Removal of Heavy Metals from Urban Stormwater Runoff Using Bioretention Media Mix

Effects of dissolved organic carbon on potentially toxic element desorption in stormwater bioretention systems

Stormwater runoff contains dissolved organic carbon (DOC) and potentially toxic elements (PTEs). Interactions between DOC and PTEs can impact PTE speciation and mobility, but are not fully understood. Soil samples were collected from a vegetated bioretention bed to investigate the effects of DOC (0, 15, and 50 mg-C/L) on the desorption of 10 PTEs captured by the soil media: Mn, Fe, Co, Cu, Zn, As, Cd, Sn, Sb, and Pb. In the absence of DOC, the desorbed PTE concentration from bioretention media into the aqueous phase ranking was as follows: Fe > Mn ∼ Zn > Cu > Pb > Sb > As > Co > Sn ∼ Cd. Increased DOC concentrations resulted in a reduction of the soil-water distribution coefficient (Kd) values. The greatest shift in Kd was observed for Cu and lowest for Sb. The PTE sorption capacities were lower for surficial soil samples (lower Kd) compared to the deep soil samples. Overall, the desorbed PTE (average midchannel 55.7 μg/g) fraction accounted for <1.1 % of the total extracted PTEs (5364 μg/g), and while this is a small percentage of the total, this is the fraction that is mobile. The extracted PTE fractions revealed that DOC reduced the organic matter-bound and carbonate-bound fractions. The PTE desorption trends suggest that reducing DOC in stormwater runoff could be an effective measure to mitigate the release of PTEs into the environment.

Evaluation of sorbent amendments used with stormwater management practices to remove contaminants: Impacts of rainfall intensity and antecedent dry periods

For a comprehensive evaluation of the suitability and efficiency of soil amendments in bioretention systems, it is crucial to investigate the capability of amendments for simultaneously serving three important functions under intermittent and variable flow conditions: removing a wide range of contaminants, supporting plant health, and maintaining media infiltration rate. However, most studies have not considered these important factors and conditions simultaneously, which may overestimate or underestimate the bioretention performance. In this study, a long-term vegetated column study was conducted to investigate the ability of various sorbent amendments- coconut coir fiber (CCF), blast furnace slag (BFS), and waste tire crumb rubber (WTCR) -for removal of metals, nutrients, and polycyclic aromatic hydrocarbons (PAHs) from stormwater. The experiments were performed under intermittent flow conditions considering different runoff intensities and antecedent dry periods (ADP). The long-term effect of bioretention usage on plant health and media infiltration rate was also investigated. All amended and unamended columns were able to remove >99 % of influent metals, except Cu, over the 7-month experiment period with different rain intensities and dry periods; modest effluent Cu concentrations occurred with higher rainfall. The performance of different media for removing PAHs such as naphthalene and acenaphthylene varied with the rain intensity. The BFS-amended media had high phosphate removal capacity (>90 %) under tested conditions. In all columns, nitrate removal was notably affected by changes in stormwater intensity and ADP, with high nitrate removal during heavy rainfall. Over the entire experiment, all media had good infiltration rate within the locally acceptable range (1-25 cm/h). The high iron and aluminum contents of BFS adversely affected the plant health in BFS-amended media. Overall, this study identifies the opportunities and challenges associated with the usage of bioretention amendments, and improves awareness among bioretention designers to consider seasonal effect on the performance of bioretention systems.

Performance of biochar mixed cement paste for removal of Cu, Pb and Zn from stormwater

Using biochar as a partial replacement of Portland cement in cementitious materials is a promising solution to mitigate negative environmental impacts. However, current studies in available literature primarily focus on the mechanical properties of composites made with cementitious materials and biochar. Therefore, this paper reports the effects of the type of biochar, the percentage of biochar addition, and the particle size of the biochar on the removal efficiency of Cu, Pb, and Zn, as well as the effect of contact time on the removal efficiency of Cu, Pb, and Zn, along with the compressive strength. The peak intensities of OH^-, CO₃^2- and Calcium Silicate Hydrate (Ca-Si-H) peaks increase with increasing biochar addition levels, reflecting increased hydration product formation. The reduction of particle size of biochar causes the polymerization of the Ca-Si-H gel. However, no significant changes were observed in heavy metal removal, irrespective of the percentage of biochar addition, the particle size of biochar, or the type of biochar added to the cement paste. Adsorption capacities above 19 mg/g, 11 mg/g and 19 mg/g for Cu, Pb and Zn were recorded in all composites at an initial pH of 6.0. The Pseudo second order model best described the kinetics of the Cu, Pb, and Zn removal. The rate of adsorptive removal increases with the decrease in the density of the adsorbents. Over 40% of Cu and Zn were removed as carbonates and hydroxides through precipitation, whereas over 80% of Pb removal was via adsorption. Heavy metals bonded with OH^-, CO₃^2- and Ca-Si-H functional groups. The results demonstrate that biochar can be used as a cement replacement without negatively impacting heavy metal removal. However, neutralization of the high pH is needed before safe discharge.

