GT/ New high-performance solar cell material
Energy & green technology biweekly vol.66, 15th March — 29th March
TL;DR
- A study unveils a novel, stable solar absorber material through a rapid computational screening method, potentially revolutionizing solar cell technology.
- Nanoscale devices leveraging the hydroelectric effect can now harvest electricity from fluid evaporation, tapping into previously untapped energy reservoirs.
- A breakthrough in efficient hydrogen storage has been achieved by a dedicated research team.
- The efficiency of hydrogen generation sees a boost with the development of a catalyst for the urea oxidation reaction.
- Concerns about climate-induced range shifts for species amidst solar energy expansion are underscored by a study focusing on Joshua trees, kit foxes, and solar developments.
- A certified world-record efficiency of 27.1% in solar energy conversion is achieved by a newly engineered triple-junction perovskite/Si tandem solar cell, showcasing remarkable progress in photovoltaic technology.
- Agricultural leftovers find a new purpose as scientists devise a sustainable method to produce high-performance plastics.
- Innovative electrochemical approaches tackle pollution from ‘forever chemicals’ like PFAS found in various products, offering hope for remediation.
- New research reveals the air-purifying capabilities of green walls, with certain plant species exhibiting superior pollution removal.
- The majority of CO2 emissions in highly alkaline waters are attributed to carbonate buffering, challenging conventional methods of tracking riverine CO2 emissions and prompting the proposal of a more accurate tracking method.
- And more!
Green Technology Market
Green technology is an applicable combination of advanced tools and solutions to conserve natural resources and environment, minimize or mitigate negative impacts from human activities on the environment, and ensure sustainability development. Green technology is also referred to as clean technology or environmental technology which includes technologies, such as IoT, AI, analytics, blockchain, digital twin, security, and cloud, which collect, integrate, and analyze data from various real-time data sources, such as sensors, cameras, and Global Positioning System (GPS).
Green technology, also known as sustainable technology, protects the environment by using various forms of sustainable energy. Some of the best examples of green technologies include solar panels, LED lighting, wind energy, electric vehicles, vertical farming, and composting.
The global Green Technology and Sustainability market size to grow from USD 11.2 billion in 2020 to USD 36.6 billion by 2025, at a Compound Annual Growth Rate (CAGR) of 26.6% during the forecast period. The growing consumer and industrial interest for the use of clean energy resources to conserve environment and increasing use of Radio Frequency Identification sensors across industries are driving the adoption of green technology and sustainability solutions and services in the market.
Latest Research
Discovery of the Zintl-phosphide BaCd2P2 as a long carrier lifetime and stable solar absorber
by Zhenkun Yuan, Diana Dahliah, Muhammad Rubaiat Hasan, et al in Joule
A Dartmouth Engineering-led study reported the discovery of an entirely new high-performance material for solar absorbers — the central part of a solar cell that turns light into electricity — that is stable and earth-abundant. The researchers used a unique high-throughput computational screening method to accelerate the discovery process and were able to quickly evaluate approximately 40,000 known candidate materials.
“This is the first example in the field of photovoltaics where a new material has been found through this type of approach with an experimental follow-up,” said Geoffroy Hautier, Dartmouth’s Hodgson Family Associate Professor of Engineering. “Most people study one or two materials at a time, and we looked at forty thousand.”
Dartmouth researcher Zhenkun Yuan is first author on the study with co-authors including research associate Yihuang Xiong, engineering PhD candidates Gideon Kassa and Andrew Pike, and engineering professors Hautier and Jifeng Liu — as well as researchers from eight other partner institutions. This research stems from an award Hautier and Liu received in 2022 as part of $540 million the US Department of Energy granted to universities and National Laboratories nationwide to develop clean-energy technologies, including new photovoltaic materials.
The solar absorber material was confirmed in the lab to be not only promising in its ability to efficiently transform light into electricity, but also highly stable in both air and water.
“You can put it out for six months and it will stay the same,” Hautier said. “When you don’t have to worry about moisture and air contamination, that significantly reduces your costs.”
The study points out that, normally, finding new solar materials is tedious and slow with an overwhelming number of options to even begin to consider.
