Functional green wall development for increasing air pollutant phytoremediation: Substrate development with coconut coir and activated carbon

Modulating phytoremediation: How drip irrigation system affect performance of active green wall and microbial community changes

A recent innovation in air phytoremediation is active green walls, which utilises biofiltration technology with airflow from mechanical ventilation. While this novel technology is gaining traction, the influence of irrigation on soil moisture, and subsequently the microbial communities that play a role in air filtration is untested. In this study, the application of drip irrigation techniques in active green walls were investigated for their influence on system performance. A modular green wall system was tested, with tests across 7 different plant species, as well as a substrate only control. Water distribution across the modules, the water-carrying capacity and airflow through the substrate were measured. The microbial community present, which is critical to the phytoremediation process, was quantified by identifying individual microbial phospholipid fatty acids (PLFA) within the substrate. Results demonstrated that the lower-speed drip irrigation reduced water consumption compared to the rapid system, and had generally more uniform moisture distribution. High flow drip irrigation resulted in a water pathway phenomenon, leading to uneven moisture distribution within the green wall, and this effect was accentuated with fibrous root plant species. Drip irrigation did not change microbial community composition across planted modules, apart from increasing fungi by 6%, but did wash out bacteria at the high flow rate used (-56.67%), thus low flow rate irrigation rate is more beneficial for both plant growth and microbial community composition. The current work provides evidence that drip irrigation has considerable effects on both substrate airflow rate and substrate microbial density: both key to system air cleaning performance.

Phytoremediation of formaldehyde by three selected non-native indoor plant species

Formaldehyde is an organic volatile compound and a commonly used chemical in various construction materials thus causing dwellers to be exposed to it inside a building. Its remediation from indoor air has been carried out through various techniques where potted plants and living walls are at the front foot. It is necessary to study plants under various conditions for their efficiency. We selected three plant species Epipremnum aureum, Chlorophytum comosum, and Spathiphyllum wallisii non-native of Bahrain. These plants were tested under normal conditions in a sealed fumigation box where formaldehyde concentration was kept ∼3 ppm, CO2 ∼ 450 ppm, light intensity 1000 Lx (equal to 13.5 µmol.m-2.s-1), irrigated with tap water. Analysis of Variance (ANOVA) statistical method was performed to test the significant differences of purification efficiencies of the tested indoor plants against HCHO. In addition, the statistical method was used to test the significant difference, if any, of the plants to CO2 emission because of absorbing HCHO. The physical health of plants and their short-term remediation ability reveals that all plants exhibited up to 70% remediation potential and tolerance to remediate the target chemical. It is evident that the impact of local environmental factors on the plants is negligible.

A review of phyto- and microbial-remediation of indoor volatile organic compounds

Volatile organic compounds (VOCs) are crucial air pollutants in indoor environments, emitted from building materials, furniture, consumer products, cleaning products, smoking, fuel combustion, cooking, and other sources. VOCs are also emitted from human beings via breath and whole-body skin. Some VOCs cause dermal/ocular irritation as well as gastrointestinal, neurological, cardiovascular, and/or carcinogenic damage to human health. Because people spend most of their time indoors, active control of indoor VOCs has garnered attention. Phytoremediation and microbial remediation, based on plant and microorganism activities, are deemed sustainable, cost-effective, and public-friendly technologies for mitigating indoor VOCs. This study presents the major sources of VOCs in indoor environments and their compositions. Various herbaceous and woody plants used to mitigate indoor VOCs are summarized and their VOCs removal performance is compared. Moreover, this paper reviews the current state of active phytoremediation and microbial remediation for the control of indoor VOCs, and discusses future directions.