Zeolite performance in removal of multicomponent heavy metal contamination from wastewater

Efficient removal of heavy metal pollutants from wastewater by ion-exchange sorbents requires knowledge and understanding of the interplay between the adsorption patterns of the different components. The present study elucidates the simultaneous adsorption characteristics of six toxic heavy metal cations (Cd²⁺, Cr³⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺) by two synthetic (13X and 4 A) and one natural (clinoptilolite) zeolite from solutions containing equimolar mixtures of the six metals. Equilibrium adsorption isotherms and equilibration dynamics were obtained by ICP-OES and complemented by EDXRF. An order of magnitude lower adsorption efficiency was exhibited by clinoptilolite (maximum of 0.12 mmol ions/g zeolite), relative to that obtained by the synthetic zeolites 13X and 4 A (a maximum of 2.9 and 1.65 mmol ions/g zeolite respectively). The strongest affinities to both zeolites were demonstrated by Pb²⁺ and Cr³⁺ (1.5 and 0.85 mmol/g zeolite respectively for 13X, and 0.8 and 0.4 mmol/g zeolite respectively for 4 A adsorbed from the highest solution concentration). The weakest affinities were observed by Cd²⁺ (0.1 mmol/g for both zeolites), Ni²⁺ (0.2 and 0.1 mmol/g for 13X and 4 A respectively), and Zn²⁺ (0.1 mmol/g for both zeolites). Large differences were observed between the two synthetic zeolites in terms of their equilibration dynamics and adsorption isotherms. Pronounced maxima were displayed in the adsorption isotherms for zeolites 13X and 4 A. The decline in adsorption of the weaker adsorbing ions with the increase in total solution concentration was attributed to the thermodynamic equilibrium between the ions adsorbed on the zeolite surface and those in the solution. Regeneration by 3 M KCL eluting solution resulted in considerable reduction in adsorption capacities following each desorption cycle.

Bioretention soil capacity for removing nutrients, metals, and polycyclic aromatic hydrocarbons; roles of co-contaminants, pH, salinity and dissolved organic carbon

Conventional bioretention soil media (BSM: e.g., loamy sand) is employed in infiltration-based stormwater management practices, but concerns exist on its limited sorption capacity. However, limited quantitative data is available, particularly considering the wide range of contaminants and water quality conditions that occur in stormwater. This study utilized batch tests to investigate the capability of conventional BSM for simultaneous removal of three nutrients (ammonium, nitrate, and phosphate), six metals (Cd, Cr, Cu, Ni, Pb and Zn), and four polycyclic aromatic hydrocarbons (PAHs: naphthalene, acenaphthylene, phenanthrene, and pyrene) from synthetic stormwater. Moreover, the effects of co-contaminants and different stormwater chemistry parameters (pH, salinity, and dissolved organic carbon (DOC)) on BSM sorption capacity were investigated. BSM was not effective for nutrients removal; however, it had good removal efficiency for metals such as Cu, Pb, and Cr which are less soluble at neutral pH values compared to metals such as Ni, Cd and Zn. Moreover, BSM was effective for removing PAHs with higher hydrophobicity such as pyrene and phenanthrene. Metals sorption capacity of BSM was greater at higher pH, lower salinity and DOC; however, the sorption capacity of BSM for PAHs was not sensitive to stormwater chemistry parameters. However, competitive sorption had a notable effect on low molecular weight PAHs, Cd, and Ni. This study provides a quantitative evaluation of the BSM performance and compares the sorption capacity to potential sorptive amendments used in stormwater management. While select sorbent amendments out-performed the BSM, this was not universal and was contaminant specific; careful consideration of water quality enhancement goals and solution chemistry are required in selecting a sorbent. Overall, this study identifies the possible limitations in BSM compositions and factors that may adversely affect BSM sorption capacity, and finally describes options to enhance BSM performance and recommendations for future research.

