1. Introduction
Pollution from stormwater can have deleterious effects on the environment if left untreated before entering local receiving waters. In urban areas, stormwater can run over significant lengths, flushing out roof surfaces, residential yards, industries, and roads, etc., collecting a wide range of contaminants including nutrients, total suspended solids, pathogens, microorganisms, and trace metals [
1]. Stormwater treatment methods generally result from three primary mechanisms: physical (such as infiltration and sedimentation), chemical (ion exchange, adsorption, and chemical reactions) and biological (biodegradation or denitrification, for example). While all these pollutants present risks and hazards, heavy metals remain targeted for reduction by many water quality guidelines [
2]. Of particular interest are four metals that are commonly found in urban runoff and pose serious risks to human health and the environment: cadmium (Cd), which is considered a human carcinogen [
3]; copper (Cu) can cause vomiting, nausea, and death in high doses [
3]; chromium (Cr) in the form of Cr(VI) (hexavalent chromium) is toxic and carcinogenic [
4]; and zinc (Zn), which can have detrimental effects in high doses [
5]. Fortunately, Low Impact Development (LID) options, which are becoming widely adopted as sustainable methods for mitigating stormwater quantity and quality, can be designed to treat heavy metals in stormwater runoff.
The literature contains several notable reviews on the removal of heavy metals from polluted waters [
6,
7,
8,
9,
10,
11,
12] describing conventional methods for removing heavy metals from wastewater, including chemical precipitation, ion-exchange, adsorption, membrane filtration, coagulation–flocculation, flotation, and electro-chemical methods [
7]. Alternative approaches to modern technologies will often focus on using waste materials to remove heavy metals [
8,
10,
12] or readily available materials such as clay soils [
11] via ion-exchange. Zhao et al. [
13] reviewed several methods available for removing heavy metals noting that among the conventional methods available, chemical precipitation—a commonly used conventional method, is only effective in high concentration metal ion solutions. This has particular implications for treating stormwater, which tends to have lower concentrations of contaminants than wastewater.
Biosorption is another method that exploits the benefits of low cost and environmentally friendly alternatives such as fungi and poplar trees etc. to provide sustainable methods for the removal of heavy metals in stormwater. Chitin and its deacetylated form chitosan, is an example of a biosorption material for water purification, and can be extracted from the shells of shrimps, lobsters and crabs, or supporting mineral deposits [
14,
15]. In biosorption research, both natural and modified absorbents are investigated. Lim et al. [
16] reviewed all the economical absorbents available in recent years based on the source of the absorbent. They note that nano-sized, zerovalent particles and minerals, such as magnetite, laterite, and cement kiln dust, etc., show great efficiency in removing arsenic. Egg, hen, and duck shells from food industries are also popular absorbents and agricultural waste such as coconut husks, rice husks, palm fruit, nut shells, and fruit bagasse are becoming promising materials for metal removals. Biosorption using calcium carbonate from seafood waste, such as oyster, clam, and crab shells, is also gaining popularity [
15].
Oyster shells are a type of mollusk shell in which over 90% of the shell’s mass is calcium carbonate with organic matrices occupying less than 5% of the shell [
17]. Calcium carbonate has three crystal forms: calcite, aragonite, and vaterite. Calcite is the most stable form, followed by aragonite, then vaterite. Aragonite is the most common mineral in a mollusk shell, followed by calcite, which are crystal forms of calcium carbonate. The mechanism by which the oyster shell (CaCO
3 micro-particles) absorbs metal ions is through ion exchange in three steps: (i) absorption of metal ions on the porous surface area (involving dissolution of partial calcium carbonate because of higher solubility compared to most of the metal carbonates, releasing Ca
2+ and CO
32−); (ii) precipitation of metal ions on the surface; and (iii) the formation of heavy metal complex nucleation and crystals on the surface [
3]. To date, mollusk shells have been used in wastewater treatment for many purposes, such as purifying wastewater by trapping particulates (by forming a filter bed with shell powder), nutrient reduction [
18]; adjusting pH to provide an alkaline environment for specific reactions; or ion substitution for removing heavy metal ions. The mechanism of using mollusk shells for water treatment is mainly by using calcium carbonate for heavy metal sedimentation while releasing calcium into the water at the same time. The original hypothesis dates from early studies on the strong adsorption ability of metal ions on calcite, a calcareous geologic counterpart [
14].