Advances in amelioration of air pollution using plants and associated microbes: An outlook on phytoremediation and other plant-based technologies

Globally, air pollution is an unfortunate aftermath of rapid industrialization and urbanization. Although the best strategy is to prevent air pollution, it is not always feasible. This makes it imperative to devise and implement techniques that can clean the air continuously. Plants and microbes have a natural potential to transform or degrade pollutants. Hence, strategies that use this potential of living biomass to remediate air pollution seem to be promising. The simplest future trend can be planting suitable plant-microbe species capable of removing air pollutants like SO2, CO2, CO, NOX and particulate matter (PM) along roadsides and inside the buildings. Established wastewater treatment strategies such as microbial fuel cells (MFC) and constructed wetlands (CW) can be suitably modified to ameliorate air pollution. Green architecture involving green walls and green roofs is facile and aesthetic, providing urban ecosystem services. Certain microbe-based bioreactors such as bioscrubbers and biofilters may be useful in small confined spaces. Several generative models have been developed to assist with planning and managing green spaces in urban locales. The physiological limitations of using living organisms can be circumvent by applying biotechnology and transgenics to improve their potential. This review provides a comprehensive update on not just the plants and associated microbes for the mitigation of air pollution, but also lists the technologies that are available and/or can be modified and used for air pollution control. The article also gives a detailed analysis of this topic in the form of strengths-weaknesses-opportunities-challenges (SWOC). The strategies mentioned in this review would help to attain corporate Environmental Social and Governance (ESG) and Sustainable Development Goals (SDGs), while reducing carbon footprint in the urban scenario. The review aims to emphasise that urbanization is possible while tackling air pollution using facile, green techniques involving plants and associated microbes.

Engineering green wall botanical biofiltration to abate indoor volatile organic compounds: A review on mechanisms, phyllosphere bioaugmentation, and modeling

Indoor air pollution affects the global population, especially in developed countries where people spend around 90% of their time indoors. The recent pandemic exacerbated the exposure by relying on indoor spaces and a teleworking lifestyle. VOCs are a group of indoor air pollutants with harmful effects on human health at low concentrations. It is widespread that plants can remove indoor VOCs. To this day, research has combined principles of phytoremediation, biofiltration, and bioremediation into a holistic and sustainable technology called botanical biofiltration. Overall, it is sustained that its main advantage is the capacity to break down and biodegrade pollutants using low energy input. This differs from traditional systems that transfer VOCs to another phase. Furthermore, it offers additional benefits like decreased indoor air health costs, enhanced work productivity, and well-being. However, many disparities exist within the field regarding the role of plants, substrate, and phyllosphere bacteria. Yet their role has been theorized; its stability is poorly known for an engineering approach. Previous research has not addressed the bioaugmentation of the phyllosphere to increase the performance, which could boost the system. Moreover, most experiments have studied passive potted plant systems at a lab scale using small chambers, making it difficult to extrapolate findings into tangible parameters to engineer the technology. Active systems are believed to be more efficient yet require more maintenance and knowledge expertize; besides, the impact of the active flow on the long term is not fully understood. Besides, modeling the system has been oversimplified, limiting the understanding and optimization. This review sheds light on the field's gains and gaps, like concepts, experiments, and modeling. We believe that embracing a multidisciplinary approach encompassing experiments, multiphysics modeling, microbial community analysis, and coworking with the indoor air sector will enable the optimization of the technology and facilitate its adoption.

Fuelling phytoremediation: gasoline degradation by green wall systems-a case study

The capacity for indoor plants including green wall systems to remove specific volatile organic compounds (VOCs) is well documented in the literature; however under realistic settings, indoor occupants are exposed to a complex mixture of harmful compounds sourced from various emission sources. Gasoline vapour is one of the key sources of these emissions, with several studies demonstrating that indoor occupants in areas surrounding gasoline stations or with residentially attached garages are exposed to far higher concentrations of harmful VOCs. Here we assess the potential of a commercial small passive green wall system, commercially named the 'LivePicture Go' from Ambius P/L, Australia, to drawdown VOCs that comprise gasoline vapour, including total VOC (TVOC) removal and specific removal of individual speciated VOCs over time. An 8-h TVOC removal efficiency of 42.45% was achieved, along with the complete removal of eicosane, 1,2,3-trimethyl-benzene, and hexadecane. Further, the green wall also effectively reduced concentrations of a range of harmful benzene derivatives and other VOCs. These results demonstrate the potential of botanical systems to simultaneously remove a wide variety of VOCs, although future research is needed to improve upon and ensure efficiency of these systems over time and within practical applications.