Health implications, distribution and source apportionment of heavy metals in road deposited dust of Jammu City in northern India

Road deposited dust (RDD) is an important indicators of heavy metal contamination in urban areas. In this study, we measured eight heavy metals (V, Cr, Mn, Fe, Ni, Cu, Zn, and Pb) in RDD collected from 34 different locations in Jammu city represented by different land uses such as industrial, urban-residential, high-density traffic, and sub-urban locations, and evaluated their ecological and health risks. The ratio of metal concentrations in RDD to their respective background levels varied as: Cu (3.94) > Pb (3.75) > Zn (3.01) > Cr (1.75) > Ni (1.51) > Mn (1.40) > V (1.35) > Fe (1.1) suggesting Cr, Ni, Cu, Zn and Pb were enriched anthropogenically. Geospatial maps revealed a heterogeneous distribution of metals in Jammu city with metal(s) specific hotspots primarily localized around high traffic density locations and industrial clusters. The index of geoaccumulation indicated 32%, 26%, 20%, 9%, and 8%, of samples belonged to "moderately polluted" category for Zn, Cu, Pb, Cr, and Ni respectively. Health index (HI) showed low non-carcinogenic hazards of metal contamination to adults but a high hazard to children. Though the values of total carcinogenic risks (TCR) (6.53E-05 to 3.71E-04) considerably exceeded the USEPA acceptable levels (1 × 10^-6 ≤ TCR <1 × 10^-4) suggesting high carcinogenic risks of metal contamination to both adults and children. Besides potential ecological risk index (PERI) revealed that 56% of samples had PERI >40 suggesting "moderate to high ecological risk" of metal contamination in the Jammu city RDD.

Improvement of heavy metal removal from urban runoff using modified pervious concrete

Heavy metals are one of the major chemical pollutant groups in urban runoff. The application of porous concrete is a potential alternative to conventional runoff management systems with the ability to remove heavy metals. Hence, a thorough understanding of the heavy metal removal mechanisms and constraints of conventional porous concrete opens a path for the development of effective modifications. This review critically discusses the major contributors in ordinary porous concrete which supports heavy metal removal. The effects of initial concentration, contact time and competing ions on heavy metal removal using porous concrete are also discussed. Additionally, the effect of decalcification, atmospheric carbonation, acid influent on heavy metal removal is reviewed. The major drawback of porous concrete is the high pH (>8.5) of the effluent water, decalcification of the porous concrete and leaching of adsorbed pollutants. Overall, the addition of adsorbent materials to the porous concrete increases removal efficiencies (7% - 65% increase) without neutralizing the effluent pH. Meanwhile, the addition of Reduced Graphene Oxide is successful in reducing the leachability of the removed heavy metals. The addition of pozzolanic materials can lower the effluent pH while maintaining similar removal efficiencies to unmodified porous concrete. Therefore, developing a novel method of neutralizing the effluent pH must be prioritized in future studies. Additionally, the toxicity that can occur due to the abrasion of modified porous concrete requires study in future research. Further, advanced characterization methods should be used in future studies to understand the mechanisms of removal via the modified porous concrete materials.

Decontamination performance of a bioretention system using a simple sand-based filler proportioning method

This study investigated the decontamination performance of a bioretention system using a sand-based filler constructed using sand and peat soil. The filler was constructed according to a simple proportioning method that considers water turnover time and organic content. Different inorganic constituents were added to the filler including zeolite, volcanic rock, coal slag, vermiculite and perlite to further improve the decontamination effect. Total suspended solids (TSS), total phosphorus (TP), ammonium nitrogen (NH4+-N), total nitrogen (TN) and chemical oxygen demand (COD) were measured in the influent and effluent. The results showed that: (1) the overall removal effect of the sand-based filler was satisfactory, indicating a certain feasibility and practicality of the method; (2) bioretention based on the sand-based filler had a better performance in removing TSS and TP with the removal rate both over 95%, and the addition of inorganic constituents to the filler was beneficial to TSS removal due to the effect of cumulative filtration capabilities of multiple materials, while phosphate was easily adsorbed by the positively charged particles of the peat soil; (3) the high removal rate of NH4+-N was due to adsorption and it could reach more than 80% by adding inorganic constituents with good adsorption such as zeolite vermiculite and perlite. Similarly, the addition of vermiculite and coal slag could increase the removal rate of COD by 15-25%. This research offers a novel alternative for guiding the selection and proportion of fillers in bioretention systems.

