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Environmental assessment of utilizing date palm ash as partial replacement of cement in mortar
Journal of Hazardous Materials 357 (2018) 175–179
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Environmental assessment of utilizing date palm ash as partial replacement of cement in mortar
T
Nawaf I. Blaisi Department of Environmental Engineering, College of Engineering, Imam Abdulrahman Bin Faisal University, PO Box 1982 Dammam, 31451, Saudi Arabia
A R T I C LE I N FO
A B S T R A C T
Keywords: Trace metals TCLP EPA method 1315 Monolithic test Leaching, date palm ash
Saudi Arabia’s date palm industry generates date palm ash (DPA) from the thermal processing of palm oil fibers and shells. This waste material has potential to be used as partial replacement of cement in structural mortar. However, no studies to date have examined its pollution potential. DPA was used as a cement replacement in Portland cement mortar (PCM) using a 10% and 100% replacement rate and then compared to an ordinary PCM control sample. Total elemental analysis, the toxicity characteristic leaching procedure (TCLP) and monolith leaching tests were conducted. Elemental analysis revealed a standard elemental profile similar to data for the comparably used wood ash. Aluminum (Al) and iron (Fe) were elements with the greatest abundance in DPA but no element exceeded regulatory thresholds. Leachability testing revealed that while concentrations of Al and Fe may appear high in DPA, they experience relatively low mobility when encapsulated in PCM matrices as indicated by their calculated leachability index. The results presented in this paper indicate that DPA poses no environmental risk to human health when used as cement replacement in PCM.
1. Introduction Beneficial use is the recycling of waste materials as ingredients for novel products or for use in manufacturing processes. Research has shown how waste material produced in large quantities such as bio solids, coal ash, or water treatment sludge is repurposed for profitable use in a variety of applications across the United States of America [1]. The environmentally friendly reuse of waste in this manner reduces landfilling and eliminates harmful manufacturing footprints. A variety of industrial waste materials are suitably explored in a number of applications around the world [2]. For example, Waste to Energy (WTE) bottom ash is beneficially used across the Eastern Hemisphere, perhaps most notably in the Netherlands and Denmark, where over 90% of WTE bottom ash is recycled [3]. Much of this ash is utilized in road construction applications as a coarse base material which provides structural support under roadways [4]. A potential target for such productive use is in the Saudi Arabian date palm industry which generates more than 200,000 tons of waste annually [5]. The waste is generated in palm oil mills and includes date palm (DP) ash, a by-product of burning extracted palm oil fibers and shells. Currently, date palm ash is not being commercially utilized in any reuse applications but it’s potential for reuse has received some recent attention [6]. Research on the chemical properties of DPA, as observed in beneficial use opportunities, is limited. Additionally, with the exception of work done by [6], no research studies have examined the factors
E-mail address: [email protected]. https://doi.org/10.1016/j.jhazmat.2018.06.013 Received 30 December 2017; Received in revised form 3 June 2018; Accepted 4 June 2018 Available online 05 June 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
affecting DPA’s performance in reuse scenarios [7]. This lack of information coupled with uncertainty in the DPA market, due to fluctuating volumes of DPA, results in a lower than nominal amount of recycling for this waste material [6]. A key to alleviating the scientific uncertainty in DPA’s reuse is dependent upon its physical properties and how well it performs in environmental tests. As citizens across the globe grow increasingly more aware of the human impact on the environment governments have responded with initiatives, as is the case in Saudi Arabia, to investigate the effective utilization of solid waste materials in an effort to decrease landfilling [8]. A potential application of DPA that has been examined in the past is its utilization as partial cement replacement, in different construction applications [9,10]. For example, Khellou et al. [11] investigated the mechanical properties of construction pavement containing DPA (4–12%) mixed gypsum-calcareous material. The author concluded from the findings that 8% DPA replacement substantially increase the compressive strength and bearing index. Additionally, W. Al-Kutti et al [7] evaluated the compressive strength development in concrete and mortar using (10 to 30) % DPA Type I cement replacement at different curing period. The author reported that 10% DPA in concrete and mortar showed significant improvement in compressive strength development in 28 to 360 days as compared to pure PCM and reached to maximum value of 85.5 MPa in 360-days. Recently, Gunarani and Chakkravarthy [12] investigated the effect of using date palm seed ash (DPSA) (4–12) % as partial replacement of conventional cement on strength, water absorption,
Journal of Hazardous Materials 357 (2018) 175–179
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cost effective use of DPA in Portland cement.
