Progress report for GNE24-327
Project Information
Across communities in the northeastern U.S., urban agriculture fosters neighborhood engagement, stimulates local economies, provides access to fresh
produce, and increases the land area utilized for agriculture, thus improving food security. Many low-income neighborhoods can benefit greatly from urban agriculture. However, a major hurdle to popularizing urban agriculture is the potential soil contamination by heavy metals, especially legacy contaminants like lead (Pb). Measurements of total Pb can be misleading since the true health risks depend on the human intake rate and the fraction of contaminant that is bioavailable. As urban agriculture becomes more popular, growers face challenges in accessing reliable soil contamination testing means and deciding on a method to remediate or amend soil to decrease contamination risks. Amendments for remediating contaminated soil, such as biochar, should be tested for their ability to reduce Pb bioaccessibility. Biochar is well-known to have metal-sorbing properties. We propose a novel method of post-processing biochar, functionalizing it by heating it in oxygenic conditions to increase its capacity to decrease bioaccessible Pb in urban soils. If this method is shown to be successful, it may be extremely useful for urban gardeners who wish to reduce the risk of Pb poisoning for themselves and their children. In support of this goal, we are also validating a novel, inexpensive analytical method for detecting the amount of bioaccessible Pb in soils.
1.1.
Our first objective is to establish methods for analyzing EPA Method 1340 assays using XRF spectroscopy. We will use benchtop XRF to measure bioaccessible Pb in liquid sample extracts. This objective comprises three main tasks: Task 1. Measuring Accuracy and Precision: We will evaluate the accuracy of XRF by comparing it to ICP, which is typically used for measuring bioaccessible Pb. We will test whether the difference between bioaccessible Pb measured by XRF and that measured by ICP for known standards does not exceed 10%. Three replicates on the same liquid samples will be used to assess precision of the method. Task 2. Creating a Regression Equation: We will develop a regression equation for bioaccessible lead measured in liquid extractions using ICP and XRF analysis on at least 30 samples. Establishing this relationship will determine whether the methods are statistically equivalent. Task 3. Regression Curve Validation: To validate the established regression curve, we will measure an additional set of soil samples. We will plot ICP measurements on bioaccessible Pb calculated versus XRF measurements on bioaccessible Pb measured to evaluate the agreement between these data. The completion of this objective will determine if XRF can be utilized as a cost-effective alternative to ICP for measuring bioaccessible Pb in liquids.
1.2.
Our second objective is to assess the efficacy of heat-functionalized biochar in immobilizing lead within soil. This objective involves several
key tasks: Task 1. Creating Functionalized Biochar: We will alter the surface properties of biochar by heating it under various experimental conditions in an air atmosphere. This process is designed to generate additional chemically active sites on the biochar’s surface which can bond with Pb. Such chemical bonds are generally considered non-reversible, significantly reducing the likelihood of Pb re-entering the solution and thereby immobilizing it within the soil. Task 2. Conditioning Soil-Biochar Mixture: The soil-biochar mixture will be conditioned for two weeks to ensure that Pb redistribution occurs effectively. Task 3. Time Series Experiment: Over six months, we will conduct a time series experiment to monitor the dynamics of bioaccessible Pb. This will help us track how the effectiveness of the biochar application changes over time. These tasks will help us evaluate the long-term effectiveness of the biochar treatment in reducing bioaccessible Pb in soil, and provide valuable insights into the potential of heat-functionalized biochar as a sustainable solution for soil remediation.
Urban agriculture and gardening is important for community well-being, offering clear social and economic advantages to low-income underprivileged communities (Hanna & Oh, 2000; Horst & Hoey, 2017). Urban agriculture fosters neighborhood engagement, stimulates local economies, and enhances access to fresh produce. However, a major challenge facing urban agriculture is soil contamination with heavy metals, particularly lead (Pb), which remains a legacy contaminant (Lusby et al., 2015; O’Shea et al., 2021). When assessing urban sites for agriculture, stakeholders must develop reliable risk assessment approaches and choose robust and sustainable mitigation strategies (Wharton et al., 2012; Kim et al., 2014; Wagner & Payne, 2019).