Stormwater treatment for reuse: Current practice and future development - A review

Stormwater harvesting is an effective measure to mitigate flooding risk and pollutant migration in our urban environment with the continuously increasing impermeable faction. Treatment of harvested stormwater also provides the fit-for-purpose water sources as an alternative to potable water supply ensuring the reliability and sustainability of the water management in the living complex. In order to provide the water management decision-maker with a broad range of related technology database and to facilitate the implementation of stormwater harvesting in the future, a comprehensive review was undertaken to understand the corresponding treatment performance, the applicable circumstances of current stormwater treatment and harvesting technologies. Technologies with promising potential for stormwater treatment were also reviewed to investigate the feasibility of being used in an integrated process. The raw stormwater quality and the required quality for different levels of stormwater reuses (irrigation, recreational, and potable) were reviewed and compared. The required level of treatment is defined for different 'fit-for-purpose' uses of harvested stormwater. Stormwater biofilter and constructed wetland as the two most advanced and widely used stormwater harvesting and treatment technologies, their main functionality, treatment performance and adequate scale of the application were reviewed based on published peer-reviewed articles and case studies. Excessive microbial effluent that exists in stormwater treated using these two technologies has restricted the stormwater reuse in most cases. Water disinfection technologies developed for wastewater and surface water treatment but with high potential to be used for stormwater treatment have been reviewed. Their feasibility and limitation for stormwater treatment are presented with respect to different levels of fit-for-purpose reuses. Implications for future implementation of stormwater treatment are made on proposing treatment trains that are suitable for different fit-for-purpose stormwater reuses.

Removal of Cd (II) Ions from Bioretention System by Clay and Soil Wettability

In this work, a silane modifier with benzyl substitutes (OFS-B) and linear substitutes (OFS-L) was used to modify bentonite clay and soil, and the results were characterized by Fourier transform-infrared absorption spectroscopy (FT-IR) and powder-X-ray diffraction (XRD) analysis. A contact angle analysis was performed to determine the wettability of modified clay and soil. The findings revealed that silane-modified OFS-L clay and soil produced wettable surfaces, while OFS-B exhibited hydrophobic properties. These clays and soils were used in a bioretention system for Cd (II) removal. In the study, seven different types of bioretention systems, including natural, OFS-L, and OFS-B modified clay and soil, as well as natural, OFS-L, and OFS-B modified soil, were applied to Cyperus alternifolius plants without an additional layer. The removal capacity of Cd (II) was measured in the following order: modified clay > modified soil > original clay/soil > no layer, i.e., 99.48%, 92.22%, 88.10/78.5%, and 30.0%, respectively. OFS-L removed more Cd (II) than OFS-B during the modification. OFS-L now improves the bioavailability and accumulation of Cd (II) in the plant (18.5 µg/g) and has a higher chlorophyll-b concentration (1.92 mg/g fresh weight) than other systems. The wettable clay exhibited clay leaching into the various levels of the bioretention system. In the bioretention system, benzyl substituted clay prevented the penetration of water and formed a Cd (II) agglomeration. When compared to non-wettable modifiers, these results indicated that wettable clay material could be a capable material for removing Cd (II).

Effect of ZnO nanoparticles on Zn, Cu, and Pb dissolution in a green bioretention system for urban stormwater remediation

Stormwater runoff from urban and suburban areas can carry hazardous pollutants directly into aquatic ecosystems. These pollutants, such as metals, nutrients, aromatic hydrocarbons, pesticides, and pharmaceuticals, are very toxic to aquatic organisms. Recently, significant amounts of zinc oxide engineered nanoparticles (ZnO-NPs) have been detected in urban stormwater and its bioretention systems. This raises concerns about a potential increase of stormwater toxicity and reduced performance of the treatment infrastructures. To tackle these issues, we developed a simple, low-cost bioretention system to remediate stormwater and retain ZnO-NPs. This system retained up to 73% Zn, 66% Cu, and >99% Pb. However, the removal efficiency for Pb was lower after adding ZnO-NPs to the system, possibly due to the remobilization of Pb phosphates. The effect of ZnO-NPs on stormwater toxicity and metal accumulation in wetland plants was also evaluated.