“We’ve been building a database of known materials — both naturally occurring and human-made — for a long time,” Hautier explained. “That’s giving us the capability to rapidly screen and make decisions on what may or may not be useful. We weren’t able to screen for stability, but we could narrow it down to approximately 20 reasonable solar materials — among the thousands and thousands of possibilities — and after talking with our colleagues, we had a feeling this one would be stable.”
The team plans to continue to improve the tools for even better screening, as well as explore the entire family of materials they call “Zintls,” which could lead to enhancements and optimizations of the discovered material.
“There are a lot of opportunities around further characterizing this material and understanding it better, such as how it absorbs light and how to make it as a thin film,” said Liu, who conducts and oversees materials-testing in his lab. “Collaboration is crucial. It takes a whole community of thinkers and many different skills to make it all work — computing, experimentation, fabrication, characterization, optimization — and you need to put all that together in a team.”
“We won’t have it as a solar panel tomorrow,” Hautier said, “but we think this family of materials is exceptional and worth looking at.”
Salinity-dependent interfacial phenomena toward hydrovoltaic device optimization
by Tarique Anwar, Giulia Tagliabue in Device
Evaporation is a natural process so ubiquitous that most of us take it for granted. In fact, roughly half of the solar energy that reaches the earth drives evaporative processes. Since 2017, researchers have been working to harness the energy potential of evaporation via the hydrovoltaic (HV) effect, which allows electricity to be harvested when fluid is passed over the charged surface of a nanoscale device. Evaporation establishes a continuous flow within nanochannels inside these devices, which act as passive pumping mechanisms. This effect is also seen in the microcapillaries of plants, where water transport occurs thanks to a combination of capillary pressure and natural evaporation.
Although hydrovoltaic devices currently exist, there is very little functional understanding of the conditions and physical phenomena that govern HV energy production at the nanoscale. It’s an information gap that Giulia Tagliabue, head of the Laboratory of Nanoscience for Energy Technology (LNET) in the School of Engineering, and PhD student Tarique Anwar wanted to fill. They leveraged a combination of experiments and multiphysics modelling to characterize fluid flows, ion flows, and electrostatic effects due to solid-liquid interactions, with the goal of optimizing HV devices.
“Thanks to our novel, highly controlled platform, this is the first study that quantifies these hydrovoltaic phenomena by highlighting the significance of various interfacial interactions. But in the process, we also made a major finding: that hydrovoltaic devices can operate over a wide range of salinities, contradicting prior understanding that highly purified water was required for best performance,” says Tagliabue.
The researchers’ device represents the first hydrovoltaic application of a technique called nanosphere colloidal lithography, which allowed them to create a hexagonal network of precisely spaced silicon nanopillars. The spaces between the nanopillars created the perfect channels for evaporating fluid samples, and could be finely tuned to better understand the effects of fluid confinement and the solid/liquid contact area.
“In most fluidic systems containing saline solutions, you have an equal number of positive and negative ions. However, when you confine the liquid to a nanochannel, only ions with a polarity opposite to that of the surface charge will remain,” Anwar explains. “This means that if you allow liquid to flow through the nanochannel, you will generate current and voltages.”
“This goes back to our major finding that the chemical equilibrium for the surface charge of the nanodevice can be exploited to extend the operation of hydrovoltaic devices across the salinity scale,” adds Tagliabue. “Indeed, as the fluid ion concentration increases, so does the surface charge of the nanodevice. As a result, we can use larger fluid channels while working with higher-concentration fluids. This makes it easier to fabricate devices for use with tap or seawater, as opposed to only purified water.”
Because evaporation can occur continuously over a wide range of temperatures and humidities — and even at night — there are many exciting potential applications for more efficient HV devices. The researchers hope to explore this potential with the support of a Swiss National Science Foundation Starting Grant, which aims to develop “a completely new paradigm for waste-heat recovery and renewable energy generation at large and small scales,” including a prototype module under real-world conditions on Lake Geneva.
And because HV devices could theoretically be operated anywhere there is liquid — or even moisture, like sweat — they could also be used to power sensors for connected devices, from smart TVs to health and fitness wearables. With the LNET’s expertise in light energy harvesting and storage systems, Tagliabue is also keen to see how light and photothermal effects could be used to control surface charges and evaporation rates in HV systems. Finally, the researchers also see important synergies between HV systems and clean water generation.