Most research to date use shell powder with specific particle sizes. Most studies have proven that shell powder works well in solutions with high concentrations of contaminants. Tudor et al. [
14] studied the application of minimally processed shells on heavy metals in high concentration industrial wastewater, and compared the difference in removal efficiency among three molluscan species: clam, oyster, and lobster. Higher initial lead concentrations required greater exposure times to reach higher removal efficiencies, but varying exposure time produced varying results with oysters having the highest removal efficiency over longer contact times. Zhang et al. [
3] compared lobster, clam, and oyster powder against natural minerals for removal capacity of silver, lead, gold, nickel, copper, zinc, cadmium, trivalent chromium, and hexavalent chromium ion solutions. Overall, seashells outperformed materials of geologic origin. Similarly, Liu et al. [
15] worked on the removal ability of bivalve mollusk shells (raw vs. pretreated) with single and mixed metals solutions in beaker experiments. Removal efficiencies were influenced by the solution’s initial concentration and exposure time. Du et al. [
19] also studied the influence of shell powder particle size, material dosage and pH on the removal of lead, zinc, and cadmium ions. The results showed that removal capacity increased dramatically with smaller particle sizes. Du et al. [
20] compared mollusk shell powder and geological calcite, both at the nano-size, for removal of copper, chromium, lead, and zinc ions. The results showed that mollusk shell nanoparticles had a higher absorption for adsorption occurring in mixed metal ion solutions. Another study [
21] focused on the use of oyster shells for treatment of copper ions in wastewater but with lower initial concentrations. A strong positive correlation was found between initial concentration and equilibrium adsorption capacity. Seco-Reigosa et al. [
22] tried to use shell ash from shell calcination to treat hexavalent chromium, arsenic, and mercury ions. Hexavalent chromium removal efficiency was poor but increased with initial concentration because of the competition between hydroxyl ions and chromium oxyanions [
23]. Moon et al. [
24] also used calcined oyster shells to treat metal ions of copper and lead. They observed a 95% reduction for copper while a mixture of calcined oyster shell and cow bones showed a 99% and 95% reduction for lead and copper ions, respectively; thus, demonstrating that the oyster shell provided the greatest proportion of removal. A table summarizing these studies is provided in
Appendix A.
All the literature reviewed used a powder form or ash form of the shells because of the large surface area provided by fine particles. There are few, if any, investigations into the feasibility of using whole oyster shells in this capacity. Using waste shells directly to avoid the energy cost associated with conversion to powder may be worthwhile when attempting to implement these methods on a large scale for practical use. Waste oyster shells are abundant in coastal cities such as Victoria, British Columbia, Canada, and using this waste for treating pollutants is gaining widespread attention for use in stormwater treatment. Given the literature, it is expected that the whole oyster shell will be less effective than powder form. In addition, most of the literature describes the performance of shells for quality mitigation of wastewater/sewage, which has a much higher concentration of contaminants and is supplied virtually continuously. However, stormwater is very different from wastewater in that the former is intermittent and has lower concentrations. In addition, wastewater treatment is typically conducted in a controlled environment where contact time, or exposure time, is controlled. An LID treating stormwater provides a contact time that depends on the LID’s design. That is, the LID is designed to retain the stormwater for a certain amount of time while transporting the contaminated water through various components of the infrastructure. Thus, this contact time or exposure time is effectively the infrastructure’s total hydraulic retention time, which is a function of the LID’s design. However, the use of oyster shells for stormwater treatment may in some circumstances be as simple as placement in a catchbasin or sediment-trap or strewn along a ditch receiving stormwater runoff. The primary objective of this paper is to determine the potential for the application of whole, unprocessed oyster shells in the treatment of heavy metals in stormwater. The efficacy of using whole, unaltered oyster shells to treat four heavy metals (Zn2+, Cu2+, Cd2+, and Cr(VI)) in stormwater is investigated with a focus on the role of contact time (herein referred to as exposure time, ET), initial concentration (IC) of incoming stormwater, surface area (SA) of the whole oyster shell in contact with the contaminated stormwater, and hydraulic retention time (HRT).