Accumulation of heavy metals in stormwater biofiltration systems augmented with zeolite and fly ash

The discharge of high levels of heavy metals into the environment is of concern due to its toxicity to aquatic life and potential human health impacts. Biofiltration systems have been used in urban environments to address nutrient contamination, but there is also evidence that such systems can be effective in reducing heavy metals concentration in stormwater. However, the accumulation pattern of heavy metals and lifespan of such systems, which are important in engineering design, have not been thoroughly explored. This study investigated the accumulation patterns of lead (Pb), copper (Cu), zinc (Zn), manganese (Mn), and iron (Fe), which are common in urban runoff, in non-vegetated filtration columns using three different types of filter media, namely sand (S), and mixtures of sand with fly ash (sand-fly ash mix, SF), and with zeolite (sand-zeolite mix, SZ). The columns were assessed in terms of infiltration rate, the mass of heavy metals accumulation at different depths, and formation of crust layer (schmutzdecke) at the surface. The results show that most of the heavy metals accumulated at the top 5-10 cm of the filter media. However, Zn was found adsorbed to a depth of 15 cm in S and SZ columns, while Mn and Fe were present in column S throughout the entire 30 cm depth of the filter media. The presence especially of Zn, Mn, and Fe in the deeper portions of the filter media before the top 5 cm layer reached its maximum adsorption capacity, hints that transport to the deeper layers is not necessarily dependent on saturation of the upper layers for these heavy metals. SF accumulated heavy metals most at the top 5 cm of the filter media layer, and retained twice the mass of heavy metals in the crust layer, compared to S and SZ columns. SF also yielded the lowest value of infiltration rate of 31 mm/h. Considering both metals accumulation and clogging potential of the filter media, the periodic maintenance of these systems is suggested to be approximately between 1.5 and 3 years.

A review on plant-microbial interactions, functions, mechanisms and emerging trends in bioretention system to improve multi-contaminated stormwater treatment

Management and treatment of multi-polluted stormwater in bioretention system have gained significant attraction recently. Besides nutrients, recent source appointment studies found elevated levels of Potentially toxic metal(loid)s (PTMs) and contaminants of emerging concern (CECs) in stormwater that highlighted many limitations in conventional media adsorption-based pollutant removal bioretention strategies. The substantial new studies include biological treatment approaches to strengthen pollutants degradation and adsorption capacity of bioretention. The knowledge on characteristics of plants and their corresponding mechanisms in various functions, e.g., rainwater interception, retention, infiltration, media clogging prevention, evapotranspiration and phytoremediation, is scattered. The microorganisms' role in facilitating vegetation and media, plant-microorganism interactions and relative performance over different functions in bioretention is still unreviewed. To uncover the underneath, it was summarised plant and microbial studies and their functionality in hydrogeochemical cycles in the bioretention system in this review, contributing to finding their interconnections and developing a more efficient bioretention system. Additionally, source characteristics of stormwater and fate of associated pollutants in the environment, the potential of genetical engineered plants, algae and fungi in bioretention system as well as performance assessment of plants and microorganisms in non-bioretention studies to propose the possible solution of un-addressed problems in bioretention system have been put forward in this review. The present review can be used as an imperative reference to enlighten the advantages of adopting multidisciplinary approaches for the environment sustainability and pollution control.

Effectiveness of two lightweight aggregates for the removal of heavy metals from contaminated urban stormwater

Contaminated runoff stormwater from urban environments carries several contaminants to water bodies, thereby affecting the health of living beings and ecological systems. Among all the contaminants, heavy metals possess high toxicity and impact water quality. The stormwater management through green infrastructures composed by adequate materials can provide an excellent solution, simultaneously ensuring the appropriate hydraulic performance and contaminant removal rate. The proposed research aims at the elimination of heavy metals (i.e. Ni, Cu, Zn, Cd and Pb) through column experiments by selecting four possible and novel treatments for urban stormwaters. Two lightweight aggregates (Arlita and Filtralite) were tested separately and in combination with CaCO₃. The study determines the efficiency and lifetime of each treatment by varying the interaction time between the filter materials and contaminated water and the type of filter. The observed removal mechanisms were closely related to the changes in pH due to the interactions between water and different materials. The reductions in heavy metal concentrations depend on the type of heavy metal, interaction time and type of filter material. Results indicate that the combined use of CaCO₃, Arlita and Filtralite did not improve the removal rates of heavy metals. However, it decreased the efficiency of the decontamination process. The significance of this study lies on the removal efficiency of Arlita and Filtralite as decontamination treatments. Both the tested lightweight aggregates led to a considerable decrease in the heavy metal concentrations in urban runoff stormwater although Filtralite was particularly efficient. After 4 weeks, the treatments were still successfully reducing and stabilising 99% of the heavy metals in the contaminated stormwater. These results confirm that the lifetime of the tested lightweight aggregates is adequate and emphasise, as a novel application of these materials, on their feasibility for the improvement of urban stormwater quality.