alkalinity and soprtivity. They found that using 4% DPSA replacement to PCC showed higher compressive strength while 2% and 8% replacement level can be effectively applied for construction requiring higher acid resistance and bond strength, respectively. Similar behavior was observed in our previous study [6] at 10% DPA as cement replacement in mortar. Palm tree signifies an important national status. Production of palm waste and ash deserves its usage as a raw supplement material in building industry. An advocate for usage of the material demands such kind of environmental assessment study to reassure its health safety issues. Therefore, in order to justify the use of DPA in PCM, multiple environmental impact assessments have to be conducted. The environmental risk a beneficially used waste material poses is assessed by examining two exposure pathways: direct human exposure and leaching to water supplies [13]. Direct human exposure is commonly evaluated by comparing total element concentrations (mg/kg) to risk thresholds based on toxicity data and exposure assumptions [14]. Leaching risk is assessed using standardized tests that are commonly used for both hazardous waste determinations and beneficial use determinations [13]. Hazardous waste determination is assessed in the United States using the Environmental Protection Agency’s (EPA) Toxicity Characteristic Leaching Procedure (TCLP), a method designed to simulate leaching conditions resulting from disposal with municipal solid waste [15]. Leaching test results are then compared to toxicity characteristic (TC) limits to evaluate whether a material should be designated as hazardous waste. Batch and monolithic leaching tests assess the pollution potential of leachate originating from size-reduced waste materials used as aggregate replacements in semi-impermeable monoliths [1]. These tests measure the mobilization of trace elements as a function of time and allow for the calculation of observed elemental diffusivity [16]. When waste materials are incorporated into media, such as PCC, or when treated with solidification and stabilization (S/S) for disposal purposes, the mobility of pollutants out of contaminated media is governed by diffusion [18]. Further background information on the protocols used during these leaching tests was outlined by [17] in a review of the methods as presented in the EPA’s Leaching Environmental Assessment Framework (LEAF). Current research on DPA used as a cement replacement in profitable application, has evaluated its civil engineering properties. However little other work has examined its pollution potential in either un-encapsulated or encapsulated forms [18]. While some research has evaluated pollutant leaching from un-encapsulated DPA no work has attempted to evaluate the concentration of trace elements originating from beneficially used DPA in its encapsulated form. This information is necessary to assess the environmental risk recycled DPA poses and to serve as a basis by which applicable waste management practices can be recommended in any future uses of the waste as a cement replacement in PCM. The work presented in this paper evaluates the environmental risk associated with pollutants leaching from DPA in its un-encapsulated and encapsulated forms. Laboratory-scale leaching experiments, using standardized EPA methodologies, quantified the pollution potential of DPA used as replacement material in PCM. Results obtained from leaching tests were compared to Florida’s Soil Cleanup Target Levels (SCTL) to evaluate whether ash amended samples should be characterized as posing a hazardous risk to humans. Batch test results from the DPA evaluated relative leaching risk of ash amended samples and were used, subsequently, to determine proper disposal methods of the waste if it was not suited for reuse. Monolithic leaching tests, conducted under diffusion control scenarios, evaluated the mobility of trace elements in encapsulated, DPA amended monoliths which simulate mortar blocks and sidewalks. Leachability index (LI) and observed elemental diffusivity were then calculated for the monoliths to quantify trace element mobility. These tests provide insight into the environmental impact of DPA reuse and can be used by stakeholders to make informed decisions regarding the productive and
2. Methods 2.1. Facility description and sample collection DPA samples were collected from a date palm recycling facility located in Saudi Arabia’s Eastern Province. The facility utilizes desiccated palm fronds in the production of coal and firewood with a maximum capacity of 2 tons per day. The facility first grinds and then thermally processes the desiccated material which produces DPA. Sampling trips consisted of collecting 40 kg sub-samples in sealed containers. Samples were then homogenized, and stored in sealed polypropylene plastic containers until analyzed. In order to preserve sample consistency particle size requirements, as outlined in EPA method 1311, were achieved by passing DPA through a 425 μm US sieve. Particle distribution of the DPA and mix design is referenced in [6]. Mortar specimens for Method 1315 test were produced using a mix design with three replacements (control, 10%, and 100% replacement) and aged for 28 days and mix design composition is listed in Section S1. The structural data such as workability, density, compressive strength, approx. setting time and chemical composition of OPC and DPA is demonstrated in section S2 (A–E). 