and particle size distribution of the samples. Furthermore, how the samples compared to each other and how representative the samples were for MSWI fly ash, in general, were investigated by multivariate modelling. MSWI fly ash samples were obtained from Denmark, Sweden and Greenland. Comparing the chemical composition of the MSWI fly ash samples with the chemical requirements for coal fly ash to be used in concrete, specified in EN 450-1 2012, indicated a low potential as SCMs. Additionally, the MSWI fly ash contained and leached more toxic metals than the allowed limits. The particle size distributions were larger than cement and indicated limited potential as filler. A principal component analysis showed that the obtained samples were chemically different materials compared to each other and would have different effects if used in cement-based materials. Additionally, the samples from Denmark were unrepresentative of MSWI fly ash, while the Greenlandic and Swedish samples were comparable to previously studied MSWI fly ash samples.

Metal‐Chitosan Nanocomposites: A Perspective Way to Preparation, Morphology, and Structural Studies

AbstractThe metal vapor synthesis is used to prepare sols of nanoparticles (Cu, Ag, and Au) in various solvents (isopropanol, triethylamine, and acetone). The sols are then used to deposit metal nanoparticles onto chitosan. The successful introduction of metal into the chitosan matrix is confirmed with X‐ray spectroscopic and diffraction methods. The morphology and size distribution of metal particles in nanocomposite materials are characterized by transmission electron microscopy and small‐angle X‐ray scattering.

Simple strategy synthesizing stable CuZnO/SiO2 methanol synthesis catalyst

The metal particle size distribution of a catalyst can affect the rate of deactivation in many chemical reaction. This work developed a facile method to prepare a well dispersed CuZnO/SiO2 catalyst via rotary evaporation assisted deposition-precipitation method (RE-CZS). By comparison, conventional deposition-precipitation method and ultrasound assisted deposition-precipitation method were applied to synthesize other catalysts (Im-CZS and Ul-CZS), in order to clarify the influence of Cu particle size distribution of the catalyst on catalytic stability of methanol synthesis via CO2 hydrogenation. RE-CZS catalyst possessed the narrowest Cu particle distribution (ca. 4.9–9.1 nm) and thus exhibited the best catalytic stability because of the least Cu particle growth. Im-CZS catalyst showed the broadest Cu particle distribution (ca. 5.3–28.7 nm) and suffered from the most serious particle growth resulting in the least catalytic stability. The Cu particle distribution and the related catalytic stability of the Ul-CZS catalyst lied between RE-CZS catalyst and Im-CZS catalyst. The origin of the tremendous effect of the Cu particle size distribution on the catalytic stability was studied via XRD, N2 adsorption/desorption, N2O chemisorption, ICP-AES, XPS, TEM and temperature-programmed techniques, which suggested that catalyst deactivation was mainly attributed to the growth of Cu particles originating from Ostwald ripening effect.

Impacts of leachate of landfill on the groundwater hydrochemistry and size distributions and heavy metal components of colloids: a case study in NE China.

Colloids associated with heavy metals are ubiquitous in contaminated groundwater; waste accumulation at imperfectly sealed landfills can produce large amounts of leachate with colloids and heavy metal contaminants, which can pollute the downstream groundwater. In this study, three sites in a landfill were sampled to reveal heavy metal particle size distributions and their chemical compositions. The > 220nm particle sizes were the predominant size in the downstream groundwater, while the < 10nm particle sizes were the predominant size in the upstream groundwater. Total Fe increased from 35.5μg/L in the upstream groundwater to 107μg/L in the downstream groundwater. This increase was attributed to the enhanced migration and accumulation of colloids in the aqueous phase. The elements and the colloid size distribution in the landfill indirectly reflected the composition and degradation of the waste. Colloids played a key role in distribution of both solid particles and aqueous contaminants in the landfill. The results of this study will contribute to the knowledge of the effect of different contaminants in the vicinity of landfills without appropriate sealing systems.