Conventional approaches to mitigating Pb pollution in soil include physical, chemical, electrokinetic, and, more recently, phytoremediation techniques (Priya et al., 2023). In comparison, the use of biochar as an agent for soil remediation has received far less attention (Amalina et al.,2022; Bashir et al., 2020). This is in part due to the tremendous variety in properties provided by natural and synthetic biochars. It is important to note that not all biochars are equivalent, and some may have better capability to retain Pb than others (Sivaranjanee et al., 2023, Tan et al., 2024). The reactivity of biochar can be enhanced through activation procedures, which have been achieved through physical, chemical and thermal methods (Amalina et al., 2022; Bashir et al., 2020). Increases in biochar reactivity can improve their ability to remove heavy metals from solution. In urban agriculture communities, compost additions are a commonly used approach to reduce (“dilute”) Pb concentration in soil, however there are limitations. Compost additions may not be effective for soils with high concentrations of lead. Moreover, previous research has indicated that compost may not efficiently retain Pb in the soil; instead, compost could increase Pb’s mobility, potentially leading to Pb discharge into groundwater during decomposition processes (Bolan et al., 2014). In fact, relying solely on compost as a remediation approach can lead to its overapplication and potential leaching of the excessive nutrients to nearby streams (Small et al., 2019).
In contrast to compost, biochar demonstrates promise as a remediation material, exhibiting high efficiency in Pb retention due to its abundant surface area and reactive functional groups (Yuan et al., 2019). However, some of the activation procedures employed to manufacture commercial activated biochars may involve strong chemicals and should be conducted at a large commercial facility (Yuan et al., 2019). We propose an inexpensive method to functionalize biochar at 300°C under an air environment. Our hypothesis is that heating biochar in the presence of oxygen for several hours will activate its surface, creating more surface functional groups, and making it more reactive towards lead adsorption. We will test this hypothesis in a laboratory setting with containers of Pb-contaminated soil mixed with functionalized biochar that has undergone heat treatment, along with control containers of soil only, and soil mixed with un-treated biochar. If the heated biochar does show superior Pb-sorbing properties, it follows that such a product would provide significant economic and health benefits as a soil amendment for all urban gardeners.
To evaluate the efficiency of biochar amendment, we will assess whether functionalized biochar reduces bioaccessible Pb levels. While the EPA regulates total Pb concentration (200 ppm screening level, as of Jan. 17, 2024, USEPA, Region 7), this measure may not accurately reflect the potential health risks related only to the lead readily available for absorption by the human body. An in vitro bioaccessibility method, such as EPA Method 1340 (EPA, 2013), will allow us to estimate the portion of lead that can be absorbed by humans and therefore provide a better understanding of its risk. Moreover, to enhance the accessibility and affordability of this analysis, we seek to develop a procedure for analyzing EPA Method 1340 assays using X-ray fluorescence (XRF) spectroscopy. Unlike the costly and labor-intensive Inductively Coupled Plasma (ICP) analysis, XRF offers a more accessible and rapid alternative, aligning with our goal to make bioaccessibility assays more rapid, feasible and affordable, especially for laboratories serving underprivileged urban communities.
To summarize, if functionalized biochar proves to considerably decrease Pb bioaccessibility in soil, its use could be incredibly beneficial for urban gardeners to mitigate the risk of Pb poisoning for themselves, their children, and their entire communities. In alignment with this proposal’s objective, we are also validating a novel, rapid and cost-effective analytical method for quantifying the amount of bioaccessible Pb in soils.
