Characterization of permeable pavement materials based on recycled rubber and chitosan

Construction and Building Materials 69 (2014) 221–231

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Characterization of permeable pavement materials based on recycled rubber and chitosan Christopher A. Murray ⇑, Kayla S. Snyder, Brooke A. Marion Department of Interdisciplinary Studies, Lakehead University, 500 University Avenue, Orillia, Ontario L3V 0B9, Canada

h i g h l i g h t s  Composite materials for use as permeable pavers were prepared from chitosan and tire crumb.  Compressive strength and hydraulic conductivity similar to conventional permeable pavers was achieved.  Sorption capacity of dissolved zinc from water was as high as 0.63 mg per gram of chitosan.  Unlike other permeable pavements, properties were not strongly dependent on binder content.  Material properties and morphology were dominated by the concentration of chitosan solution used.

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Article history: Received 13 March 2014 Received in revised form 11 July 2014 Accepted 17 July 2014 Available online 12 August 2014 Keywords: Tire crumb Chitosan Permeable pavement Hydraulic conductivity Compressive strength Zinc

a b s t r a c t A variety of composite permeable pavement materials were prepared from crumb rubber embedded in a matrix of the biopolymer chitosan, which is a waste product of the seafood industry. We have characterized the hydraulic conductivity, mechanical properties, and the capability of these materials to remove particulate and dissolved pollutants (including zinc) from water. The dependence of material properties on process parameters such as binding polymer content differs from what is typical of binder-based permeable pavement, due to the mechanism by which chitosan is introduced, and in many cases the stability of the composite material increases with decreased binding polymer content. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction One of the most important areas of focus where water quality is concerned is management of stormwater and runoff. As development increases and natural vegetative groundcover is replaced by pavement and roofing, rainfall that would have otherwise been absorbed and slowly released by plants and soil is concentrated into fast-moving streams in pipes and gutters, dramatically increasing its ability to erode soil, carry pollutants and overflow into other water management systems such as wastewater systems. In addition to increasing the fraction of impermeable space, development is typically associated with increased soil vulnerability – removing the vegetative groundcover for development makes soil susceptible to erosion. Developing countries and regions responding to disaster are similarly vulnerable to pollutants

⇑ Corresponding author. Tel.: +1 705 330 4008x2651; fax: +1 705 329 4035. E-mail address: [email protected] (C.A. Murray). http://dx.doi.org/10.1016/j.conbuildmat.2014.07.047 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.

carried by stormwater, and when existing stormwater systems are overtaxed or destroyed, a major health risk is caused by uncontrolled flood water mixing with sewage. A simple solution to these problems involves increasing the capacity of developed areas to infiltrate rainwater rather than conveying it over impermeable surfaces to rivers, lakes and streams. Permeable or porous pavement is a family of passive technologies that address the problem of runoff by providing pathways for water to infiltrate down through walkways, parking lots and roads [1–3]. Rainwater may be directed over an expanse of permeable pavement, which (if large enough) can allow all of the runoff to infiltrate down into groundwater. This reduces the buildup of ice in cold conditions, and may achieve physical filtration and pollutant removal [4,5], but its most important role is to reduce the impact on other stormwater conveyance systems by reducing the volume of runoff and its capacity for erosion. Two major categories of permeable pavement that include a large fraction of aggregate materials are (1) permeable cement or

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asphalt and (2) resin- or elastomer-bound pavement. Materials falling into the former category are typically prepared by omitting or limiting a void-filling component of what would normally be impermeable concrete or asphalt [1,6] (Fig. 1) and those falling into the latter category are made by bonding granular and/or aggregate material within a polymeric matrix that inherently leaves spaces [7]. Cement- or asphalt-based permeable pavement is usually heavy and capable of supporting similar compression as their impermeable counterparts [6]. Binder-based permeable pavers are light, often flexible and are more likely found in walkways than driveways because of their lesser resistance to compression. Crumb rubber is a common filler material for both types of permeable paver [7,8], as it is low in cost and can often impart elasticity and resilience that is desirable in walkways. Crumb rubber is generally made from waste car and truck tires that have failed or have exceeded their usable lifetime [9]. More than 300 million waste tires accumulate each year in the USA and the majority of the approximately 80% that are reused are used as fuel. Less than 5% of the tire is typically eroded from the surface before the tread depth becomes insufficient to ensure safe use and the tire is discarded. Because the rubber used in tires is chemically crosslinked, it cannot be reformed into new rubber without significant energy input that generally makes such recycling uneconomical. Tires brought to a tire recycling facility are cryogenically or mechanically crumbled and ground, and the steel belting and fiber is removed magnetically and through the flow of air, respectively. The remaining crumb rubber is sized for the appropriate application and most often there is no washing or cleaning step involved in the preparation of the final crumb product. Primary applications for tire crumb include filler, athletic and equestrian running surfaces, road resurfacing [6,10,11], playing surfaces [12] and garden mulch. In some of these applications, significant cleaning of the rubber is required after it has been purchased from the tire recycling facility, because of the contaminants present in tiresmost notably organic compounds and heavy metals such as zinc, lead and cadmium [12,13]. These metals eventually leach from untreated crumb rubber into water, and similarly pose a threat to water along roadways, where the tread has been abraded off in the form of tiny, high-surface area particles that eventually get swept from the road into ditches and other stormwater conveyances. Because of the environmental impact of tire waste, significant interest exists in developing new applications for crumb rubber. While most existing applications treat tire crumb as a commodity filling material, some researchers have pursued more exotic applications that might increase its value and indirectly motivate more progressive approaches to tire recycling. Crumb rubber has been used as an experimental filter media [14–16] that becomes more compressed with increased depth in the filter, leading to decreasing pore sizes ideal for physical filtration (where large particles are stopped first, smaller particles make their way further into the filter and ultimate filter clogging involves a full filter, rather