“Natural evaporation is used to drive desalination processes, as fresh water can be harvested from saltwater by condensing the vapor produced by an evaporative surface. Now, you could imagine using an HV system both to produce clean water and harness electricity at the same time,” Anwar explains.
Small-pore hydridic frameworks store densely packed hydrogen
by Hyunchul Oh, Nikolay Tumanov, Voraksmy Ban, Xiao Li, Bo Richter, et al in Nature Chemistry
A groundbreaking development in efficient hydrogen storage has been reported by Professor Hyunchul Oh in the Department of Chemistry at UNIST, marking a significant advancement in future energy systems. This innovative research centers around a nanoporous magnesium borohydride structure (Mg(BH₄)₂), showcasing the remarkable capability to store hydrogen at high densities even under normal atmospheric pressure.
The research team, under the leadership of Professor Oh, has successfully tackled the challenge of low hydrogen storage capacity by leveraging advanced high-density adsorption technology. Through the synthesis of a nanoporous complex hydride comprising magnesium hydride, solid boron hydride (BH4)2, and magnesium cation (Mg+), the developed material enables the storage of five hydrogen molecules in a three-dimensional arrangement, achieving unprecedented high-density hydrogen storage.
The reported material exhibits an impressive hydrogen storage capacity of 144 g/L per volume of pores, surpassing traditional methods, such as storing hydrogen as a gas in a liquid state (70.8 g/L). Additionally, the density of hydrogen molecules within the material exceeds that of the solid state, highlighting the efficiency of this novel storage approach.
Professor Oh emphasizes the significance of this breakthrough, stating, “Our innovative material represents a paradigm shift in the realm of hydrogen storage, offering a compelling alternative to traditional approaches.” This transformative development not only enhances the efficiency and economic viability of hydrogen energy utilization but also addresses critical challenges in large-scale hydrogen storage for public transportation applications.
Accessible Ni‐Fe‐Oxalate Framework for Electrochemical Urea Oxidation with Radically Enhanced Kinetics
by Jiseon Kim, Min‐Cheol Kim, Sang Soo Han, Kangwoo Cho in Advanced Functional Materials
Professor Kangwoo Cho and PhD candidate Jiseon Kim from the Division of Environmental Science & Engineering at Pohang University of Science and Technology (POSTECH) collaborated with the Korea Institute of Science and Technology (KIST) to devise a novel catalyst aimed at enhancing the efficiency of reactions using contaminated municipal sewage to produce hydrogen — a green energy source.
With the growing environmental concerns of pollution associated with fossil fuel, hydrogen has garnered increased interest. Water electrolysis technology is a sustainable process that leverages Earth’s abundant water to produce hydrogen. However, the concurrent oxygen evolution reaction during hydrogen production is notably slow, resulting in a considerably low energy conversion efficiency.
Lately, the academic community has been tackling this issue by integrating the urea oxidation reaction with the hydrogen generation reaction. Urea, a pollutant found in urine, releases a significant amount of energy during its oxidation process, offering a potential means to enhance both the efficiency of hydrogen generation and the purification of toilet wastewater. Ultimately, it is necessary to find a catalyst that can effectively drive the urea oxidation reaction, thereby amplifying the efficiency of both hydrogen generation and wastewater treatment.
In pursuit of increased efficiency in the urea oxidation reaction, the team created a catalyst known as nickel-iron-oxalate (O-NFF). This catalyst combines iron (Fe) and oxalate on nickel (Ni) metal, resulting in an expansive surface area characterized by nanometer-sized particles in fragment form. This unique property enables the catalyst to adsorb more reactants, facilitating an accelerated urea oxidation reaction.
In experiments, the O-NFF catalyst devised by the team successfully lowered the voltage required for hydrogen generation to 1.47 V RHE (at 0.5 A/cm2) and exhibited a high reaction rate even when tested in a mixed solution of potassium hydroxide (1 M) and urea (0.33 M) with a Tafel slope of 12.1 mV/dec. The researchers further validated the catalyst’s efficacy by confirming its promotion of the urea oxidation reaction through photoelectron/X-ray absorption spectroscopy using a radiation photo accelerator.
Professor Kangwoo Cho who led the research stated, “We have developed a catalyst capable of purifying municipal sewage while simultaneously enhancing the efficiency of hydrogen production, a green energy source.” He added, “We anticipate that O-NFF catalysts, synthesized from metals and organics, will contribute to the improved efficiency of industrial electrolysis hydrogen production.”