Research
- 1. Establishing a procedure for analyzing EPA Method 1340 assays by XRF spectroscopy
Liquid EPA Method 1340 Extractions
This method involves subjecting soil samples to simulated physiological conditions, mimicking the acidic environment of the human stomach, to determine the portion of lead that can potentially be absorbed by the body. The extraction entails dissolving 1 g of soil in 0.4 M glycine solution at pH 1.5, shaking at 30 rpm and 37˚C for 1.5 hours, and finally filtering the solution to remove any soil particles. The amount of lead dissolved in the solution is then quantified, typically by Inductively Coupled Plasma (ICP) spectroscopy. The amount of lead released from the soil under these conditions provides an estimate of its bioaccessibility. In this work, instead of using ICP, we will develop an XRF-based approach as a faster and more affordable method to measure bioaccessible Pb via EPA 1340 extractions.
XRF Spectrometry on EPA 1340 Extractions
X-ray fluorescence (XRF) is a non-destructive method of elemental analysis that provides concentrations of elements ranging from sodium to uranium. Samples are most commonly prepared as powders (for lower quality results), flattened pellets, or fused beads (for highest quality results). X-rays strike the sample, causing electrons to be ejected from the atoms’ inner orbitals. Electrons from energetically higher orbitals must then replace the ejected electrons, and energy is released in the form of measurable x-ray fluorescence. Because the amount of energy released is different for different elements, we can efficiently identify metals and quantify their concentrations. In this work, we aim to investigate the potential utility of XRF for measuring heavy metals in liquid samples rather than solid samples. In this project, we plan to first establish the detection limits of XRF for determining lead concentrations in aqueous samples. Second, we will test the precision and accuracy of XRF as compared to ICP. We will extract 7 different soil standards in a wide concentration range (64-5532 ppm Pb) following the EPA Method 1340 procedure. The IVBA results from the standards will be used as ground truth values to ensure both analyses are providing accurate measurements. In addition, a NIST standard is extracted and tested alongside every EPA Method 1340 extraction batch to ensure quality control of the extraction procedure.
Using between 30 and 100 urban soil samples from Philadelphia, PA, we will construct a simple linear regression model comparing ICP vs. XRF IVBA measurements. Approximately 3 mL of extraction fluid will be reserved for XRF analysis. The fluid will be placed into XRF sample cups covered with polypropylene thin film, and then analyzed under an air atmosphere (in contrast to helium or vacuum). Following analysis by XRF, we will send the remaining extraction fluid to Penn State’s Agricultural Analytical Services Laboratory (AASL) for ICP analysis to validate the results. With results from both XRF and ICP, we can construct statistical regression analyses to compare the two methods. If we can extract and analyze sufficiently many soil samples, we may reserve 20% of the dataset for model validation.
- 2. Establishing the effectiveness of heat-functionalized biochar for immobilizing lead in soil.
Functionalized Biochar Preparation Procedure
We chose to use sustainable wood-based biochar from a company in cenl Pennsylvania, Metzler Biochar, due to their products being certified according to International Biochar Initiative requirements. Metzler Biochar has provided us with 1 cubic foot of their PureCHAR biochar to use in our experiments free of charge. First, we will functionalize the biochar by heating it in a muffle furnace in the presence of oxygen. In preliminary experiments, we have found that this process decreases biochar pH (from approximately pH 9 to 7 after 1 hour of heating at 300˚C), likely by adding acidic functional groups to the biochar surface. The increase in functional groups should allow for higher sorption capacity of lead, while also having the benefit of not significantly decreasing soil pH below a liming point where further changes are not desired. To assess the potential increase of functionalized biochar reactivity, surface area and porosity will be measured by N2 adsorption at a collaborator’s laboratory. A few trials will be made to establish the best conditions (time, temperature) to produce optimum biochar with tailored surface area and porosity to maximize their ability to immobilize bioaccesible Pb (Fig. 1-1, attached below).