Botanical filters for the abatement of indoor air pollutants

Nowadays, people spend 80-90% of their time indoors, while recent policies on energy efficient and safe buildings require reduced building ventilation rates and locked windows. These facts have raised a growing concern on indoor air quality, which is currently receiving even more attention than outdoors pollution. Prevention is the first and most cost-effective strategy to improve indoor air quality, but once pollution is generated, a battery of physicochemical technologies is typically implemented to improve air quality with a questionable efficiency and at high operating costs. Biotechnologies have emerged as promising alternatives to abate indoor air pollutants, but current bioreactor configurations and the low concentrations of indoor air pollutants limit their widespread implementation in homes, offices and public buildings. In this context, recent investigations have shown that potted plants can aid in the removal of a wide range of indoor air pollutants, especially volatile organic compounds (VOCs), and can be engineered in aesthetically attractive configurations. The original investigations conducted by NASA, along with recent advances in technology and design, have resulted in a new generation of botanical biofilters with the potential to effectively mitigate indoor air pollution, with increasing public aesthetics acceptance. This article presents a review of the research on active botanical filters as sustainable alternatives to purify indoor air.

A systematic review on mitigation of common indoor air pollutants using plant-based methods: a phytoremediation approach

Environmental pollution, especially indoor air pollution, has become a global issue and affects nearly all domains of life. Being both natural and anthropogenic substances, indoor air pollutants lead to the deterioration of the ecosystem and have a negative impact on human health. Cost-effective plant-based approaches can help to improve indoor air quality (IAQ), regulate temperature, and protect humans from potential health risks. Thus, in this review, we have highlighted the common indoor air pollutants and their mitigation through plant-based approaches. Potted plants, green walls, and their combination with bio-filtration are such emerging approaches that can efficiently purify the indoor air. Moreover, we have discussed the pathways or mechanisms of phytoremediation, which involve the aerial parts of the plants (phyllosphere), growth media, and roots along with their associated microorganisms (rhizosphere). In conclusion, plants and their associated microbial communities can be key solutions for reducing indoor air pollution. However, there is a dire need to explore advanced omics technologies to get in-depth knowledge of the molecular mechanisms associated with plant-based reduction of indoor air pollutants.

Phytoremediation for the indoor environment: a state-of-the-art review

Poor indoor air quality has become of particular concern within the built environment due to the time people spend indoors, and the associated health burden. Volatile organic compounds (VOCs) off-gassing from synthetic materials, nitrogen dioxide and harmful outdoor VOCs such benzene, toluene, ethyl-benzene and xylene penetrate into the indoor environment through ventilation and are the main contributors to poor indoor air quality with health effects. A considerable body of literature over the last four decades has demonstrate the removal of gaseous contaminants through phytoremediation, a technology that relies on plant material and technologies to remediate contaminated air streams. In this review we present a state-of-the-art on indoor phytoremediation over the last decade. Here we present a review of 38 research articles on both active and passive phytoremediation, and describe the specific chemical removal efficiency of different systems. The literature clearly indicates the efficacy of these systems for the removal of gaseous contaminants in the indoor environment, however it is evident that the application of phytoremediation technologies for research purposes in-situ is currently significantly under studied. In addition, it is common for research studies to assess the removal of single chemical species under controlled conditions, with little relevancy to real-world settings easily concluded. The authors therefore recommend that future phytoremediation research be conducted both in-situ and on chemical sources of a mixed nature, such as those experienced in the urban environment like petroleum vapour, vehicle emissions, and mixed synthetic furnishings off-gassing. The assessment of these systems both in static chambers for their theoretical performance, and in-situ for these mixed chemical sources is essential for the progression of this research field and the widespread adoption of this technology.