Aligning renewable energy expansion with climate-driven range shifts
by Uzma Ashraf, Toni Lyn Morelli, Adam B. Smith, Rebecca R. Hernandez in Nature Climate Change
Climate change is driving both the loss of biodiversity and the need for clean, renewable energy. It is also shifting where species are expected to live in the future. Yet these realities are rarely considered together. Where can clean energy projects be built without impacting the future habitat ranges of threatened and endangered species?
A study from the University of California, Davis, examines this question by overlaying renewable energy siting maps with the ranges of two species in the southwestern United States: the iconic and climate-vulnerable Joshua tree and federally endangered San Joaquin kit fox. The study found that Joshua trees are expected to lose 31% of their habitat while kit foxes lose 81% by 2070. That’s with climate change alone, under a moderate emissions scenario. When overlayed with existing and proposed renewable energy projects, an additional 1.7% of Joshua tree habitat and 3.9% of kit fox habitat could be lost.
“This study describes how we need to use more renewable energy to fight climate change, but it also warns us that as we expand renewable energy, we are going to overlap with biodiversity hotspots,” said first author Uzma Ashraf, a postdoctoral scholar with the UC Davis Wild Energy Center and the Department of Land, Air and Water Resources. “We show how advanced computer modeling can be applied to improve our understanding of how to site renewable energy resources in ways that benefit biodiversity and their shifting ranges.”
Globally, 290 gigawatts (GW) of renewable energy capacity were developed in 2021. The world needs to ramp that up to 1,120 GW every year between now and 2030 to meet net zero emissions goals by 2050. Meanwhile, animal populations have declined by two-thirds in the past 50 years, mostly due to habitat losses, which are exacerbated by climate change, the study notes. Altering the landscape could damage places that would otherwise serve as climate refugia under future climate conditions.
San Joaquin kit foxes have been known to use solar facilities for habitat, which scientists attribute to the shade the facilities provide. The study said this suggests there may be ways to minimize impacts to the species through careful attention to its ecological needs.
Corresponding author and Associate Professor Rebecca R. Hernandez directs the Wild Energy Center at UC Davis. She said her center is working to develop a framework to help clean energy developers make future-facing decisions on siting that consider expected range shifts of animals.
“There is a current moonshot for solar and wind energy development,” Hernandez said. “It is one where the footprint of the transition takes hold fast but in a manner that reinforces goals for biodiversity conservation and social justice. Species maps are now dynamic over time under climate change. Our team uses state-of-the-art computational tools to chart a safe passage for renewables.”
Triple-junction solar cells with cyanate in ultrawide bandgap perovskites
by Shunchang Liu, Yue Lu, Cao Yu, Jia Li, Ran Luo, Renjun Guo, Haoming Liang, Xiangkun Jia, Xiao Guo, Yu-Duan Wang, Qilin Zhou, Xi Wang, Shaofei Yang, Manling Sui, Peter Müller-Buschbaum, Yi Hou in Nature
Scientists from the National University of Singapore (NUS) have developed a novel triple-junction perovskite/Si tandem solar cell that can achieve a certified world-record power conversion efficiency of 27.1 per cent across a solar energy absorption area of 1 sq cm, representing the best-performing triple-junction perovskite/Si tandem solar cell thus far. To achieve this, the team engineered a new cyanate-integrated perovskite solar cell that is stable and energy efficient.
Solar cells can be fabricated in more than two layers and assembled to form multi-junction solar cells to increase efficiency. Each layer is made of different photovoltaic materials and absorbs solar energy within a different range. However, current multi-junction solar cell technologies pose many issues, such as energy loss which leads to low voltage and instability of the device during operation.
To overcome these challenges, Assistant Professor Hou Yi led a team of scientists from NUS College of Design and Engineering (CDE) and Solar Energy Research Institute of Singapore (SERIS) to demonstrate, for the first time, the successful integration of cyanate into a perovskite solar cell to develop a cutting-edge triple-junction perovskite/Si tandem solar cell that surpasses the performance of other similar multi-junction solar cells. Asst Prof Hou is a Presidential Young Professor at the Department of Chemical and Biomolecular Engineering under CDE as well as a Group Leader at SERIS, a university-level research institute in NUS.