Amendment of Contaminated Soil with Biochar
First, a trial experiment was performed using previously-collected soil from a community garden in Philadelphia, PA. The dried, sieved soil was a sandy loam (per the hydrometer method) with a pH of 7.44 and 6.5% organic matter (LOI). The total lead concentration was measured as 576 ppm using XRF on pressed pellet samples. 32 g of soil was placed into each of 36 plastic beakers. The beakers were divided into three treatments: control soil, soil with 5% w/w unheated biochar, and soil with 5% w/w heated biochar. The total lead in the unheated and heated biochar mixtures was remeasured (549 and 550 ppm, respectively). Each beaker was covered in perforated parafilm and maintained at 40% moisture by mass (as discussed later, this moisture level was quite high and may have affected our results). At approximately 0, 2, 8, and 13 weeks, three beakers were removed for bioaccessibility and pH tests. The EPA Method 1340: In-Vitro Bioaccessibility (IVBA) Assay for Lead in Soil procedure was used (with modification: soil was sieved to 250 μm). The resulting extracts were tested with ICP-OES (MDL = 0.005 and LOQ = 0.025). IVBA calculations were carried out as follows: (Extracted Pb*100)/(Total Pb * Mass of Sample).
For the second experiment, contaminated soil was collected the Lancaster area. We collaborated with local agency Green and Healthy Homes Initiative to find a specific location for this task. Our contact, Darren Palmer, provided us with one composite soil sample, approximately one 5-gallon bucket in volume, obtained from the topsoil of a 2.5 x 22 feet strip on a residential property. Initial portable XRF screening of the soil gave a result of over 2,000 ppm Pb.
Learning from the difficulties encountered in our trial experiment, we plan to perform an experiment to test further conditions under which the heated biochar might significantly reduce lead bioaccessibility in soil. We will first characterize soil, analyzing total lead (using EPA Method 3050B), LOI organic matter content, pH, field capacity, and soil texture. Because this soil is extremely contaminated (~10x the EPA's regulatory limit), if necessary, the soil may be diluted. For our experiment, in the laboratory we will set up containers of ~250 g of soil (~30 g for each replicate) with: (a) control soil without additions; (b) soil with untreated biochar; and (c) soil with treated biochars. The soil will be kept moist (using the mass of water needed to keep the soil at field capacity) using weekly adjustments and allowed to react with the biochar over 2 weeks. Adjusted moisture conditions and increased replicates as compared to our trial experiment will hopefully allow us to see whether the slight decrease in IVBA in heated biochar conditions shows statistical significance.
Over a course of 6 months (emulating a growing season), we will take measurements of pH and bioaccessible lead (using EPA Method 1340 and ICP). In order to have samples for measuring each month, we will require 6 additional replicates of each treatment. For each sampling time we will have five subsamples to evaluate the consistency of the extraction values (Fig. 1-3).
Biochar Functionalization Results
3-hour heated biochar showed the most dramatic changes in physical and chemical properties, and so was chosen for use in our IVBA experiments. Biochar pH decreased from 10.26 to 8.79 following 3 hours of heating. This pH shift supports the conclusion that heating in oxygenic conditions increases acidic surface groups on biochar surfaces. The surface area also increased from 502.76 m2/g to 659.24 m2/g (31.1% change), while pore volume increased from 0.227 cm3/g to 0.336 cm3/g (48.0% change).
Pb in-vitro Bioaccessibility Results
While the biochar-soil experiments using the highly contaminated Lancaster soil have not yet been performed, our trial experiment using Philadelphia soils is complete. As shown in Figure 2, lower bioaccessibility was observed within the first week of the experiment: 77.58% (control), 75.59% (unheated biochar), and 75.62% (heated biochar). After 2 weeks bioaccessibility increased to 89.91% (control), 88.06% (unheated biochar), and 88.51% (heated biochar). As pH was not significantly lowered across time and treatment, this increase in bioaccessible Pb is not likely due to pH-dependent changes in solubility. Instead, the increase may be due to factors such as localized anaerobic conditions releasing Pb from iron oxides, or an initial increase in microbial activity breaking down insoluble organometallic complexes.
The data from 8 and 13 weeks show all samples remaining between about 83% and 86% bioaccessibility. Within these timepoints, both the unheated and heated biochar treatments had significantly lower IVBA measurements than the control soil (p < 0.000; Figure 3). And while not statistically significant, there was a small decrease in bioaccessible Pb in the heated biochar treatment compared to the unheated biochar treatment.