Effects of airflow rate and plant species on formaldehyde removal by active green walls

Formaldehyde is a hazardous volatile organic compound (VOC) listed as a Group 1 carcinogen by the International Agency for Research on Cancer. The active green wall system is a promising technology that utilizes active airflow passing through plants grown along a vertical alignment to increase their mass exposure to pollutants. However, few studies have investigated the effect of airflow rate on their efficacy for formaldehyde removal, and plant-mediated effects are unknown. This study assessed the formaldehyde removal ability of the active green wall using dynamic experiments. Three levels of airflow rate (30, 50, and 65 m³·h^-1) and inlet formaldehyde concentration (1.0, 2.0, and 3.5 mg·m^-3) were used and three plant species were investigated. The removal of formaldehyde by active green walls was significantly (P < 0.01) affected by the airflow rate, formaldehyde concentration, and plant species. The single pass removal efficiency varying from 38.18 to 94.42% decreased as the airflow rate and formaldehyde concentration increased. The elimination capacity varied from 189 to 1154 mg·m^-2·h^-1 and increased with the inlet formaldehyde loading rate. Significant differences in formaldehyde removal effectiveness among the plant species were observed with Chlorophytum comosum performing the best, followed by Schefflera octophylla, with Chamaedorea elegans being the worst.

Phytoremediation of indoor formaldehyde by plants and plant material

Formaldehyde evolves from various household items and is of environmental and public health concern. Removal of this contaminant from the indoor air is of utmost importance and currently, various practices are in the field. Among these practices, indoor plants are of particular importance because they help in controlling indoor temperature, moisture, and oxygen concentration. Plants and plant materials studied for the purpose have been reviewed hereunder. The main topics of the review are, mechanism of phytoremediation, plants and their benefits, plant material in formaldehyde remediation, and airtight environmental and health issues. Future research in the field is also highlighted which will help new researches to plan for the remediation of formaldehyde in indoor air. The remediation capacity of several plants has been tabulated and compared, which gives easy access to assess various plants for remediation of the target pollutant. Challenges and issues in the phytoremediation of formaldehyde are also discussed.Novelty statement: Phytoremediation is a well-known technique to mitigate various organic and inorganic pollutants. The technique has been used by various researchers for maintaining indoor air quality but its efficiency under real-world conditions and human activities is still a question and is vastly affected relative to laboratory conditions. Several modifications in the field are in progress, here in this review article we have summarized and highlighted new directions in the field which could be a better solution to the problem in the future.

The botanical biofiltration of volatile organic compounds and particulate matter derived from cigarette smoke

Despite the growing use of control measures, environmental tobacco smoke (ETS) remains a significant pollutant source in indoor air in many areas of the world. Current control methods for reducing ETS exposure are inadequate to protect public health in environments where cigarettes are smoked. An alternative solution is botanical biofiltration which has previously been shown to lower concentrations of volatile organic compounds (VOCs) and particulate matter (PM) from a range of polluted air streams. This study is the first to assess the potential of a botanical biofilter with the species Spathiphyllum wallisii (Peace Lily) for the removal of cigarette-derived VOCs and all size fractions of PM. Single pass removal efficiencies of 43.26% for total VOCs and 34.37% for total suspended particles were achieved. The botanical biofilter reduced the concentrations of a range of harmful ETS chemicals including nicotine, limonene, and toluene. Evaluation of the re-emission of ETS constituents filtered by the botanical biofilter revealed no particle resuspension or off gassing. The results demonstrate the potential of botanical biofilters to reduce public ETS exposure, although further research is needed to improve upon and ensure the efficiency of these systems for practical applications.

Phytoremediation: The Sustainable Strategy for Improving Indoor and Outdoor Air Quality

Most of the world’s population is exposed to highly polluted air conditions exceeding the WHO limits, causing various human diseases that lead towards increased morbidity as well as mortality. Expenditures on air purification and costs spent on the related health issues are rapidly increasing. To overcome this burden, plants are potential candidates to remove pollutants through diverse biological mechanisms involving accumulation, immobilization, volatilization, and degradation. This eco-friendly, cost-effective, and non-invasive method is considered as a complementary or alternative tool compared to engineering-based remediation techniques. Various plant species remove indoor and outdoor air pollutants, depending on their morphology, growth condition, and microbial communities. Hence, appropriate plant selection with optimized growth conditions can enhance the remediation capacity significantly. Furthermore, suitable supplementary treatments, or finding the best combination junction with other methods, can optimize the phytoremediation process.