2. Materials and Methods
The first phase of the research involves lab-scale experiments to investigate the effect of different ETs (from one hour to 7 days) using single shells in beakers (where the shell’s mass, SA and volume are known) with a range of initial concentration solutions containing a single metal. The second phase of the research involves a mid-scale device that works with a layer of many shells to test the impact of elapsed time and HRT on removal efficiency. The mid-scale device will allow the information on a single shell to be scaled-up to a group of shells that might be used in a catchbasin, for example.
2.1. Oyster Shell Characterization
A large quantity of whole and unprocessed oyster shells was obtained from a storage site in Victoria, BC. These shells were originally collected from coastal areas of Victoria and are mainly Fanny Bay oysters from Baynes Sound. In total, 132 shells were obtained for use in this research. Each shell was cleaned carefully with a brush and then air dried. The mass, volume, and SA of each shell was determined before treatment. Because of the shells’ irregular shape, a manual method was developed to measure the SA of each shell as precisely as possible [
25]. In this method, sheets of aluminum foil were carefully molded to each side of the oyster shell. The foil mold was cut precisely to the edge of the shell, and then flattened out onto a sheet of paper in order to determine the surface area from a photograph. This is illustrated in
Figure 1a,b. The top side of each shell was measured separately from the bottom side (which would be the inside of a closed shell when the oyster was alive in the shell) and all shells were measured and labeled. As shown in
Figure 1c, the shells ranged from 26.7 cm
2 to 194.6 cm
2 in surface area (mean of 82.6 cm
2), with mass ranging from 5.8 g to 79.1 g (mean of 23.0 g), and volumes ranging from 3.5 mL to 34 mL (mean of 13.8 mL).
Figure 1c shows a histogram of the shell SA and
Figure 1d,e are scatter plots of the shell data.
Figure 1d,e show a planar relationship between SA, mass, and volume. To investigate the relationship between SA, mass, and volume for this species of oyster (in order to scale up results from the lab study to the field application), these data were fitted with a function intentionally defined to be dimensionally homogenous with variables
M (mass),
V (volume), and SA (surface area). In Equation (1a),
M has units of grams,
V has units of cm
3 and SA has units of cm
2. Density
ρ =
M/
V has units of g/cm
3 and the mean shell density was 1.73 g/cm
3 with a standard deviation of 0.59 g/cm
3 and a median shell density of 1.64 g/cm
3. The proposed equation is
where
c and
d are constants with units of density (g/cm
3) and the constant
e has units of grams. The following fitted equation (using Matlab
®) produces an R
2 of 0.77:
In Equation (1a) the values of
c,
d, and
e should be some portion of the average density of the oyster shells, with
e being a reflection of the scatter in
Figure 1d,e. Notice that
c,
d, and
e sum to a value similar to the median density. While both the volume and the mass have a bearing on the value of surface area for each oyster shell, for practical reasons, LIDs designed with raw oyster shells would likely be designed with a certain minimum mass of shells. A simple linear regression between SA (cm
2) in terms of mass
M (g) produces the following expression with an R
2 = 0.67:
or
Equation (2) is for practical purposes and suggests a rule of thumb of doubling the amount of mass in grams to get a rough idea of the surface area needed in cm2.
2.2. Individual Experiments for Cu2+, Zn2+, Cd2+, and Cr(VI)
Concentrations of individual contaminants within a stormwater sample will vary widely with the location and event [
1]. Thus, a lab-scale experiment targeting each metal individually was conducted. To study the effect of oyster shell SA and metal solution IC on removal efficiency (RE), 4 × 4 individual experiments (including one blank) were proposed for each metal. ET ranged from 1 h, 6 h, 18 h, 36 h, 3 d, 5 d, and 7 d.