Although these differences have been identified between treatments, they may or may not be of practical significance.
ICP vs. XRF
We have completed analysis of the 7 soil standards, and are now working on extracting and analyzing the 30-100 Philadelphia soil samples that will be used in our regression analysis.
Education & Outreach Activities and Participation Summary
Participation Summary:
Outreach Task 1:
A common skepticism about biochar use is that its performance varies based on the application purpose or soil type. According to biochar-us.org, effective biochar application to improve soil quality involves three key components: the right biochar source, the right application rate, and the right placement in soil. Our project investigates a specific type of biochar designed to reduce lead (Pb) bioavailability in soil. We will host a webinar to discuss the importance of selecting the appropriate biochar type for metal remediation and how to optimize its properties for maximum effectiveness. This webinar will be illustrated with results from our experiment, offered free of charge on the Penn State Extension website. We anticipate 50-80 growers will attend, potentially shifting their perspectives on the efficacy of biochar as a soil amendment.
For our experimental trials, we tested different biochar application rates which is one of the three key components (source, application rate, placement) for the optimum biochar use. This data will be included in the webinar materials.
Outreach Task 2:
Growers often need assistance interpreting soil test results, especially concerning metal pollutants. Recent EPA guidelines have lowered the Pb toxicity threshold from 400 ppm to 200 ppm. Our project's findings on bioavailable versus total Pb will aid growers in making informed decisions about potential risks. We will create a factsheet detailing these two forms of Pb and introducing the use of portable XRF devices to estimate bioaccessible Pb. This will help contextualize soil pollution levels and necessary remediation efforts.
Using our research data we will prepare the fact sheet with the tentative title: “Understanding Lead in Soil: Total vs. Bioavailable Lead for Growers and Gardeners.” While the data are still being collected, we start to plan the structure and main sections of the factsheet. It will consist of the following sections:
1. Introduction: Brief overview of soil lead contamination issues and importance of understanding lead levels for growers and gardeners
2. Lead in Soil: compare Total vs. Bioavailable - give definitions and significance for each.
3. Updated EPA Guidelines: New Pb toxicity threshold: Lowered from 400 ppm to 200 ppm, Rationale behind the change and implications for soil management and remediation efforts
4. XRF Technology: basic principles of X-ray fluorescence, measurement as powders and liquids, advantages and disadvantages.
5. Interpreting XRF Results: (1) relationships between bioavailable and total Pb through regression analysis to adjust to the regulations based on total concentrations. (2) bioavailable Pb - ICP vs XRF – evaluation of XRF method on liquid samples.
6. Relevance to Growers: How to use XRF data for informed decision-making, assessing potential risks to crops and human health
7. Best Practices for Soil Testing: proper sampling techniques for XRF analysis, frequency of testing recommendations, when to seek professional lab analysis if needed to comply with regulations)
8. Additional Resources: Links to EPA methods (e.g., Method 6200), Relevant standards and guidelines, Local soil testing services and support, Ag Analytical, SRCL
10. Glossary of Terms: Definitions of key technical terms used in the fact sheet
Outreach Task 3:
The high cost of biochar, especially in comparison to compost, can deter small farm owners. We will gather information on mobile biochar production systems, such as the CharBoss developed by the USDA Forest Service, suitable for smaller operations. These systems can reduce transportation costs and allow for the utilization of crop residues, mitigating CO2 emissions from their decomposition. The fact sheet, "Portable Biochar Production Unit and Their Role in Improving the Environment," will be available on the Penn State Extension website and promoted through our existing networks. It will be formatted for both print and online access, with an expected reach of approximately 75 urban growers.
For this task we started to explore the currently available producers of portable biochar production units on the market. All biochar units will compare and ranked based on the capacity, mobility and CO2 sequestration potential. Principles of operation for each units and production conditions will be discussed.
Outreach Task 4:
Translation Services. Educational materials from Tasks 1-3 will be translated into Spanish to broaden accessibility. These materials will be available on the same platforms as their English versions, aiming to reach 85 urban growers.