Evandro B. da Silva

Mechanisms of metal sorption by biochars: Biochar characteristics and modifications

Biochar produced by thermal decomposition of biomass under oxygen-limited conditions has received increasing attention as a cost-effective sorbent to treat metal-contaminated waters. However, there is a lack of information on the roles of different sorption mechanisms for different metals and recent development of biochar modification to enhance metal sorption capacity, which is critical for biochar field application. This review summarizes the characteristics of biochar (e.g., surface area, porosity, pH, surface charge, functional groups, and mineral components) and main mechanisms governing sorption of As, Cr, Cd, Pb, and Hg by biochar. Biochar properties vary considerably with feedstock material and pyrolysis temperature, with high temperature producing biochars with higher surface area, porosity, pH, and mineral contents, but less functional groups. Different mechanisms dominate sorption of As (complexation and electrostatic interactions), Cr (electrostatic interactions, reduction, and complexation), Cd and Pb (complexation, cation exchange, and precipitation), and Hg (complexation and reduction). Besides sorption mechanisms, recent advance in modifying biochar by loading with minerals, reductants, organic functional groups, and nanoparticles, and activation with alkali solution to enhance metal sorption capacity is discussed. Future research needs for field application of biochar include competitive sorption mechanisms of co-existing metals, biochar reuse, and cost reduction of biochar production.

Metal leachability from coal combustion residuals under different pHs and liquid/solid ratios

Coal combustion residuals (CCRs) contain variable amounts of trace metals, which can negatively impact the environment. We analyzed metal concentrations and leachability of CCRs from seven coal-fired power plants from Florida. The purpose of this study was to characterize and assess metal leachability in representative CCRs samples from coal-fired power plants, including As, Ba, Cd, Cr, Pb, and Se. The specific objectives were to: (1) measure metal leachability under different pH conditions and liquid-to-solid ratios using USEPA Leaching Environmental Assessment Framework (LEAF) Methods 1313 and 1316, and (2) compare their leachability with those obtained by the Synthetic Precipitation Leaching Procedure (SPLP). All metals excluding Cd showed amphoteric behavior, presenting higher concentrations at low and high pH using LEAF Method 1313. The highest Cd leaching was observed at pH 2-4 and decreased at pH>7. SPLP results were highly variable when compared to the LEAF data. All metals except Ba exceeded the Florida Groundwater Cleanup Target Levels at all pH levels, however, metal leaching was low at typical soil pH of 4-9. Metal concentrations in fly ash decreased in most cases with increasing LS ratio. Therefore, due to potential leaching of some metals, evaluation is needed before beneficial use of CCRs.

Human exposure to polycyclic aromatic hydrocarbons: Metabolomics perspective

Polycyclic aromatic hydrocarbons (PAHs) are organic contaminants exhibiting carcinogenic toxicity. They are widespread in the environment, especially in urban areas. Humans are exposed to PAHs via inhalation, ingestion and dermal contact. Though much research has investigated their toxicity, little is known regarding the metabolic responses in humans after exposing to PAHs. However, those studies are important since PAHs become carcinogenic after metabolic activation by humans as indirect-acting carcinogens. As such, it is important to study their metabolism in humans based on metabolomics analysis. The goal of metabolomics study is to obtain a comprehensive view of metabolic reactions in humans after exposing to PAHs to better control the underlying metabolisms and reduce their genotoxicity. This article reviewed the biomarkers, analytical techniques including nuclear magnetic resonance and mass spectrometry, big data multivariate statistical analysis, and animal models that have been utilized to better understand the biological effects of PAHs, PAH-derivatives, and their metabolites and biotransformation products on humans.

Metal concentrations in traditional and herbal teas and their potential risks to human health

Food and beverage consumption is an important route for human exposure to metals. Traditional tea (Camellia sinensis) is a widely-consumed beverage, which may contain toxic metals. This study determined total and infusion concentrations of 5 metals including Al, As, Cd, Cr, and Pb in 47 traditional and herbal teas from 13 countries and assessed their potential risks to human health. The data showed that herbal teas exhibited higher As (0.26mgkg-1), Cd (0.19mgkg-1) and Pb (2.32mgkg-1) than traditional teas. Black tea from India had high Cr at 31mgkg-1 while white tea from China had low Cr at 0.39mgkg-1. Arsenic, Cd and Pb did not exceed the WHO limit for medicinal plants excluding one herbal tea with 1.1mgkg-1 As and 26.4mgkg-1 Pb. However, Cr in 47% herbal teas and 73% traditional teas exceeded the Canada limit of 2mgkg-1. Metal concentrations in tea infusions were below the MCL for drinking water except for Al. Total Al and its infusion was lower in herbal teas (47-1745mgkg-1 and 0.09-3.95mgL-1) than traditional teas (50.3-2517mgkg-1 and 0.02-7.51mgL-1), with 0.9-22% and 4-49% of the Al being soluble in infusion. The Al concentrations in infusion in all black tea and 83, 75 and 25% of the green, oolong and herbal teas exceeded the secondary MCL for drinking water at 0.2mgL-1. However, the weekly intake of Al from drinking tea (0.001-0.39 and 0.003-0.56mgkg-1 for children and adults) was lower than the provisional tolerable weekly intake for Al at 1.0mgkg-1. Our data showed that it is important to consider metal intake from tea consumptions, especially for Cr and Al in heavy tea drinkers.