“Remarkably, after 15 years of ongoing research in the field of perovskite-based solar cells, this work constitutes the first experimental evidence for the inclusion of cyanate into perovskites to boost the stability of its structure and improve power conversion efficiency,” said Asst Prof Hou.
The interactions between the components of the perovskite structure determine the energy range that it can reach. Adjusting the proportion of these components or finding a direct substitute can help modify the perovskite’s energy range. However, prior research has yet to produce a perovskite recipe with an ultrawide energy range and high efficiency.
In this recently published work, the NUS team experimented on cyanate, a novel pseudohalide, as a substitute for bromide — an ion from the halide group that is commonly used in perovskites. Dr Liu Shunchang, Research Fellow in Asst Prof Hou’s team, employed various analytical methods to confirm the successful integration of cyanate into the perovskite structure, and fabricated a cyanate-integrated perovskite solar cell. Further analysis of the new perovskite’s atomic structure provided — for the first time — experimental evidence that incorporating cyanate helped to stabilise its structure and form key interactions within the perovskite, demonstrating how it is a viable substitute for halides in perovskite-based solar cells.
When assessing performance, the NUS scientists found that perovskite solar cells incorporated with cyanate can achieve a higher voltage of 1.422 volts compared to 1.357 volts for conventional perovskite solar cells, with a significant reduction in energy loss. The researchers also tested the newly engineered perovskite solar cell by continuously operating it at maximum power for 300 hours under controlled conditions. After the test period, the solar cell remained stable and functioned above 96 per cent capacity.
Encouraged by the impressive performance of the cyanate-integrated perovskite solar cells, the NUS team took their ground-breaking discovery to the next step by using it to assemble a triple-junction perovskite/Si tandem solar cell. The researchers stacked a perovskite solar cell and a silicon solar cell to create a dual-junction half-cell, providing an ideal base for the attachment of the cyanate-integrated perovskite solar cell.
Once assembled, the researchers demonstrated that despite the complexity of the triple-junction perovskite/Si tandem solar cell structure, it remained stable and attained a certified world-record efficiency of 27.1 per cent from an accredited independent photovoltaic calibration laboratory.
“Collectively, these advancements offer ground-breaking insights into mitigating energy loss in perovskite solar cells and set a new course for the further development of perovskite-based triple junction solar technology,” said Asst Prof Hou.
Theoretical efficiency of triple-junction perovskite/Si tandem solar cells exceeds 50 per cent, presenting significant potential for further enhancements, especially in applications where installation space is limited. Going forward, the NUS team aims to upscale this technology to larger modules without compromising efficiency and stability. Future research will focus on innovations at the interfaces and composition of perovskite — these are key areas identified by the team to further advance this technology.
Performance polyamides built on a sustainable carbohydrate core
by Lorenz P. Manker, Maxime A. Hedou, Clement Broggi, Marie J. Jones, Kristoffer Kortsen, Kalaiyarasi Puvanenthiran, Yildiz Kupper, Holger Frauenrath, François Marechal, Veronique Michaud, Roger Marti, Michael P. Shaver, Jeremy S. Luterbacher in Nature Sustainability
In our rapidly industrialized world, the quest for sustainable materials has never been more urgent. Plastics, ubiquitous in daily life, pose significant environmental challenges, primarily due to their fossil fuel origins and problematic disposal.
Now, a study led by Jeremy Luterbacher’s team at EPFL unveils a pioneering approach to producing high-performance plastics from renewable resources. The research introduces a novel method for creating polyamides — a class of plastics known for their strength and durability, the most famous of which are nylons — using a sugar core derived from agricultural waste. The new method leverages a renewable resource, and also achieves this transformation efficiently and with minimal environmental impact.
“Typical, fossil-based plastics need aromatic groups to give rigidity to their plastics — this gives them performance properties like hardness, strength and high temperature resistance,” says Luterbacher. “Here, we get similar results but use a sugar structure, which is ubiquitous in nature and generally completely non-toxic, to provide rigidity and performance properties.”
Lorenz Manker, the study’s lead-author, and his colleagues developed a catalyst-free process to convert dimethyl glyoxylate xylose, a stabilized carbohydrate made directly from biomass such as wood or corn cobs, into high-quality polyamides. The process achieves an impressive atom efficiency of 97%, meaning almost all the starting material is used in the final product, which drastically reduces waste.