The Common Approaches of Nitrogen Removal in Bioretention System

Bioretention is considered one of the best management practices (BMPS) for managing stormwater quality and quantity. The bioretention system has proven good performance in removing total suspended solids, oil, and heavy metals. The nitrogen (N) removal efficiency of the bioretention system is insufficient, however, due to the complex forms of nitrogen. Therefore, this paper aims to review recent enhancement approaches to nitrogen (N) removal and to discuss the factors influencing bioretention efficiency. To improve bioretention efficiency, several factors should be considered when designing bioretention systems, including nitrogen concentration, climate factors, and hydrological factors. Further, soil and plant selection should be appropriate for environmental conditions. Three design improvement approaches have been reviewed. The first is the inclusion of a saturated zone (SZ), which has been used widely. The SZ is shown to have the best performance in nitrogen removal. The second approach (which is less popular) is the usage of additives in the form of a mixture with soil media or as a separated layer. This concept is intended to be applied in tropical regions with wet soil conditions and a short dry period. The third approach combines the previous two approaches (enhanced filter media and applying a SZ). This approach is more efficient and has recently attracted more attention. This study suggests that further studies on the third approach should be carried out. Applying amendment material through filter media and integrating it with SZ provides appropriate conditions to complete the nitrogen cycle. This approach is considered a promising method to enhance nitrogen removal. In general, the bioretention system offers a promising tool for improving stormwater quality.

Effect of shell powder on removal of metals and volatile organic compounds (VOCs) from resin in an atmospheric-pressure microwave plasma reactor

Resin has been widely used for thermosetting printed circuit boards (PCBs) and is a key part of e-waste from scrap PCBs. It requires appropriate treatment because of its harmful elements (metals and metalloids) and organic compounds that are toxic to human health and the environment. The purpose of this study is to eliminate volatile organic compounds (VOCs) and elements (metals and metalloids) in resin via the use of powdered snail shell (Babylonia formosae) in an atmospheric-pressure microwave plasma reactor. Shell powder plays a significant role in the destruction of benzene and toluene with removal efficiency 98.8 % and 100 %, respectively, compared to quartz sand with removal efficiency 44.9 %. A high ratio of shell powder increases the inertization of metals and metalloids by more than 96 %. The crystalline structures of these materials are dominated by calcite formations (CaCO₃), confirming the elimination of metals and metalloids. Raman spectroscopy shows that the shell powder vitrifies these elements. The use of shell powder is thus recommended to degrade hazardous substances and to vitrify elements from resin in plasma pyrolysis.

The impact of media, plants and their interactions on bioretention performance: A review

Bioretention systems have gained considerable popularity as a more natural approach to stormwater management in urban environments. The choice of bioretention media is frequently cited as one of the critical design parameters with the ultimate impact on the performance of the system. The goal of this review is to highlight data that challenge the importance of media as being the dominant design parameter and argue that the long-term performance is shaped by the interactions between media and the living components of a bioretention system, especially vegetation. Some of the key interactions are related to the impact of plant roots on media pore structure, which has implications on infiltration, storage capacity, and treatment. Another relevant interaction pertains to evapotranspiration and the associated impacts on the water balance and the water quality performance of bioretention systems. The impacts of vegetation on the media are highlighted and actual, as well as potential, impacts of plant-media interactions on bioretention performance are presented.