2.2. Total concentration Total elemental analysis was conducted using the total environmentally available procedure outlined in EPA method 3050b at a constant temperature of 95 ± 5 °C [19]. Prior to conducting the procedure, DPA was ground to a powder using a ceramic ball mill. Throughout the course of this study, samples were analyzed in triplicate. Quality control protocols were used for all laboratory analysis in this research. 2.3. Leaching tests All leachate generated from leaching experiments were digested in accordance to EPA Method 3010 A. Batch leaching test Toxicity Characteristic Leaching Procedure (TCLP) was conducted on DPA and control samples as outlined by EPA Method 1311 [20]. TCLP extraction fluid was determined using the final pH of DPA samples after preliminary testing was completed (EPA method 1311). Extraction Fluid #2 was used because pH was > 5.0. TCLP test Fluid #2 is an acetic acid, sodium hydroxide solution with a pH of 4.93 ± 0.05 [20]. The TCLP test utilizes a liquid to solid (L/S) ratio of 20:1 mL reagent water/ g-dry sample and an extraction period of 18 ± 2 h at 28 rpm. Following extraction, samples were filtered using borosilicate glass fiber filter. Some samples were centrifuged at 4000 ± 100 rpm for 10 min when high suspended solid content was observed. Triplicate monolith testing was conducted on PAA (100% DPA replacement), PAAO (10% DPA replacement) and PCM (100% Portland cement) samples as outlined by EPA method 1315. Cylindrical samples were submerged in reagent water and measured 10.1 cm in diameter and ranged in height from 10.3 to 10.6 cm. Reagent water was renewed after 0.08, 1.0, 2.0, 7.0, 14, 28 42, 49, and 63 days [21] using a liquid to exposed surface area ratio of 9 ± 1 mL reagent water/cm2 of sample area. Equations for calculating cumulative mass release from monolith testing are included by [14]. Leachability Index is calculated by taking the –log10 (Diobs) and has units of cm2/s. High mobility waste has an LI < 6.5 while moderate and limited mobility is understood as 6.5 < LI < 8.0 and LI > 8.0, respectively [22]. 2.4. Trace elemental analysis Total elemental analysis for arsenic, cadmium, iron, lead, selenium, silver, and vanadium was evaluated using Inductively Coupled Plasma176
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Mass Spectrometry (ICP-MS) (Agilent Technologies 7700X) as outlined by EPA Method 6020 A [23]. Total elemental analysis for aluminum, barium, copper and chromium was evaluated using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) (Horiba Scientific, ACTIVA S) as outlined in EPA Method 6010D [24]. Leachate samples were analyzed for aluminum, arsenic, barium, beryllium, cadmium, chromium, copper, iron, lead, selenium, silver, and zinc using ICP-OES.
Table 2 TCLP trace elemental concentrations on DPA (values in mg/L).
2.5. Risk assessment approach to assessing reuse Risk thresholds, such as Florida’s SCTL, were developed to protect human health from direct contact with harmful pollutants and are often used to determine the suitability of drinking water (Solo-Gabriele et al. [25]). Their use in the context of evaluating both total and leachable concentrations of beneficially used waste is well documented (Townsend et al. [13]). Florida’s SCTL are classified as being either industrial or residential; both risk threshold categories were used in this study [26]. An upper confidence limit (UCL) is used to account for uncertainties in data when comparing experimental results to published risk thresholds.
Material characterization of DPA produced a solids content of 99.8%; a particle size distribution of the “as used” sample can be found in [7]. Total elemental concentration of the DPA is shown in Table 1. These values were compared to concentrations of another thermal treatment residual found in the literature: wood ash [13]. Trace elemental concentrations for DPA were obtained and the resultant data are shown in Table 1 alongside the elemental composition data of wood ash, a comparable and conventionally used material. Aluminum and iron were found in DPA in the highest abundance. The concentration of these trace elements in DPA is found to be 4300 and 9530 mg/kg, respectively. Other trace elements content in DPA including arsenic, barium, copper, lead, silver and zinc exits in low level i.e. < 80 mg/kg. Their order of concentration level was appeared as Cd > Ba > Zn > Cu > Cr > V > Ni > Pb > As > Ag (Table 1). DPA’s elemental composition is dependent on the occurrence of those elements existing naturally in source material such as date palm shells, its combustion temperature, and whether or not fertilizers used in the growing process included heavy metals such as iron [27,28]. Moreover, the varied range of concentration of arsenic, zinc, and lead in DPA as
Florida Residential SCTLs
Florida Industrial SCTLs
Ag Alc
< 0.1 4300
410 80,000
8,200 –
As Ba Cd Cr (total) Cu Fec
2.0 32.0 76.0 20.5 22.5 9530
2.1 120 82 210 150 53000
12 130,000 1,700 470 89,000 –
Ni Pb V Zn
16.0 14.0 17.5 25.0
0.2-0.4 40004500 42-53 220-300 5.5-6.1 12-14 41-46 59006100 6-8 29-35
340 400 67 26,000
35,000 1,400 10,000 630,000
a b c
380-420
Ag Al As Ba Cd Cr Cu Ni Pb V
< 0.1 350 (35.0) 0.7 (0.050) 17.0 (1.5) < 0.1 3.50 (0.95) 3.0 (0.85) 3.80 (1.10) 0.5 (0.0085) 15.0 (1.6)
5 – 5 100 1 5 – – 5 –
Regulatory TC limits are not provided for Al,Cu,Ni, and V.