than one blocked by a small amount of material at the top). Crumb rubber has also been considered as an absorbent of the very heavy metals and other contaminants it is known to contribute to surface water. In this paper we present results describing the preparation of stormwater filter materials suitable for permeable pavement from crumb rubber that is untreated following collection from the tire recycling facility. The binding material we have used to hold the matrix of rubber particles together is the biopolymer chitosan, which is derived from chitin. Chitin is the second-most abundant polysaccharide found on Earth, and plays a structural role in most invertebrate exoskeletons. Chitosan can degrade in a landfill (with suitable temperatures, moisture and enzyme content) and correspondingly is a candidate material for many single-use packaging applications that promises to relieve stress on overfilling landfills. Chitosan is a waste product of the seafood industry and, like tire rubber, it is underutilized because of difficulties associated with its processing. Unlike common thermoplastics, the high degree of inter- and intra-molecular bonding in chitosan prevents it from melting, and only solution-based processing methods are available. Dilute solutions of chitosan (dissolved in weakly acidic, aqueous solutions) can be dried to form films, coatings and membranes but chitosan is rarely used in a structural capacity (in spite of this being one of its roles as a biomolecule in nature). Instead, chitosan currently finds application in specialty products requiring antimicrobial properties, biocompatibility (chitosan can be formed into implants that resist rejection from the body) and fat-binding capacity: chitosan can be eaten but not digested, and it can bind to fats and prevent their digestion. Because of hydroxyl and amine side groups on the chitosan monomer, it is reactive and readily binds with metals [17]. In developed countries, rubber and chitosan are both industrial waste products for which much-needed recycling technologies may be encouraged by higher value applications. We have used largely untreated rubber and chitosan as it is available from largequantity manufacturers to demonstrate that this type of technology can be made available not only to the developed world where stormwater is increasingly of concern, but also to developing nations and areas recovering from disaster. Cleanliness of water is of primary concern in such areas, and it is our hope that because both chitosan and rubber are easily available and because the processing techniques described below are simple and inexpensive this type of technology may find use in such areas where water quality and pollution are of more critical concern. 2. Materials and methods We have prepared a variety of candidate permeable paving materials by mixing crumb rubber and weakly acidic aqueous chitosan solutions, using various solution concentrations and ratios of solution to rubber. A subset of these samples were exposed to sodium tripolyphosphate solution to achieve chemical crosslinking (but without the toxic consequences of typical crosslinkers such as glutaraldehyde), as some form of crosslinking of chitosan is necessary to prevent dissolution should

Fig. 1. Schematic representation of cement-based permeable pavement, which differs from conventional concrete (left) in that the cement content is reduced, leaving a continuous network of pores between aggregate materials (right).

C.A. Murray et al. / Construction and Building Materials 69 (2014) 221–231 these materials encounter acidic environments. The ability of these materials to perform as permeable pavers was characterized by measuring their hydraulic conductivity, their ability to remove fine sediment from water, their mechanical compressibility and their ability to remove zinc from aqueous solution. 2.1. Sample preparation Crumb rubber (with average particle diameter of approximately 2 mm) was donated by Emterra Rubber (Mississauga, Ontario). The crumb rubber was prepared from a typical mix of used car and truck tires, and was used in all preparations without any cleaning or other treatment beyond the mechanical and magnetic sizing and separation carried out at the plant. As such, fine particles, impurities such as fiber and wire were still present. A high-molecular weight sample Chitosan was provided by Imtex membranes Corp., and had an average molecular weight (as measured through viscometry of weakly acidic solutions [18]) of 450,000 g/mol, and a degree of acetylation (which ranges from 0% to 100% for pure chitosan and chitin, respectively) of DA = 81%, as measured using potentiometric titration [19] and UV–vis spectroscopy [20]. Before mixing with crumb rubber, chitosan was dissolved in solutions of 5% acetic acid in water, at concentrations between 1% and 8% by mass of chitosan. Each sample was prepared by mixing 10 g of crumb rubber with a well-defined amount of chitosan solution (ranging from 5 to 40 g) in a cylindrical mold with inner diameter of 3.8 cm (see Fig. 2). The mixture was left to dry in a dehydrator and sample mass was periodically measured until no change within a 5 hour period was observed, at which point drying of the sample was judged to be complete. Once dried, the samples were removed from the molds, photographed and qualitatively examined. Depending on the ratio of chitosan to rubber used and the initial chitosan concentration the initial sample height ranged from 1.5 cm to 3 cm. As described below, initial compression testing was performed before further treatment and characterization. Following drying and initial compression testing, half of the samples were neutralized by replacing them in their molds and exposing them to an excess amount of 0.1% NaOH solution for 48 h. The samples were thoroughly rinsed with deionized water to remove any precipitated salts and then dried. The other half of the samples were simultaneously neutralized/insolubilized and crosslinked through exposure to an excess amount of 5% sodium tripolyphosphate (STPP) in water for 48 h, after which samples were thoroughly rinsed with deionized water and dried. A small number of ‘‘conventional’’ permeable paver material samples were prepared by mixing the same crumb rubber with polyurethane adhesive at various ratios and allowing these similarly sized coupons to cure. 2.2. Hydraulic conductivity measurement Hydraulic conductivity of the samples was measured using a modified fallinghead type permeability test [21] in which the sample was secured in a pipe (sealed around the edge using silicone caulking to prevent water from bypassing the sample). The apparatus allowed measurement of pressure drop across the coupon (through a clear manometer tube in communication with the upstream side of the sample chamber) as water was allowed to flow into the top of the pipe and through the sample. Flow rate was measured using a stopwatch and a large beaker to collect water on the downstream side of the sample. The mass of water collected in a given amount of time was measured using a balance and flow rate was determined for each sample as a function of headloss (the height of water above the sample).