Starting with Cu2+, 16 one-liter beakers were prepared with four IC (including a blank with distilled water) and four different SA ranges. After the shells were measured with the method indicated, a comparative analysis amongst the shells divided the 132 shells into four distinct groups such that every shell in a group had a surface area that did not differ by more than 3 cm2 from every other shell in the group. Thus, for all intents and purpose, all shells in a group had roughly the same SA. Given the distribution of 132 shells, the following four differently sized groups naturally arose from the sample: (69 cm2, 85 cm2, 99.5 cm2, and 108 cm2). These separate groups were then used to determine the impacts of SA on efficiency of removing Cu2+ in the initial stages of the experiment. However, initially, the difference in some of the SA groups was too small to see the impact of this variable; thus, one more experiment for Cu2+ with IC = 0.20 ppm with six different SA group sizes ranging from 38 cm2, 121.6 cm2, 223 cm2, 320.8 cm2, 443 cm2, and 550 cm2 was conducted. The initial concentrations for Cu2+ included a blank solution (0 ppm), and three low concentration solutions: 0.23 ppm, 0.93 ppm, 2.79 ppm. During the experiment, the beakers were covered with a piece of plastic wrap to prevent water from evaporating. Four parameters, including pH, temperature, electric conductivity (EC) and concentration were monitored for each ET. The values of pH, temperature and EC were tested with a YSI Sonde EXO2 six parameter probe with accuracies of ±0.2 °C on temperature, ±0.1 of pH and ±1% of reading on EC. A LaMotte Smart Spectro 2 Spectrophotometer was used to measure concentrations with Diphenylcarbohydrazide reagent for Cr(VI); Bicinchoninic Acid reagent for Cu2+; 1-(2-pyridylazo)-2-naphthol reagent for Cd2+, and Zincon reagent for Zn2+. Although initial concentrations were prepared based on molar calculations to achieve a desired IC, when measured with the spectrophotometer, deviations from the expected values existed. Thus, the concentration recorded for IC was the one produced by the spectrophotometer and not that determined from molar calculations.
Similarly, for Zn2+, Cr(VI) and Cd2+, 3 × 3 individual experiments were devised. For Zn2+, 1 ppm, 0.5 ppm, and 0.2 ppm IC were tested with 35 cm2, 154 cm2 and 303 cm2 sized shells (same ET range as Cu2+). For Cr(VI) the same concentrations were tested with 37.8 cm2, 151 cm2, and 300.5 cm2 and similarly for Cd2+ but with shell sizes of 43 cm2, 152.8 cm2, and 300 cm2. Again, the initial concentrations recorded by the spectrophotometer were used in the analysis.
The removal efficiency (RE) in this work was calculated as
where
Co is the initial concentration (mg/L) and
C(
t) is the concentration at time
t (h) [
1,
26]. In addition, because several processes in wastewater treatment follow a first order kinetic, modelling was conducted to determine if the removal process for each of these metals by oyster shells followed a first order kinetic model. The general model of a first order kinetic [
26] is:
where
k is the reaction rate in units of h
−1. In addition, to investigate the possibility of a higher than first order reaction, the following was used:
where a second order reaction is being explored.
2.3. Mid-Scale Experiment
In wastewater treatment, hydraulic retention time (HRT) is an important parameter related to inflow rate and reactor volume. HRT in the field of environmental hydraulics [
26] is a parameter in completely mixed flow reactors (CMFR) or plug flow reactors—two models used to understand the transport and retention of pollutants in large water bodies like lakes. In the implementation of whole oyster shells for use in the field to treat stormwater, exposure time (ET), which is related to HRT, needs consideration as the type of exposure seen in wastewater treatment is not the kind of exposure expected in LID systems. To treat stormwater with a biosorption material, the LID must be designed with a minimum HRT, noting that stormwater is intermittent. In large scale systems like LIDs, ET is intimately connected with HRT but generally,
where
Ve is the effective volume of the reactor or container (m
3) and
Q is the effective outflow rate (m
3/h) giving an HRT in hours [
26]. To explore a larger scale application above the lab scale, and which could allow the exploration of HRT values that may exist in actual LID systems, a mid-scale device was designed and fabricated to mimic real conditions as closely as possible. This device was composed of five plexiglass pieces and an additional piece that is movable inside of tank to change the tank volume. The base is on adjustable pedals allowing the tank to achieve a desired slope to permit drainage by gravity. The overall dimensions are approximately 1 m length × 0.6 m height × 0.3 m width. The design of the mid-scale tank is shown in
Figure 2.