Interactive effects of chromate and arsenate on their uptake and speciation in Pteris ensiformis

Background and aimsArsenate (AsV) and chromate (CrVI) inhibit each other’s uptake and translocation in As-hyperaccumulator Pteris vittata. In the present study, we extended the research to As-sensitive plant Pteris ensiformis to better understand the mechanism of their interactions.MethodsPlants were exposed to 0, 0.75 or 7.5 mg L−1 AsV and 0, 0.52, or 5.2 mg L−1 CrVI for 7 d in hydroponics. Arsenic and Cr speciation were determined in nutrient solutions and plant biomass.ResultsP. ensiformis accumulated high levels of As and Cr in the rhizomes and roots with low levels in the fronds. However, P. ensiformis was much more effective in taking up Cr than As, as much more Cr was accumulated in the roots (306–6015 vs. 87–642 mg kg−1). AsV and CrVI increased each other’s uptake in the rhizomes and roots when co-present. The AsV and CrVI taken up by P. ensiformis were reduced to arsenite (AsIII) and chromite (CrIII), possibly serving as detoxification mechanism.ConclusionsUptake of As and Cr induced oxidative stress as indicated by increased lipid peroxidation and electrical conductivity. Arsenic and Cr increased each other’s uptake by P. ensiformis.

PAHs in urban soils of two Florida cities: Background concentrations, distribution, and sources

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous organic contaminants, which are found in soils throughout the U.S. The objective of this study was to determine the background concentrations, distributions, and sources of 16 USEPA priority PAHs in two urban soils. A total of 114 soil samples were collected from two large cities in Florida: Orlando and Tampa. The results showed that soils were dominated by high molecular weight PAHs in both cities. The average ∑16-PAHs in Orlando and Tampa soils were 3227 and 4562 μg kg-1, respectively. The averages of 7 carcinogenic PAHs based on the benzo[a]pyrene-equivalent (BaP-EQ) concentrations in the two cities were 452 and 802 μg kg-1. BaP-EQ concentrations in 60-62% of samples were higher than the Florida Soil Cleanup Target Level (FSCTL) for residential soils at 100 μg kg-1 and 20-25% of samples were higher than FSCTL for industrial soils at 700 μg kg-1. Based on molecular diagnostic ratios and PMF modeling, major sources of soil PAHs in both cities were similar, mainly from pyrogenic sources including vehicle emissions, and biomass and coal combustion. Based on ArcGIS mapping, PAH concentrations in soils near business districts and high traffic roads were higher. Thus, it is important to consider background PAH concentrations in urban soils when considering soil remediation.

Assessment of trace metals in five most-consumed vegetables in the US: Conventional vs. organic

Metal concentrations (As, Cd, Pb, Cr, Ba, Co, Ni, Cu, and Zn) in conventional and organic produce were assessed, specifically, five most-consumed vegetables from the US including potato, lettuce, tomato, carrot and onion. They were from four representative supermarkets in a college town in Florida. All vegetables contained detectable metals, while As, Cd, Pb, Cr, and Ba are toxic metals, Co, Ni, Cu, and Zn are nutrients for humans. The mean concentrations of As, Cd, Pb, Cr and Ba in five vegetables were 7.86, 9.17, 12.1, 44.8 and 410 μg/kg for organic produce, slightly lower than conventional produce at 7.29, 15.3, 17.9, 46.3 and 423 μg/kg. The mean concentrations of Co, Ni, Cu, and Zn in five vegetables were 3.86, 58.5, 632, and 2528 μg/kg for organic produce, comparable to conventional produce at 5.94, 68.2, 577, and 2354 μg/kg. For toxic metals, the order followed tomato < lettuce < onion < carrot < potato, with root vegetables being the highest. All metals in vegetables were lower than the allowable concentrations by FAO/WHO. Health risks associated with vegetable consumption based on daily intake and non-carcinogenic risk based on hazard quotient were lower than allowable limits. For the five most-consumed vegetables in the US, metal contents in conventional produce were slightly greater than organic produce, especially for Cd and Pb.

Speciation, bioaccessibility and potential risk of chromium in Amazon forest soils

Even though the Amazon region is widely studied, there is still a gap regarding Cr exposure and its risk to human health. The objectives of this study were to 1) determine Cr concentrations in seven chemical fractions and 6 particle sizes in Amazon soils, 2) quantify hexavalent Cr (CrVI) concentrations using an alkaline extraction, 3) determine the oral and lung bioaccessible Cr, and 4) assess Cr exposure risks based on total and bioaccessible Cr in soils. The total Cr in both A (0-20 cm) and B (80-100 cm) horizons was high at 2346 and 1864 mg kg-1. However, sequential extraction indicated that available Cr fraction was low compared to total Cr, with Cr in the residual fraction being the highest (74-76%). There was little difference in total Cr concentrations among particle sizes. Hexavalent Cr concentration was also low, averaging 0.72 and 2.05 mg kg-1 in A and B horizon. In addition, both gastrointestinal (21-22 mg kg-1) and lung (0.95-1.25 mg kg-1) bioaccessible Cr were low (<1.2%). The low bioavailability of soil Cr and its uniform distribution in different particle sizes indicated that Cr was probably of geogenic origin. Exposure based on total Cr resulted in daily intake > the oral reference dose for children, but not when using CrVI or bioaccessible Cr. The data indicated that it is important to consider both Cr speciation and bioaccessibility when evaluating risk from Cr in Amazon soils.