Characterization and temporal variation of urban runoff in a cold climate - design implications for SuDS

ABSTRACTThis paper characterizes stormwater runoff in a coastal cold climate urban area regarding metal fractionation and particle size distribution to understand the implications for sustainable urban drainage system (SuDS) design. Strong temporal and spatial variations were found for all studied parameters. Suspended and dissolved solids showed up to 10-fold and 44-fold increase, respectively, during salting periods (air temperature <0 °C). Metals were nearly solely associated with the suspended fraction (>1.2 µm), which increased during the salting periods (>97%). Use of snow-tyres and de-icers (gritting sand and NaCl) are more likely to explain the temporal fluctuations, rather than contributions from nearby snow piles. Residential streets might serve as a source of pollutants during the winter months when street sweeping is minimal. These results imply that protection against clogging from excessive sediment loads and use of salts during winter should be the primary focus for SuDS design.

Influence of silica sources on structural property and activity of Pd-supported on mesoporous MCM-41 synthesized with an aid of microwave heating for partial hydrogenation of soybean methyl esters

MCM-41 has been hydrothermally synthesized using fumed silica (SiO2) and silatrane [Si(TEA)2] as silica sources. Pd nanoparticles were successfully impregnated to the mesoporous MCM-41 supports. Soybean oil methyl ester was partially hydrogenated by the Pd/MCM-41-SiO2 and Pd/MCM-41-silatrane catalysts under a mild condition (low temperature and pressure). Both catalysts could rapidly and selectively convert the polyunsaturated fatty acid methyl esters (C18:3 and C18:2) to monounsaturated fatty acid methyl esters (C18:1) at 100 °C and 0.4 MPa H2 within 4 h. The results verified that the Pd/MCM-41-silatrane catalyst with the greater surface of Pd active sites had higher catalytic activity, representing in term of turnover frequency (TOF), in partial hydrogenation than Pd/MCM-41-SiO2 under both C18:2 conversions of 40% and 60%. Even if the lower selectivity toward cis-C18:1 was obtained for the former. In addition to the better stable structure of MCM-41-silatrane support as compared to MCM-41-SiO2 support, silatrane precursor is more favorable to diminish the extent of complete hydrogenation than fumed silica as it provided the lower index. Even though, this silica precursor sues for more synthetic step. Due to the higher oxidative stability of Pd/MCM-41-SiO2, MCM-41-SiO2 support was further studied for the later research. When varying the Pd loadings in 0.5–2 wt.% on MCM-41-SiO2 support, the Pd(2)/MCM-41-SiO2 catalyst gave the highest performance (TOF) whether the complete hydrogenation was concurrently accompanied. Results signify that the nature of the silica source and the Pd concentration modified the surface active sites and size distribution of metallic particles, which determine the catalytic reactivity and selectivity.

Particle Size Distribution of Heavy Metals and Magnetic Susceptibility in an Industrial Site.

This study was conducted to explore the relationships between magnetic susceptibility and some soil heavy metals concentrations in various particle sizes in an industrial site, central Iran. Soils were partitioned into five fractions (< 28, 28-75, 75-150, 150-300, and 300-2000µm). Heavy metals concentrations including Zn, Pb, Fe, Cu, Ni and Mn and magnetic susceptibility were determined in bulk soil samples and all fractions in 60 soil samples collected from the depth of 0-5cm. The studied heavy metals except for Pb and Fe displayed a substantial enrichment in the < 28µm. These two elements seemed to be independent of the selected size fractions. Magnetic minerals are specially linked with medium size fractions including 28-75, 75-150 and 150-300µm. The highest correlations were found for < 28µm and heavy metals followed by 150-300µm fraction which are susceptible to wind erosion risk in an arid environment.