The bio-based polyamides exhibit properties that can compete with their fossil counterparts, offering a promising alternative for various applications. What’s more, the materials demonstrated significant resilience through multiple cycles of mechanical recycling, maintaining their integrity and performance, which is a crucial factor for managing the lifecycle of sustainable materials.
The potential applications for these innovative polyamides are vast, ranging from automotive parts to consumer goods, all with a significantly reduced carbon footprint. The team’s techno-economic analysis and life-cycle assessment suggest these materials could be competitively priced against traditional polyamides including nylons (e.g. nylon 66), with a global warming potential reduction of up to 75%.
Complete electrocatalytic defluorination of perfluorooctane sulfonate in aqueous solution with nonprecious materials
by Ziyi Meng, Madeleine K. Wilsey, Connor P. Cox, Astrid M. Müller in Journal of Catalysis
Scientists from the University of Rochester have developed new electrochemical approaches to clean up pollution from “forever chemicals” found in clothing, food packaging, firefighting foams, and a wide array of other products. A new study describes nanocatalysts developed to remediate per- and polyfluoroalkyl substances, known as PFAS.
The researchers, led by assistant professor of chemical engineering Astrid Müller, focused on a specific type of PFAS called Perfluorooctane sulfonate (PFOS), which was once widely used for stain-resistant products but is now banned in much of the world for its harm to human and animal health. PFOS is still widespread and persistent in the environment despite being phased out by US manufacturers in the early 2000s, continuing to show up in water supplies.
Müller and her team of materials science PhD students created the nanocatalysts using her unique combination of expertise in ultrafast lasers, materials science, chemistry, and chemical engineering.
“Using pulsed laser in liquid synthesis, we can control the surface chemistry of these catalysts in ways you cannot do in traditional wet chemistry methods,” says Müller. “You can control the size of the resulting nanoparticles through the light-matter interaction, basically blasting them apart.”
The scientists then adhere the nanoparticles to carbon paper that is hydrophilic, or attracted to water molecules. That provides a cheap substrate with a high surface area. Using lithium hydroxide at high concentrations, they completely defluorinated the PFOS chemicals.
Müller says that for the process to work at a large scale, they will need to treat at least a cubic meter at a time. Crucially, their novel approach uses all nonprecious metals, unlike existing methods that require boron-doped diamond. By their calculations, treating a cubic meter of polluted water using boron-doped diamond would cost $8.5 million; the new method is nearly 100 times cheaper.
In future studies, Müller hopes to understand why lithium hydroxide works so well and whether even less expensive, more abundant materials can be substituted to bring the cost down further. She also wants to apply the method to an array of PFAS chemicals that are still prevalently used but have been linked to health issues ranging from development in babies to kidney cancer. Müller says that despite their issues, outright banning all PFAS chemicals and substances is not practical because of their usefulness in not only consumer products, but in green technologies as well.
“I would argue that in the end, a lot of decarbonization efforts — from geothermal heat pumps to efficient refrigeration to solar cells — depend on the availability of PFAS,” says Müller. “I believe it’s possible to use PFAS in a circular, sustainable way if we can leverage electrocatalytic solutions to break fluorocarbon bonds and get the fluoride back out safely without putting it into the environment.”
Although commercialization is a long way off, Müller filed a patent with support from URVentures, and foresees it being used at wastewater treatment facilities and by companies to clean up contaminated sites where they used to produce these PFAS chemicals. She also calls it a social justice issue.
“Often in areas with lower income across the globe, there’s more pollution,” says Muller. “An advantage of an electrocatalytic approach is that you can use it in a distributed fashion with a small footprint using electricity from solar panels.”
Exploring the interplay between particulate matter capture, wash-off, and leaf traits in green wall species
by Mamatha Tomson, Prashant Kumar, K.V. Abhijith, John F. Watts in Science of The Total Environment
Green walls can strip pollution from the air — and some plants do it better than others, according to new research from the University of Surrey.
Researchers planted 10 species on a custom-built 1.4m green wall beside the A3 in Guildford.
Mamatha Tomson, postgraduate researcher at the University of Surrey, said: “By planting vertically on a green wall, communities can clean up their air without taking up too much street space.