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2.3. Filtration performance Following hydraulic testing, the filtration performance of a subset of samples was measured. Influent water with concentration of finely ground silica particles (sil-co-sil 106) [22] (average particle size of 22 lm) of 200 mg/l was allowed to flow into the hydraulic testing cell, and inlet and outlet samples were collected and measured using a modified Total Suspended Solids (TSS) removal test procedure [23]. Trans-sample pressure drop and flow rate were recorded (as described above). The samples were not tested for sediment loading capacity, and no pressure change that might be associated with clogging was observed over the course of the measurements. Each collected sample was filtered through a pre-weighed glass fiber filter (with nominal pore size of 1 lm) under light vacuum, and the sample collection beaker was rinsed with deionized water into the filter until no trace of the sediment was observed. The filter was dried in an oven at 85 °C and weighed to obtain a measure of the amount of sediment in the sample. Control samples with well-defined sediment concentration were interspersed with the experimental samples to confirm accuracy of the measurement.

2.4. Mechanical (compressive) strength The mechanical response of the samples was evaluated by measuring stress and strain of samples with a procedure modeled after ASTM D575 [24], using a custombuilt device consisting of a screw and force transducer, which was capable of applying in excess of 1330 N of force to the samples with a resolution of 4 N, and allowed simultaneous measurement of compression with a resolution of 0.03 mm. The apparatus was calibrated by applying compression to the limits of the device without sample, and the stress/strain behavior of the force transducer was subtracted from the measurements made with samples in place. Following release from the molds and examination, preliminary measurements of the mechanical response of the samples was made, so that any change accompanying crosslinking, hydraulic or filter testing could be discerned. Compression was kept to a minimum to avoid any destruction of the samples. After neutralizing/crosslinking and the hydraulic and filter testing described above, samples were replaced in the compression testing apparatus and exposed to progressively higher values of compressive force while the displacement of the samples was measured. The samples were thus cycled in pressure until either the sample disintegrated or the limits of the testing device were reached. As described in standardized compression testing procedures [24], every value of displacement was held for approximately 3 s before pressure was measured, to allow for a consistent amount of stress relaxation in the samples.

2.5. Zinc removal Samples were tested for their ability to remove dissolved zinc from water using UV–vis spectroscopy. Both neutralized and crosslinked samples were left to soak under solutions (100 g of solution for each test, which covered the sample in small beakers) of zinc at concentration of 3 mg/l chosen to be high but appropriate for accelerated capacity testing. After times of 1, 2, 4 and 8 days, aliquots of the solution surrounding the various samples were collected and measured using a Dithizone-based spectroscopic method [25], and removal (or leaching) was determined by comparison with the original concentration of zinc in the solution.

Fig. 2. Digital images of the sample coupons resulting from varying the concentration of chitosan solution used (vertical axis) and the amount of chitosan solution (horizontal axis) in combination with 10 g of crumb rubber.

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3. Results and discussion 3.1. Qualitative observations Dramatic differences were observed in the samples as a function of the amount and concentration of chitosan solution added to the rubber (see Fig. 2). Because of the means by which chitosan solution dries, high ratios of solution to rubber caused crumb rubber particles to be widely dispersed in the final dried material, leading to a more porous sample with less mechanical integrity than when a smaller amount of solution was used. On average, the void space formed by the gaps between 10 g of crumb rubber particles is 12–13 ml. As such, when the volume of solution used in preparing the sample, Vsol, is greater than 12 ml, rubber particles are forced to separate. As the amount of solution used increases beyond this void space amount, the resulting structure becomes increasingly fragile and includes larger gaps between rubber particles. Because the chitosan solution at the surface of the sample dries and becomes immobile before the solution in the bulk of the sample has time to dry and contract, in samples prepared using large amounts of chitosan solution rubber particles become immobilized near the surface (where a crust formed) and at the bottom (where chitosan remains mobile the longest), leaving a large open space in the center of the sample (Fig. 3a). If smaller amounts of more concentrated solution are used, a much denser, stronger sample resulted (see Fig. 3b). Though only one sample prepared using a chitosan solution concentration of 1% was sufficiently stable for mechanical testing, several samples prepared using solutions of higher concentration but smaller absolute amounts of chitosan were stable. This result is contrary to what is expected of conventionally prepared ‘‘binder-based’’ permeable materials, in which more binding polymer generally leads to a stronger material that is less porous. Clearly, there must be a lower limit to the amount of chitosan that can be used and still maintain sufficient sample integrity. In the samples prepared for this study, the smallest amount of chitosan that when used resulted in a cohesive sample was 0.1 g