Effective reduction of roadside air pollution with botanical biofiltration

Currently no sustainable, economical and scalable systems have been developed for the direct removal of roadside air pollutants at their source. Here we present a simple and effective air filtering technology: botanical biofiltration, and the first field assessment of three different botanical biofilter designs for the filtration of traffic associated air pollutants - NO₂, O₃ and PM_2.5 - from roadside ambient air in Sydney, Australia. Over two six month research campaigns, we show that all of the tested systems filtered NO₂, O₃ and PM_2.5 with average single pass removal efficiencies of up to 71.5%, 28.1% and 22.1% respectively. Clean air delivery rates of up to 121 m³/h, 50 m³/h and 40 m³/h per m² of active green wall biofilter were achieved for the three pollutants respectively, with pollutant removal efficiency positively correlated with their ambient concentrations. We propose that large scale field trials of this technology are warranted to promote sustainable urban development and improved public health outcomes.

Pediatric Asthma Attack and Home Paint Exposure

Although asthma mortality has been declining for the past several decades, asthma morbidity is on the rise, largely due to deteriorating indoor air quality and comorbidities, such as allergies. Consumer products and building materials including paints emit volatile organic compounds (VOCs), such as propylene glycol (PG), which is shown to dehydrate respiratory tracts and can contributor to airway remodeling. We hypothesize that paint exposure increases the risk of asthma attacks among children because high levels of VOCs persist indoors for many weeks after painting. Children 1-15 years old visiting two of the University of Miami general pediatric clinics were screened for their history of asthma and paint exposure by interviewing their parents and/or guardians accompanying them to the clinic. They were also asked questions about asthma diagnosis, severity of asthma and allergies and their sociodemographics. The risk of asthma attack among asthmatic children was modeled with respect to paint exposure adjusting for potential confounders using multivariate logistic regressions. Of 163 children, 36 (22%) reported physician-diagnosed asthma and of these, 13 (33%) had an asthma attack during the last one year. Paint exposure was marginally significant in the univariate analysis (OR = 4.04; 95% CI = 0.90-18.87; p < 0.1). However, exposed asthmatic children were 10 times more likely to experience an asthma attack than unexposed asthmatic children (OR = 10.49; CI = 1.16-94.85, p < 0.05) when adjusted for other risk factors. Given paint is one of the sources of indoor VOCs, multiple strategies are warranted to manage the health effects of VOC exposure from paint, including the use of zero-VOC water-based paint, exposure avoidance and clinical interventions.

Current State of Indoor Air Phytoremediation Using Potted Plants and Green Walls

Urban civilization has a high impact on the environment and human health. The pollution level of indoor air can be 2–5 times higher than the outdoor air pollution, and sometimes it reaches up to 100 times or more in natural/mechanical ventilated buildings. Even though people spend about 90% of their time indoors, the importance of indoor air quality is less noticed. Indoor air pollution can be treated with techniques such as chemical purification, ventilation, isolation, and removing pollutions by plants (phytoremediation). Among these techniques, phytoremediation is not given proper attention and, therefore, is the focus of our review paper. Phytoremediation is an affordable and more environmentally friendly means to purify polluted indoor air. Furthermore, studies show that indoor plants can be used to regulate building temperature, decrease noise levels, and alleviate social stress. Sources of indoor air pollutants and their impact on human health are briefly discussed in this paper. The available literature on phytoremediation, including experimental works for removing volatile organic compound (VOC) and particulate matter from the indoor air and associated challenges and opportunities, are reviewed. Phytoremediation of indoor air depends on the physical properties of plants such as interfacial areas, the moisture content, and the type (hydrophobicity) as well as pollutant characteristics such as the size of particulate matter (PM). A comprehensive summary of plant species that can remove pollutants such as VOCs and PM is provided. Sources of indoor air pollutants, as well as their impact on human health, are described. Phytoremediation and its mechanism of cleaning indoor air are discussed. The potential role of green walls and potted-plants for improving indoor air quality is examined. A list of plant species suitable for indoor air phytoremediation is proposed. This review will help in making informed decisions about integrating plants into the interior building design.