3.2. Leaching test Leachable concentrations (mg/L) were determined using the TCLP (Fluid #2) test (mean initial pH of DPA was 9.05). TCLP testing data for DPA was found below the U.S. toxicity characteristic (TC) criteria intended for hazardous waste materials. Fluid #2 was used for the TCLP test because initial mean pH of DPA was 9.05. DPA did not meet the criteria for classification as hazardous as defined by the US EPA’s Resource Conservation and Recovery Act (RCRA). For example, average TCLP lead concentrations (0.5 mg/L) in DPA samples were lower than RCRA and CCR hazardous waste regulatory limits (5.0 mg/L). TCLP is designed to mimic co-disposal with MSW and does not provide adequate information for a regulatory agency to use in a risk assessment approach. Future research that attempts to further assess the beneficial use of DPA should do so by replicating scenario specific conditions and conduct additional monolithic testing to mimic reuse applications that utilize DPA in its encapsulated form. EPA method 1315 was carried out to investigate Al release from the mortar to: (a) differentiate the quantity release between DPA amended mortar and PCM (b) compare the cumulative mass release [MCumu(mg−COPC/kg-dry-pavement)] between DPA amended mortar and PCM, (c) determine the leachability index (LI) to measure the release rates of diffusion controlled elements. This semi-dynamic leaching procedure was conducted under diffusion-controlled release conditions on ash-amended PAAO, ordinary PCM and PAA samples (Table 3). During testing ordinary PCM (control) had a mean initial pH of 10.0 and a final pH of 8.50. A similar trend was observed for the PAAO and PAA samples, with initial pH values of 10.5 and 10.4 and final pH values of 8.51 and 8.65 respectively (Table 3). The analogous pH response of DPA mortar can be associated to the components within the PCM drove the elevated pH seen for the DPAamended and PCM (control) and not due to the incorporation of the DPA. Similar results was also reported in previous study using bottom ash amended mortar [16]. Of all elements evaluated only the Aluminum concentrations (∼0.2 mg/L) in PAA samples were elevated and compared to GCTL across all time periods and samples. The elevated Al concentrations occurred despite the similarities in pH for each sample.
Table 1 Trace elemental composition of DPA (values in mg/kg-dry). Wood Ashb
TC (mg/L)a
compared to wood ash (Table 1) may also associated to the elemental volatilization during thermal processing supported by hypothesis made by Moustakas et al. [29]. Therefore, speciation during thermal processing of DPA may explain why certain elements are more volatiles than others, however analyzing the mechanisms that induce speciation was not the intent of this study [30]. Lastly, the concentration of available elements found in DPA were compared to Florida’s industrial and residential SCTLs. No element, except for arsenic, exceeded Florida’s SCTL. Environmental availability of Arsenic (2.2 mg/kg) was marginally higher than residential SCTL (2.1 mg/kg). The data in Table 1 demonstrates that using DPA in commercial or industrial reuse applications poses little to no concern with regard to elevated human exposure risk (Table 2).
3.1. Material characterization and total elemental composition
DPA 95% UCL
Concentration (Std deviation)
a
3. Results and discussion
DPAa
DPA
Concentrations are given for all analyzed elements in this study. Data obtained from [24]. No SCTLs limits are published for Al and Fe. 177
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Table 3 EPA method 1315 elemental concentrations on DPA. Cumulative leaching time
PAAO
PAA
PCM
Days
pH
Al concentration (mg/L)
pH
Al concentration (mg/L)
pH
Al concentration (mg/L)
0.08 1.04 2 7 14 28 42 49 63
10.50 10.9 10.5 10.4 10.1 9.85 8.86 8.75 8.50
0.38 0.57 0.58 0.69 0.72 0.75 0.72 0.67 0.65
10.4 11.1 10.3 10.1 9.97 9.67 8.77 8.70 8.60
0.41 0.81 0.66 1.13 1.20 1.25 1.21 1.18 1.16
10.0 10.3 10.2 10.15 9.85 9.62 9.54 9.52 9.50
0.31 0.33 0.33 0.35 0.34 0.30 0.28 0.28 0.26
these results in real world circumstances. Future studies may seek to emulate local conditions and take into consideration hydrogeological conditions such as aquifer depth and thickness paired alongside an analysis of the thermal processing used to generate the waste material. Analyzing how the DPA is generated can contribute to scientific work examining the volatilization of elements and thus provide an opportunity to recommend best practices to waste generators. A holistic examination of DPA must be conducted prior to recommending its use in real world developments.