Fig. 3. Schematic diagram indicating how composite samples prepared using large amounts of dilute solution can lead to a fragile, open structure upon drying (a), while for samples prepared using smaller amounts of more concentrated solution, a denser sample may be obtained (b).

(5 g of a 2% solution), but twice this amount, when introduced as 20 g of 1% solution, resulted in a sample with too little stability for any kind of testing. It is likely that an even smaller amount of chitosan than 0.1 g could be used to prepare a stable sample, if solutions of higher concentrations were used. This qualitatively different mechanism of composite material preparation leads to a much richer behavior than what is observed with traditional resin-bound permeable pavement. Because the extra degree of freedom afforded by variable solution concentration strongly affects the final sample structure, much more variation can be achieved through adjustment of a few process parameters. In the ‘‘conventional’’ samples prepared by mixing polyurethane adhesive with crumb rubber, the stiffness and durability of the sample increased with polyurethane content, and there was very little change in the mass (<1%) of the sample upon drying/curing. The highest amount of polyurethane added was only barely sufficient to fill the void space between the rubber particles, and as such overall sample dimensions were not strongly affected by the amount of polyurethane used. The thickness of the polyurethane/rubber samples were all nominally identical (16 ± 1 mm). Because there is little change in resin volume associated with curing, the properties of the polyurethane/rubber composite material are a relatively simple function of binder content: porosity decreases as more binder is added, and when the void spaces between the rubber particles are completely occupied by the binder, a solid composite material without pores is formed upon curing. There was no qualitative difference observed between the chitosan/rubber samples that had been neutralized with NaOH exposure and those that had been simultaneously neutralized and crosslinked through exposure to STPP. 3.2. Hydraulic conductivity Results obtained from chitosan/rubber samples using the modified falling head method [21] are shown in Fig. 4, where flow rate through a cross-sectional area of 11.3 cm2 is shown as a function of pressure drop across the sample. Consistent with the qualitative observations made above, the amount of chitosan in the sample did not strongly affect the ease with which water passed through the sample. The data shown in Fig. 4 was analyzed using two methods: first, it was used to calculate an average value of hydraulic conductivity K (as is typical when characterizing the flow of water

Fig. 4. Flow rate of water measured through various chitosan/rubber samples as a function of the height of driving head (in cm).

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through granular media, such as gravel, sand, and other materials used for stormwater filtration) through the use of Eq. (1) [21]:

used to calculate an equivalent hole diameter using the orifice equation [26], Eq. (2):

K ¼ QL=Ah;

pffiffiffiffiffiffiffiffi Q ¼ C d A 2gh;

ð1Þ

where Q is the flow rate, L is the thickness of the sample, A is the cross-sectional area of the sample and h is the height of the driving head of water. The resulting values of hydraulic conductivity are shown as a function of the amount of chitosan in the sample in Fig. 5a, as a function of the amount of solution used in preparing the sample in Fig. 5b, and as a function of solution concentration in Fig. 5c (a dashed line is added to indicate hydraulic conductivity values typical of sand or gravel). Clearly, the hydraulic properties of the sample depend more strongly on the amount of solution and solution concentration than on amount of chitosan in the sample. This is consistent with the qualitative observations that porosity of the sample varies strongly with the amount of chitosan solution used, though for Vsol < 12 ml the porosity of the sample should decrease as the amount of chitosan increases. In Fig. 5a the hydraulic conductivity for samples for which Vsol < 12 ml are differentiated from those for which Vsol > 12 ml, and when either of these subsets of data is considered separately, the dependence of hydraulic conductivity on the amount of chitosan is clear. To facilitate comparison with other devices used for management of stormwater quantity, the data shown in Fig. 4 were also

ð2Þ

where Q is flow rate, Cd is the coefficient of discharge, A is the crosssectional area of the hole, g is the acceleration due to gravity and h is the height of driving water. In this way, an estimate of the hydraulic behavior of the sample was made by calculating the size of a sharp-rimmed orifice (Cd = 0.62) that would demonstrate the same dependence of flow rate on trans-sample pressure. A sample curve from Fig. 4 is shown in Fig. 6, with a fit to the orifice model (Eq. (2)) shown. The results of this calculation are shown in Fig. 7, where Fig. 7a shows this calculated equivalent hole size as a function of the amount of chitosan in the sample, Fig. 7b shows the equivalent hole size as a function of the amount of chitosan solution used in preparation of the sample and Fig. 7c shows what is the strongest correlation – effective hole size as a function of solution concentration. For comparison, a representative dataset of flow rate versus trans-sample pressure for a polyurethane/rubber sample (where 2 g of polyurethane is combined with 10 g of rubber) is shown with that of a chitosan/rubber sample (in which 10 g of 2% chitosan solution is combined with 10 g of rubber) in Fig. 8. These two samples have quantitatively similar hydraulic behavior but very