The mid-scale experiment differs from the lab-scale beaker experiments in that the latter only uses standing water. The mid-scale experiment is intended to mimic runoff to an urban infrastructure with an inflow and outflow. The exposure time, or HRT would all be related to the elapsed time from the start of the storm which enters the tank in 20 min increments. The inflow combines and mixes with treated water exposed to the shells, and 60 mL is extracted every 20 min to mimic outflow. The outflow would then be a mixture of new and treated inflow. It is difficult to know the extent to which the inflow is in contact with the shells and what portion remains untreated, but some mixing is assumed, as it would be in real-world LID applications.
The mid-scale experiment was conducted according to the 6-h Duration Design Storm from the District of Saanich (in Victoria, British Columbia, Canada) Stormwater Modeling Standards [
27] but 20 min increments were used instead of 10 min increments (see
Figure 3). The inflow volume every twenty minutes was calculated based on rain depth and the bottom surface area of the tank (2787 cm
2). Thus, if the initial inflow (rain depth) is 0.36 mm, this produces 0.1 L when multiplied by 2787 cm
2 and the first flush would be 100 mL. The specific data are shown in
Figure 3. A 10.465 L sample solution was prepared mixed and confirmed by testing with 0.5 mg/L Zn
2+, 0.55 mg/L of Cd
2+, 0.33 mg/L of Cr(VI), and 0.62 mg/L Cu
2+ in one solution. A layer of shells was put on the bottom of tank with a total surface area of 9437 cm
2, mass of 2660 g and volume of 1600 mL shown in
Figure 2b. The concentrations of Zn
2+, Cu
2+, and Cr(VI) were tested with the spectrophotometer, but a separate Cd
2+ test from LaMotte was used to test Cd
2+ with 0.1 ppm accuracy. When the experiment began, every corresponding volume of water was poured into the water jar and flowed into the tank from the inlet continuously for twenty minutes. At every 20 min increment, 60 mL from the furthest outlet was collected and tested for various parameters. Outflow was monitored again at 1, 4, and 7 days, to see if anymore contact time could increase removal. Therefore, in the tank experiment, elapsed time is not the same as elapsed time in the beaker experiment (in which elapsed time is also equal to contact time).
Considering the formula of HRT in Equation (6) for the system, the volume
Ve is divided by either inflow rate or by the outflow rate (
Q in Equation (6)). For HRT calculations in wastewater treatment,
Q is the inflow rate which varies and thus,
Ve also varies because it is a function of
Q. Therefore, HRT was calculated every twenty minutes, and the volume of water at that time would be considered as
Ve and
Q is the inflow rate at that twenty minutes. For a CMFR or PFR (plug flow reactor [
26]) system representing large water bodies and LIDs,
Q is the outflow rate, which is 60 mL/20 min equal to a constant 3 mL/min.
Ve is the cumulative volume minus cumulative outflow.
The equation governing both a CFMR and a PFR system is
where
m is the mass of the pollutant in the system,
is the mass flux in,
is the mass flux out and
is the mass rate of change solely due to the reaction. For a first order decay reaction,
and
Ve is that of Equation (6),
k is that in Equation (4) and
C is the concentration of the pollutant in the tank at time
t.