Particle Size Distribution and Human Health Risk Assessment of Heavy Metals in Atmospheric Particles from Beijing and Xinxiang During Summer

Under a condition of good air quality (AQI:55-90, PM10:37-97 μg·m-3, PM2.5:17-76 μg·m-3), six groups of 54 samples were collected using an Andersen cascade impactor from both the indoor and outdoor stations in Beijing and Xinxiang from June to August in 2016. The samples were digested by microwave digestion, and nine heavy metal elements (Pb, Cr, Ni, Cu, Zn, As, Cd, Mn, and Co) in the atmospheric particles were determined with an inductively coupled plasma source mass spectrometer (ICP-MS). The results showed that the enrichment index (0-3) of most elements were low in both cities except for Cd[15.0 (Beijing) and 8.47 (Xinxiang)]. Cr, Co, Cu, and Mn in the atmospheric particles from Beijing park, Cd, Pb, and Mn in the atmospheric particles from the Beijing office, Cr, Co, Ni, and As in the atmospheric particles from Xinxiang park, and all nine heavy metal elements in the atmospheric particles from roads in both cities were found to be more concentrated in the coarse fractions; however, Pb, Zn, Cd, Ni, and As in the atmospheric particles from Beijing park, Co, Zn, Ni, Cr, As, and Cu in the atmospheric particles from the Beijing office, Pb, Zn, Cd, Cu, and Mn in the atmospheric particles from Xinxiang park, and all nine metal elements in the atmospheric particles from the Beijing office showed the opposite pattern. The result of a human health risk assessment indicated that the carcinogenic risk of the five carcinogenic elements were all less than 10-4, but a lower potential cancer risk would also occur under long term exposure. For the four non-carcinogenic elements (Pb, Zn, Mn, and Cu), the non-carcinogenic health risk values of Pb, Zn, Mn, and Cu in the atmospheric particulates in Beijing were all far less than 1, which means the corresponding non-carcinogenic risk was negligible; and, except for Mn, there was no obvious non-carcinogenic risk from Pb, Zn, and Cu in the atmospheric particles of Xinxiang.

Measuring and interpreting quantum efficiency for hydrogen photo-production using Pt-titania catalysts

A series of samples where platinum is deposited onto titania P25 using a chemical deposition method was tested in the photo-production of hydrogen using methanol as a sacrificial agent. The optimization of the platinum titania contact was analyzed as a function of the synthesis conditions. The catalysts’ structural and electronic properties were measured using a combination of porosimetry, X-ray diffraction, X-ray photoelectron, photoluminescence, and UV–visible spectroscopies, together with a microscopy study of the noble metal particle size distribution and an in situ infrared study of the catalysts under reaction conditions. The measurement of the optical properties of the catalyst suspensions and the subsequent calculation of the true quantum efficiency were used to show that a significant enhancement of ca. 250% can be achieved through the series of Pt-containing catalysts. The study provides solid evidence concerning the physical roots of the photoactivity behavior. For catalysts having equal Pt properties in terms of primary particle size and electron handling capability, a synergy between the noble metal and the oxide support at interface sites is shown to promote the conversion of carboxylate-type surface species into carbon dioxide and maximize the hydrogen yield of the reaction.

Bioaccessibility of Ba, Cu, Pb, and Zn in urban garden and orchard soils

Exposure of young children to toxic metals in urban environments is largely due to soil and dust ingestion. Soil particle size distribution and concentrations of toxic metals in different particle sizes are important risk factors in addition to bioaccessibility of these metals in the particles. Analysis of particle size distribution and metals concentrations for 13 soils, 12 sampled from urban gardens and 1 from orchard found that fine particles (<105 μm) comprised from 22 to 66% by weight of the tested soils, with Ba, Cu, Pb and Zn generally at higher concentrations in the finer particles. However, metal bioaccessibility was generally lower in finer particles, a trend most pronounced for Ba and Pb. Gastric was higher than gastrointestinal bioaccessibility for all metals except Cu. The lower bioaccessibility of Pb in urban garden soils compared to orchard soil is attributable to the higher organic matter content of the garden soils.