Fig. 5. Hydraulic conductivity, K, calculated for chitosan/rubber composite samples by application of Eq. (1) to the data shown in Fig. 4, as a function of the amount of chitosan in the resulting sample (a), the amount of chitosan solution used in the preparation of the sample (b) and the concentration of the chitosan solution used in preparing the sample (c). In (a) the results obtained using samples prepared using volume of solution Vsol less than the void space between rubber particles of approximately 12 ml are differentiated from those measured for samples with Vsol > 12 ml. The dashed line indicates a hydraulic conductivity of K = 0.001 m/s, typical of sand or gravel used for filtration.

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Fig. 6. One example data set from those shown in Fig. 4, with a fit to Eq. (2) shown, indicating the degree to which the hydraulic behavior of the composite sample can be described by that of a sharp orifice with 6.7 mm diameter.

different polymer contents, indicating that the same hydraulic behavior can be achieved with an order of magnitude less chitosan than what would be required of from a similar sample containing polyurethane. The two samples shown together in Fig. 8 demonstrate that similar values of hydraulic conductivity and porosity are possible to achieve in these two types of permeable materials, but the very different mechanisms by which they are prepared gives rise to a more complex dependence of hydraulic conductivity on process parameters for the chitosan/rubber sample than what exists for traditional resin-bound samples. When the volatile fraction of the binder resin and associated binder resin shrinkage with curing are low (as is the case with polyurethane resin), the porosity of the sample is a relatively smooth function of binder content: once the void space formed by the rubber particles is completely filled by the binder, the sample is completely impermeable. In contrast, because most of the chitosan solution volume is lost during curing, it is not trivial to achieve a completely impermeable sample using the chitosan/rubber combination of materials. Because pore size in the chitosan/rubber samples is not strictly a function of polymer content, a relatively small amount of polymer can yield a dense sample with low hydraulic conductivity, whereas a large amount of polymer can (if introduced as a large volume of low

Fig. 7. Diameter of an orifice with equivalent hydraulic properties to that of the chitosan/rubber samples, made by fitting Eq. (2) to the data shown in Fig. 4, as a function of the amount of chitosan in the resulting sample (a), the amount of chitosan solution used in the preparation of the sample (b) and the concentration of the chitosan solution used in preparing the sample (c).

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Fig. 8. Comparison of flow rate, Q, versus driving head height of water, for a chitosan/rubber composite sample and a conventional polyurethane/rubber sample.

concentration solution) give rise to high porosity and high hydraulic conductivity. Consistent with qualitative observations, there was no systematic dependence of hydraulic conductivity on whether the samples were neutralized with NaOH or simultaneously neutralized and crosslinked using STPP. 3.3. Filtration testing The fraction of fine particles removed from 200 mg/l inlet water by passing through the chitosan/rubber samples is shown in Fig. 9, as a function of the solution concentration used in the preparation of the sample. The TSS removal efficiency [23] of the samples is calculated by comparing the inlet concentration to that measured in the outlet samples (Eq. (3)):

% TSS Removal ¼ 100  ð½C i   ½C o Þ=½C i ;

ð3Þ

where [Ci] and [Co] are the concentrations of the inlet and outlet samples, respectively.

Fig. 9. Total Suspended Solids (TSS) removal efficiency measured for chitosan/ rubber samples as a function of the concentration of chitosan solution used in their preparation. Inset: TSS removal efficiency as a function of hydraulic conductivity K, calculated using the data shown in Fig. 4.

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Since there is no reason to expect these samples to remove sediment by a mechanism beyond physical filtration, it is perhaps not surprising that there is a strong correlation between hydraulic conductivity and filter performance (inset to Fig. 9). It is worth noting that the removal efficiency of these materials is comparable to that of typical stormwater treatment practices [27]. A typical commercially-available stormwater filter can achieve approximately 80% removal of similar influent sediment (i.e. sediment of similar particle size and concentration) after passing through 20 cm of granular filter media at a specific flow rate of under 1 l/sm2. The ‘‘slowest’’ samples tested demonstrated a removal efficiency of approximately 65% for an equivalent specific flow rate of 0.88 l/sm2 after passing through less than 3 cm of sample. It must be noted that no measurements were performed which might characterize the capacity for sediment loading in these composite samples, and that commercial stormwater treatment filters must not only achieve adequate filtration and hydraulic conductivity, but must maintain this performance over long periods of time without maintenance. As such, the measurements presented here only provide preliminary support for the use of these chitosan/crumb rubber composite materials as candidate for permeable pavers, and do not necessarily indicate that they would perform comparably to commercial products over long periods of use. 3.4. Mechanical testing The results of compression testing of samples immediately after preparation are shown in Fig. 10, for maximum strains between 7% and 25%. For comparison, a line is shown on Fig. 10 indicating 200 kPa, a pressure typical of foot traffic that a permeable paving material might be expected to withstand without significant deterioration. An average compressive modulus was determined for each sample and, similarly to the hydraulic and filtration measurements, the mechanical response of the chitosan/rubber composite materials (when all samples are considered together) displayed a much clearer dependence on the concentration of the chitosan solution used in preparing the sample than on the amount of chitosan contained in the sample. Initial compressive bulk modulus values ranging from 502 kPa to 2.6 MPa are shown in Fig. 11 as a function of the total amount of chitosan used (Fig. 11a), the amount of solution used to prepare the sample (Fig. 11b) and the solution concentration used to prepare the sample (Fig. 11c). As was demonstrated in measurements of hydraulic conductivity,

Fig. 10. Initial measurements of pressure applied versus strain for a subset of chitosan/rubber samples. The dashed line shows a pressure of 200 kPa, typical of foot traffic.