Dissolved metal adsorption capacities and fractionation in filter materials for use in stormwater bioretention facilities

The dissolved metal adsorption and association was determined for ten different filter materials recommended and/or implemented in bioretention facilities. Batch adsorption and batch kinetic experiments were performed at lab-scale using both single and multi-metal solutions. Metal strengths and association were determined by sequential extraction analysis. All materials adsorbed metals and 90% of adsorption occurred within 1 h. However, as metal solutions became more complex, adsorption behavior changed. Generally, filter materials classified as sand with a naturally high pH, relatively low organic matter (OM) content and large specific surface area seem to be good choices for removing dissolved metals. Additionally, a chalk additive might improve metal adsorption whereas biochar did not significantly improve metal retention and may be an unwanted (due to degradation over time) extra source of OM. Regardless of filter material, metals primarily adsorbed to the exchangeable form which indicates that metal adsorption might not be permanent, but rather substantially reversible in some cases. More research is needed to assess whether dissolved metals adsorbed in filter materials of bioretention systems pose a delayed threat instead of an immediate threat. Finally, the authors strongly recommend filter materials intended for stormwater bioretention facilities to be tested prior to implementation.

Performance Assessment of a Laboratory Scale Prototype Biofiltration System in Tropical Region

Biofiltration systems, as one of the best management practices, have good potentials to improve stormwater quality and hydrology of urban catchments. While biofiltration systems are well-studied in developed countries, the majority of those studies are conducted for temperate climate and there is a lack of lab-scale and field-scale studies on such systems under tropical conditions. This paper focuses on the performance of a lab-scale prototype biofiltration systems in stormwater retention efficiency as well as pollutants removal (including heavy metals and nutrients) from synthetic stormwater reproducing tropical rainfall events. A three-layer sand-based filter media with two different native plants including Pedilanthus tithymaloides and Cyperus alternifolius was selected for this study. Results showed that the system with Cyperus has a better stormwater retention capacity compared to the one with Pedilanthus. In addition, the observed infiltration rate in Cyperus and Pedilanthus were 338 mm/h and 267 mm/h, respectively. The better hydraulic performance in the system with Cyperus was attributed to the deeper and more extensive root penetration of this plant (as deep as 800 mm) compared to Pedilanthus (as deep as 250 mm). While both systems failed to perform well in removing total nitrogen, they performed significantly better in removing total phosphorus (Cyperus and Pedilanthus removed 67.3% and 62.5% of total phosphorus, respectively). The statistical analysis of results showed that the top 100 mm layer of filter media is the main contributor to total phosphorus removal. However, no major differences were observed between the two systems in phosphorus removal. Moreover, both systems were also capable of removing the available heavy metals (i.e., Fe, Cu, Mn, Ni, Pb, and Zn) as the removal efficiencies exceeded 90%, except for Fe (76%). Similar to phosphorus, it was concluded that the top layer is the major contributor to the heavy metals removal. Overall, the biofiltration system using Cyperus was found to be a successful system for operating under tropical conditions.

Simultaneous Adsorption of Heavy Metals from Roadway Stormwater Runoff Using Different Filter Media in Column Studies

Stormwater runoff from roadways often contains a variety of contaminants such as heavy metals, which can adversely impact receiving waters. The filter media in stormwater filtration/infiltration systems play a significant role in the simultaneous removal of multiple pollutants. In this study, the capacity of five filter media—natural quartz sand (QS), sandy soil (SS) and three mineral-based technical filter media (TF-I, TF-II and TF-III)—to adsorb heavy metals (Cu, Pb and Zn) frequently detected in stormwater, as well as remobilization due to de-icing salt (NaCl), were evaluated in column experiments. The column breakthrough data were used to predict lifespan of the filter media. Column experiment operated under high hydraulic load showed that all technical filters and sandy soil achieved >97%, 94% and >80% of Pb, Cu and Zn load removals, respectively, while natural quartz sand (QS) showed very poor performance. Furthermore, treatment of synthetic stormwater by the soil and technical filter media met the requirements of the Austrian regulation regarding maximum effluent concentrations and minimum removal efficiencies for groundwater protection. The results showed that application of NaCl had only a minor impact on the remobilization of heavy metals from the soil and technical filter media, while the largest release of metals was observed from the QS column. Breakthrough analysis indicated that load removal efficiencies at column exhaustion (SS, TF-I, TF-II and TF-III) were >95% for Cu and Pb and 80–97% for Zn. Based on the adsorption capacities, filtration systems could be sized to 0.4 to 1% (TF-I, TF-II and TF-III) and 3.5% (SS) of their impervious catchment area and predicated lifespan of each filter media was at least 35, 36, 41 and 29 years for SS, TF-I, TF-II and TF-III, respectively. The findings of this study demonstrate that soil—based and technical filter media are effective in removing heavy metals and can be utilized in full-stormwater filtration systems.