Pulmonary Health Effects of Indoor Volatile Organic Compounds-A Meta-Analysis

Volatile organic compounds (VOCs) are commonly found in consumer products, including furniture, sealants and paints. Thus, indoor VOCs have become a public health concern, especially in high-income countries (HICs), where people spend most of their time indoors, and indoor and outdoor air exchange is minimal due to a lack of ventilation. VOCs produce high levels of reaction with the airway epithelium and mucosa membrane and is linked with pulmonary diseases. This paper takes a stock of the literature to assess the strength of association (measured by effect size) between VOCs and pulmonary diseases with the focus on asthma and its related symptoms by conducting a meta-analysis. The literature was searched using the PubMed database. A total of 49 studies that measured VOCs or VOC types and pulmonary health outcomes were included in the analysis. The results of these studies were tabulated, and standard effect size of each study was computed. Most studies were conducted in high-income countries, including France (n = 7), Japan (n = 7) and the United States (n = 6). Our analysis suggests that VOCs have a medium-sized effect on pulmonary diseases, including the onset of asthma (effect size (or Cohen's d) ~0.37; 95% confidence interval (CI) = 0.25-0.49; n = 23) and wheezing (effective size ~0.26; 95% CI = 0.10-0.42; n = 10). The effect size also varied by country, age and disease type. Multiple stakeholders must be engaged in strategies to mitigate and manage VOC exposure and its associated pulmonary disease burden.

Modified coir pith with glucose syrup as a supporter in non-external nutrient supplied biofilter for benzene removal by Bacillus megaterium

Coir pith glucose syrup beads were used as a supporter in a biofilter system. The modified coir pith beads provided a carbon source and controlled humidity for microorganism growth for long-term operation without external nutrient supplementation. For the screening, Bacillus spp. were immobilised on coir pith beads and used for benzene bioremediation. The result showed that coir pith beads immobilised with Bacillus megaterium can remove on average 85-100% of the benzene (215-day operation). In addition, B. megaterium presented the ability to transform benzene to catechol. For an up-scaled application, a 25-L biofilter system was developed and tested in a closed 24-m³ container re-injected with 0.6 ppm benzene for 8 cycles. The system presented the ability to remove 100% of the benzene. This biofilter has the potential to be applied in a real benzene-contaminated site.

An Assessment of the Suitability of Active Green Walls for NO2 Reduction in Green Buildings Using a Closed-Loop Flow Reactor

Nitrogen dioxide (NO2) is a common urban air pollutant that is associated with several adverse human health effects from both short and long term exposure. Additionally, NO2 is highly reactive and can influence the mixing ratios of nitrogen oxide (NO) and ozone (O3). Active green walls can filter numerous air pollutants whilst using little energy, and are thus a candidate for inclusion in green buildings, however, the remediation of NO2 by active green walls remains untested. This work assessed the capacity of replicate active green walls to filter NO2 at both ambient and elevated concentrations within a closed-loop flow reactor, while the concentrations of NO and O3 were simultaneously monitored. Comparisons of each pollutant’s decay rate were made for green walls containing two plant species (Spathiphyllum wallisii and Syngonium podophyllum) and two lighting conditions (indoor and ultraviolet). Biofilter treatments for both plant species exhibited exponential decay for the biofiltration of all three pollutants at ambient concentrations. Furthermore, both treatments removed elevated concentrations of NO and NO2, (average NO2 clean air delivery rate of 661.32 and 550.8 m3∙h−1∙m−3 of biofilter substrate for the respective plant species), although plant species and lighting conditions influenced the degree of NOx removal. Elevated concentrations of NOx compromised the removal efficiency of O3. Whilst the current work provided evidence that effective filtration of NOx is possible with green wall technology, long-term experiments under in situ conditions are needed to establish practical removal rates and plant health effects from prolonged exposure to air pollution.