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Fig. 11. Compressive modulus, calculated by linear regression of the data shown in Fig. 10, as a function of the amount of chitosan in the resulting sample (a), the amount of chitosan solution used in the preparation of the sample (b) and the concentration of the chitosan solution used in preparing the sample (c). In (a) the results obtained using samples prepared using Vsol < 12 ml are differentiated from those measured for samples with Vsol > 12 ml.

for samples prepared using more than 12 ml of chitosan solution a more porous final structure results, in which (on average) crosssections of the sample decreasingly intersect rubber particles as pore size increases. While more concentrated solution generally leads to a stronger sample, a greater amount of solution does not. In Fig. 11a modulus values measured for samples prepared with Vsol < 12 ml are differentiated from those obtained for samples with Vsol > 12 ml, and it is clear that above and below this critical solution volume the dependence of sample strength on the amount of chitosan is consistent with the overall trend shown in Fig. 11c. Comparison between preliminary mechanical testing and the first cycles of mechanical testing after hydraulic and filtration performance measurement showed no significant change in the samples. There was little or no difference in the results of compression tests performed on samples that had been neutralized and that of those that had been simultaneously crosslinked and neutralized using STPP. As the samples were cycled to progressively higher displacements and pressures, the hysteresis typical of rubber materials was observed, with higher pressures for a given displacement as pressure was increased than was measured when pressure was decreased. A representative compression/decompression curve is shown in Fig. 12, showing nine compression/decompression cycles, with strains ranging from 9% to 40% at the limit of the cycle. In Fig. 12, the initial and final cycles are highlighted.

Fig. 12. One representative set of compression/decompression cycles for a chitosan/rubber sample. The first compression cycle (shown by open triangles) and final compression cycle (shown by open circles) are highlighted.

On any individual cycle, initial compressive modulus was lower than what was measured as the sample became increasingly compressed, an observation qualitatively consistent with the porous

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nature of the sample, which becomes more dense with compression. As the maximum pressure achieved on each cycle was increased, the sample was observed to become softer (as the chitosan matrix began to break down), such that the initial compressive modulus decreased with each cycle. In spite of this deterioration, the maximum modulus increased as a function of cycling, due to the increasing compression. Cyclic mechanical testing of many different samples was consistent with the abovementioned observation that the properties of the sample are dominated more strongly by the concentration of chitosan solution used during preparation than the overall amount of solution or of chitosan in the sample. A comparison of the mechanical testing results of four samples with identical amounts of chitosan (but varying amounts of solution and concentration of solution) is shown in Fig. 13, and shows that sample strength increases as a function of solution concentration (or decreases as a function of amount of solution used). A comparison of the mechanical test results of samples prepared with the same amount of solution but different concentration is shown in Fig. 14, and indicates that the strength of the material increases with increased solution concentration. In contrast, comparison of results obtained for samples prepared using solution of the same concentration (2%) but different amounts (5, 10, and 20 g) does not show any systematic dependence of strength on solution amount (data not shown), as the greatest amount of chitosan solution corresponded to the most porous and fragile sample. As was the case for the initial mechanical measurements before hydraulic or filter testing the multi-cycle compression testing showed a strong dependence on the concentration of the chitosan solution used and on the amount of solution used, but not on the amount of chitosan in the sample. In Fig. 15, average compressive modulus values are shown (calculated as an overall average of all stress/strain measurements over all cycles) as a function of the amount of chitosan in the final sample (Fig. 15a), the amount of solution used in preparing the sample (Fig. 15b) and solution concentration (Fig. 15c). As was observed when considering hydraulic conductivity (shown in Fig. 5) and the initial measurement of compressive strength (shown in Fig. 11), the dependence of ultimate mechanical strength on the amount of chitosan in the sample and on the amount of solution used in preparing the sample is not straightforward when all the data is considered together. In Fig. 15a the

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Fig. 14. The results of compression testing of three chitosan/rubber samples prepared with the same amount of chitosan solution but different solution concentrations, indicating greater compressive strength for samples prepared with higher concentration solution.