Given the physical size of the tank, and the way inflow and outflow were generated, in the early stages of the storm, the system is assumed to be most represented by a non-steady state CFMR with first-order decay. This results in the following expressions in their differential and integral forms:
where
Qin is the inflow rate,
Cin is the concentration in the inflow,
Qout is the outflow rate, and
Cout is the centration of the pollutant in the outflow. Given the inflow follows the design storm, the inflow concentration and the outflow concentration are both fixed, and assuming that the concentration
C in the tank at any time
t is equal to the concentration in the outflow
Cout at that time
t, and converting to discrete form, Equation (9b) reduces to
where
t =
nΔ
t, Δ
t is the time increment of 20 min (or 0.333 h) and
n = 1, …, N where NΔ
t marks the end of the experiment. A simple spreadsheet can be used to compute
k as all parameters are either known or observed. The parameters
Ve and
Qin at every time
t are effectively given in
Figure 3 and
Cout is measured at the outflow.
4. Conclusions
Initial concentration IC, surface area SA, and exposure time ET, were the factors monitored in the lab-scale treatment of Cu
2+, Zn
2+, Cd
2+, and Cr(VI) by whole oyster shells. The lab-based experiments showed that even at the lowest initial concentration with 24 h of exposure time, a 79% removal rate can be reached for treating Cu
2+ in a beaker; 33% RE for Zn
2+, 58% RE for Cd
2+, and 14% RE for Cr(VI). This suggests that under similar conditions, the adsorption rate of metals is rated as follows: Cu
2+ > Cd
2+ > Zn
2+ > Cr(VI). This order is similar to what was observed in the mixed solution sample under continuous inflow in the mid-scale experiment at the end of runoff (six hours). Generally, ET is positively related to removal efficiency: longer times can increase removal efficiency, and this is generally consistent with the literature [
15]. However, desorption can easily occur over longer times, particularly those times seen in field applications. Moreover, for most of the metals in the lab experiment, the most effective reaction time is the first few hours. Although an increase in removal can be seen with longer times, the increase was negligible. There is positive relationship between IC and removal efficiency (RE) for Cu
2+ and Zn
2+, but a negative relationship was found for hexavalent chromium. No clear relationship between IC and RE was seen for Cd
2+. A strong positive relationship was observed between SA and RE for all metals; however, the effect seems negligible when SA is between a certain range and smaller when surface area is over certain amount. It was also found that smaller shells can also reach high removal efficiencies but will take a longer time than a larger shell. In general, a first order decay rate could effectively describe the reductions of Cu
2+, Zn
2+, and Cd
2+ in the lab-scale experiments. Modelling also suggested that after a threshold of surface area is reached, ET is not a dominant factor in treatment.
In the mid-scale experiment, which mimicked real-world conditions with a 6 h storm, treatment of Cu2+ was observed in the first few hours of the storm but there was leaching of Zn2+ almost immediately and some leaching of Cd2+. The bulk of the treatment occurred in the post-storm hours while the stormwater was retained with the oyster shells in a completely mixed flow reactor with no inflow. The modelling showed that at least for Cu2+, when in combination with other metal contaminants in stormwater, an unsteady, completely mixed flow reactor with a reaction term can provide reasonable description of the treatment of Cu2+ in the field.
The laboratory experiments represented controlled experiments observing how the concentrations of ions of a single metal changed as a function of surface area, initial concentration and exposure time. It differs from the mid-scale experiment that was designed to more closely mimic conditions in the field in which incoming stormwater would include more than one type of metal and organic material. Thus, the lab-scale experiments cannot observe ion competition arising in a mixture. The intention of the laboratory experiments was to determine practical relationships between the surface area of whole, unprocessed oyster shells and the reduction in specific metal ion concentrations in order to suggest a minimum mass recommended for practical use in the field; and as well to add to the literature on the use of this material in LID designs. The lab-scale experiments would more closely resemble the mid-scale experiment after the storm was over, and for an influx of stormwater containing predominantly one metal.
Recommendations for future research should involve expansion of both the lab-scale experiments and the mid-scale experiment to involve solutions containing components typically found with metals in stormwater, specifically organics. Further exploration into the models and the coefficients relating removal efficiencies and material characteristics should be researched. Validating the relationship for surface area and exploring this relationship when using other types of waste shell materials, is recommended. In addition, with time, the shells within an LID, such as a catchbasin or ditch, will likely form biofilm on the surface and collect debris over time. This will impact the ion exchange process and metal ion mitigation, and thus maintenance routines should be explored in practice.