“Our study suggests that this process depends not only on the shape of its leaves but on the micromorphological properties of their surfaces. We think a good mixture of species will produce the most effective green walls — and look forward to carrying out further research to see if we’re right.”
Plants remove air pollution in two steps. First, they catch particles of pollution on their leaves. Then, the rain washes them safely to the ground. Evergreen Candytuft and Ivy leaves were found to be especially good at trapping pollutant particles, large and small. Meanwhile, rain was able to wash most of the pollution off the hairy leaves of Lavender. Candytuft and Marjoram also performed well in washing off smaller particles of pollution.
Professor Prashant Kumar, director of the University’s Global Centre for Clean Air Research (GCARE), said: “We hope that town planners and infrastructure experts can use our findings to think more carefully about what they plant. “Having a green wall is a great way of removing pollution — but what you plant on it can make a big difference to how successful it will be.”
Impacts of Carbonate Buffering on Atmospheric Equilibration of CO2, δ13CDIC, and Δ14CDIC in Rivers and Streams
by Matthew J. Winnick, Brian Saccardi in Global Biogeochemical Cycles
A team of researchers from the University of Massachusetts Amherst that specializes in accounting for the carbon dioxide release by streams, rivers and lakes recently demonstrated that the chemical process known as “carbonate buffering” can account for the majority of emissions in highly alkaline waters. Furthermore, carbonate buffering distorts the most commonly used method of tracking the origins of CO2 in streams. The research proposes a better method for tracking the origin of riverine CO2 emissions.
Inland waters, including streams, rivers and lakes, account for roughly 5.5 gigatons of CO2 emissions annually — about 15% of what humans emit. But current climate models have trouble accounting for this carbon, in part because, says Matthew Winnick, assistant professor of Earth, Geographic and Climate Sciences at UMass Amherst and the paper’s lead author, much of this carbon seems to be produced cryptically, through carbonate buffering.
“The process is a little weird,” says Winnick. “It acts as a kind of hidden reserve pool of CO2, replenishing carbon that is lost to the atmosphere, and ultimately increasing the amount of CO2 available for off-gassing.”
To show how this hidden pool operates, Winnick and his co-author, then-UMass graduate student Brian Saccardi, looked to studies that focused on the carbon content of the oceans. “Carbonate buffering is a really well-known phenomenon in the ocean,” says Winnick, “and even though oceans work differently from inland waters, we were able to borrow the geochemical equations to build a series of models that could account for a wide range of river and stream conditions.”
So what is carbonate buffering? It begins with CO2 — which is everywhere: in the air, in the soil and in water. When CO2 dissolves in water, it can react to form carbonic acid, which, through further reactions, can then become bicarbonate and carbonate. This reaction can also run in reverse, which means that high levels of bicarbonate and carbonate can act as reserve pools of CO2, driving emissions. This entire balance of CO2, water and carbonate is called “carbonate buffering,” and the carbonate reserves can be emitted as a greenhouse gas from stream systems. Indeed, Winnick and Saccardi found that this hidden pool can account for more than 60% of CO2 emissions under alkaline conditions.
There’s yet another trick that carbonate buffering has up its sleeve. In the era of global warming, it is critically important to know both how much carbon is being emitted overall and where this carbon is coming from. “While we don’t think stream emissions contribute to global warming, there is a big question about whether these emissions will change as climate warms, which could amplify warming in the future. To predict changes, we need to know where the CO2 is coming from,” says Winnick. But figuring out which molecule of CO2 came from which source is not a simple task. To track carbon, especially carbon emitted by bodies of water, scientists often use carbon isotopes, or versions of carbon with different masses, which act as a sort of forensic signature that can indicate the carbon’s origin.
However, Winnick and Saccardi discovered that isotope signals in streams are highly sensitive to carbonate buffering reactions. “The primary way we use isotopes to track sources is through their relationship with CO2 concentrations, but carbonate buffering causes these relationships to break down,” says Winnick. This breakdown can point to the wrong carbon culprit if not properly accounted for.
One way to account for carbonate buffering is to measure multiple isotopes of carbon, the new study suggests. Scientists typically only focus on one of the two tracer isotopes, because of the high cost of analyzing both, but the team has found that tracking the origins of both isotopes can help unmask the hidden sources of CO2.
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