samples for which less than 12 ml of chitosan solution was used are differentiated from those samples where more solution was used. For the samples where less solution was used and the rubber particles did not, on average, become separated by large pores upon curing, an expected positive correlation between mechanical strength and the amount of chitosan in the sample is observed. For samples in which more solution was used the behavior is less straightforward: all have relatively low compressive strength, as is expected for a more fragile structure, and the mechanical strength does not seem to be strongly affected by the amount of chitosan in the sample. As a function of binding polymer used, the polyurethane/rubber samples used for comparison were all much stiffer than the rubber/ chitosan samples. For comparison, a sample prepared using 0.5 g of polyurethane and 10 g of rubber demonstrated an average modulus of 2.3 MPa over eight cycles, and a similar average modulus of 2.3 MPa was measured for a sample prepared using 10 g of chitosan solution at a concentration of 2% (for a total polymer of 0.2 g) over five cycles. With the above hydraulic conductivity results in mind, this indicates that while mechanical properties similar to conventional permeable pavers can be achieved using similar amounts of polymer, where hydraulic conductivity and filtration are of primary interest much less polymer need be used to achieve the same effect. Conversely, a chitosan/rubber sample that has similar mechanical properties as a polyurethane/rubber sample has a much lower hydraulic conductivity and a correspondingly higher sediment removal capability. There was no systematic difference in mechanical strength observed between the samples neutralized by exposure to NaOH and those simultaneously neutralized and crosslinked with STPP. 3.5. Zinc absorption capacity

Fig. 13. The results of compression testing of four chitosan/rubber samples with equivalent amount of chitosan, prepared using different solution concentrations and different amounts of solution, indicating greater compressive strength for samples prepared with smaller amounts of higher concentration solution.

Two varieties of the chitosan/rubber composite samples described above were used to measure zinc absorption capacity: samples prepared using 5 g of 2% solution and 10 g of 2% solution, with 10 g of rubber. Half of this set of samples were crosslinked using STPP and half were neutralized through NaOH exposure. The absorption capacity of the media is shown in Fig. 16, and indicates a capacity of up to 0.63 mg of Zn absorbed per gram of chitosan, after 96 h of exposure. For longer absorption times, however, the Zn concentration after exposure is much higher than

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Fig. 15. Compressive modulus calculated by averaging all pressure/strain data for each sample, over several compressive cycles, as a function of the amount of chitosan in the sample (a), the amount of chitosan solution used in the preparation of the sample (b) and the concentration of the chitosan solution used in preparing the sample (c). In (a) the results obtained using samples prepared using Vsol < 12 ml are differentiated from those measured for samples with Vsol > 12 ml.

prepared with amounts of chitosan that differed by a factor of 2, suggesting that all the chitosan in the sample was similarly involved in adsorption of zinc (as opposed to some chitosan being screened in the samples with higher polymer concentration). Measurement of zinc removal by chitosan/rubber samples crosslinked using STPP were inconclusive, possibly due to interference of the colorimetric analysis and complexes formed in the crosslinking process. 4. Conclusions

Fig. 16. Sorption capacity of dissolved zinc, shown as mg of zinc removed for each gram of chitosan in the chitosan/rubber sample used. Capacity is shown as a function of time spend soaking in a zinc solution of 3 mg/l concentration, and indicates that after 96 h the capacity of the sample has been reached and zinc leaching from the crumb rubber can be observed.

the initial concentration, suggesting that the chitosan has reached its sorptive capacity and zinc from the rubber continues to leach from the rubber. Similar capacity values were obtained for samples

The results of hydraulic conductivity, TSS removal, compressive strength and metals removal testing of chitosan and crumb rubber composites described here suggest that this type of composite may be a promising candidate material for permeable pavement. Because of the mechanism by which the chitosan solution binder cures, the dependence of hydraulic and mechanical performance of these materials does not follow a dependence on polymer content that is typical of binder-based permeable pavement. Instead, much more varied final material properties can be achieved and much more complex dependence of those properties on a small number of process parameters is observed. When large amounts of solution are used in the preparation of a sample (beyond the void space in a compact sample of rubber particles) the final sample becomes highly porous and brittle, and exhibits associated high

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hydraulic conductivity and mechanical fragility. When solution volume below this void space amount are used, expected dependence of material properties on the amount of chitosan is observed. In general, stronger materials capable of improved physical filtration can be achieved with a small amount of polymer introduced as high concentration solution than when large amounts of chitosan are introduced in the form of low concentration solution. Removal of dissolved zinc by these materials suggests that appropriate sizing of devices incorporating this combination of waste products may present a viable option for of pollution removal from stormwater. Considering the ease with which these composite materials may be prepared, the ubiquity of the source materials and the impressive properties of the resulting cured materials, the combination of chitosan and crumb rubber offers a potentially low-cost, effective means of treating surface water in a wide variety of scenarios. The following conclusions can be drawn from the results presented here:  Compressive strength, hydraulic conductivity and physical filtration efficiency quantitatively similar to that of accepted permeable pavers and stormwater treatment practices can be achieved in composite materials prepared from these two ubiquitous waste products.  Unlike more traditional binder-based permeable pavement materials, the use of a polymer binder in dilute solution provides a new degree of control in sample preparation that strongly impacts the resulting material behavior and enables a wide range of material properties without qualitative change to the preparation process.  The material properties are dominated by the porosity of the sample, which discontinuously increases when enough polymer solution is added to more than fill the void space between the compact rubber particles. Above and below this critical solution volume the hydraulic properties of the samples exhibit expected dependence on the amount of chitosan in the sample, though there is a large variation between results obtained from samples made with solution above and below this critical volume.  Mechanical properties are similarly a discontinuous function of the amount of chitosan in the sample, but only below this critical solution volume are they a clear function of the amount of chitosan. For larger amounts of solution, the fragility of the sample due to large pores dominates the mechanical response.  Removal of dissolved zinc from water was demonstrated, and results suggest a capacity similar to other metals removal media may be achieved in chitosan/crumb rubber composite materials. Beyond that capacity for absorption, the composite materials leach zinc into the water.  While there was no systematic dependence of the hydraulic or mechanical properties of the samples on crosslinking through STPP exposure, zinc removal values measured using these samples were inconclusive, suggesting possible interference with the measurement.

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References [1] Imran HM, Akib S, Karim MR. Permeable pavement and stormwater management systems: a review. Environ Technol 2010;34(18):2649–56. [2] Scholz M, Grabowiecki P. Review of permeable pavement systems. Build Environ 2007;42:3830–6. [3] Brattebo B, Booth DB. Long-term stormwater quantity and quality performance of permeable pavement systems. Water Res 2003;37:4369–76. [4] Pratt CJ, Mantle JDG, Schofield PA. Urban stormwater reduction and quality improvement through the use of permeable pavements. Water Sci Technol 1989;21(8–9):769–78. [5] Newman AP, Aitken D, Antizar-Ladislao B. Stormwater quality performance of a macro-pervious pavement car park installation equipped with channel drain based oil and silt retention devices. Water Res 2013;47:7327–36. [6] Maguesvari MU, Narasimha VL. Studies on characterization of pervious concrete for pavement applications. Proc Soc Behav Sci 2013;104:198–207. [7] Meiarashi S. Porous elastic road surface as urban highway noise measure. Transport Res Rec 2004;1880:151–7. [8] Cetin A. Effects of crumb rubber size and concentration on performance of porous asphalt mixtures. Int J Polym Sci 2013:1–10. [9] Sunthonpagasit N, Duffey MR. Scrap tires to crumb rubber: feasibility analysis for processing facilities. Resour Conserv Recycl 2004;40:281–99. [10] Paje SE, Luong J, Vázquez VF, Bueno M, Miró R. Road pavement rehabilitation using a binder with a high content of crumb rubber: influence on noise reduction. Constr Build Mater 2013;47:789–98. [11] Richardson AE, Coventry KA, Ward G. Freeze/thaw protection of concrete with optimum rubber crumb content. J Clean Prod 2012;23:96–103. [12] Menichini E et al. Artificial-turf playing fields: contents of metals, PAHs, PCBs, PCDDs and PCDFs, inhalation exposure to PAHs and related preliminary risk assessment. Sci Total Environ 2011;409:4950–7. [13] Llompart M, Sanchez-Prado L, Lamas JP, Garcia-Jares C, Roca E, Dagnac T. Hazardous organic chemicals in rubber recycled tire playgrounds and pavers. Chemosphere 2013;90:423–31. [14] Tang Z, Butkus MA, Xie YF. Crumb rubber filtration: a potential technology for ballast water treatment. Mar Environ Res 2006;61:410–23. [15] Valdes JR, Liang SH. Stress-controlled filtration with compressible particles. J Geotech Geoenviron 2006;132:861–8. [16] Xie Y, Killian BA, Gaul AS. Filter media: crumb rubber for wastewater filtration. Filtr Sep 2007;44:30–2Xie Y. Method of using waste tires as a filter media. US Patent No. US6969469; 2005. [17] Muzzarelli RAA. Selective collection of trace metal ions by precipitation of chitosan, and new derivatives of chitosan. Anal Chim Acta 1971;54(1):133–42. [18] Knaul JZ, Kasaai MR, Bui VT, Creber KAM. Characterization of deacetylated chitosan and chitosan molecular weight review. Can J Chem 1998;76:1699–706. [19] Broussignac P. Un polymere natural pecu cannu dans 1’ industrie e chitosane. Chim Ind Genie Chim 1970;99:1241–7. [20] Tan SC, Khor E, Tan TK, Wong SM. The degree of deacetylation of chitosan: advocating the first derivative UV-spectrophotometry method of determination. Talanta 1998;45:713–9. [21] Erlingsson S. Measurement techniques for water flow. In: Dawson A, editor. Water in road structures: movement, drainage and effects. New York: Springer; 2009. p. 45–67. [22] Ash M, Ash I. Handbook of fillers, extenders, and diluents. 2nd ed. Synapse Information Resources Inc.; 2007. [23] USEPA. Method 160.2: residue, non-filterable, gravimetric, dried at 103–105 °C (TSS). In: Methods for chemical analysis of water and wastes, EPA-600/4-79020; 1983. [24] ASTM standard D575-91. Standard test methods for rubber properties in compression. West Conshohocken: ASTM international; 2007. [25] American Public Health Association (APHA). Standard methods of water and wastewater, 18th ed. Washington (DC): American Public Health Association, American Water Works Association, Water Environment Federation publication; 1992. [26] Swamee PK, Swamee N. Discharge equation of a circular sharp-crested orifice. J Hydraul Res 2010;48:106–7. [27] Winer R. National pollutant removal performance database for stormwater treatment practices. Ellicott City (MD): Center for Watershed Protection; 2000.