Possible explanations for the greater leachability potential of Al in PAA samples include differing starting concentrations of Al or a higher occurrence of the speciated form of Al. 3.3. Mass release in monolith testing Cumulative mass release of Al for each sample tested with EPA method 1315 are presented in Table 3. Mean MCumu of Al from the PAAO, PCM, and PAA samples was 15.1, 7.30, and 25.3 mg/kg respectively. It was observed that with 10% and 100% DPA replacement as cement (PAAO and PAA), the cumulative Al release is increased from 7.30 mg/kg to (15.1 and 25.3) mg/kg, respectively. The results clearly confirm that the incorporation of DPA as cement replacement showed an increase in cumulative mass release of Al. Similar, result was reported on leaching of bottom ash concrete and determined that Al (6.50 mg/kg) leaching was higher in bottom ash amended samples when compared to control [14].
Acknowledgements The authors would like to thank the College of Engineering at Imam Abdulrahman Bin Faisal University. The author would like to thank Mr. Muhammad Nasir Mr. Iehab Mohamed and Mr. Mukarram Zabair for their exceptional assistance with the experimental aspects of the study. Appendix A. Supplementary data
3.4. Leachability index (LI) Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2018.06.013.
Mean LI values of Al for each of three samples are presented in the SI section. LI values on the low end were 10.3 and steadily increased to 12.4, an indication of Al having low mobility (LI > 8) in its encapsulated form. The LI of Al in ash-amended samples exceeded the LI of Al in PCM supporting the hypothesis that the addition of DPA in Portland cement concrete results in greater Al mobility out of monoliths when compared to controls. However, it should be noted that while there was an increased mobility of Al out of ash amended samples, the calculated concentrations of Al that would theoretically leach into the surrounding environment did not exceed GCTL as similarly observed by [14] who used bottom ash amended concrete samples.
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4. Conclusion Saudi Arabia’s date palm industry generates date palm ash from the thermal processing of palm oil fibers and shells. This waste material has great potential in beneficial use as a cement replacement up to 10%. This study examined the pollution potential of DPA in its encapsulated form as part of a PCM matrix using standardized EPA test methods. The elemental composition of DPA revealed a standard elemental profile compared to similarly used wood ash. Al and Fe were elements with the greatest abundance in DPA but no element exceeded Florida’s industrial or residential SCTL. Concentrations of arsenic in DPA approached but did not exceed residential SCTL. Leaching tests were then completed to better understand the mobility of heavy elements out of DPA amended PCM samples. Leachability testing revealed that while concentrations of elements may appear high in DPA, they experience relatively low mobility when encapsulated in PCM matrices. The preliminary results presented in this paper indicate that DPA poses no environmental risk to human health when used as cement replacement in PCM. However, further analysis is still required to determine the overall applicability of 178
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179
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Environmental assessment of utilizing date palm ash as partial replacement of cement in mortar
T
Nawaf I. Blaisi Department of Environmental Engineering, College of Engineering, Imam Abdulrahman Bin Faisal University, PO Box 1982 Dammam, 31451, Saudi Arabia
A R T I C LE I N FO
A B S T R A C T
Keywords: Trace metals TCLP EPA method 1315 Monolithic test Leaching, date palm ash
Saudi Arabia’s date palm industry generates date palm ash (DPA) from the thermal processing of palm oil fibers and shells. This waste material has potential to be used as partial replacement of cement in structural mortar. However, no studies to date have examined its pollution potential. DPA was used as a cement replacement in Portland cement mortar (PCM) using a 10% and 100% replacement rate and then compared to an ordinary PCM control sample. Total elemental analysis, the toxicity characteristic leaching procedure (TCLP) and monolith leaching tests were conducted. Elemental analysis revealed a standard elemental profile similar to data for the comparably used wood ash. Aluminum (Al) and iron (Fe) were elements with the greatest abundance in DPA but no element exceeded regulatory thresholds. Leachability testing revealed that while concentrations of Al and Fe may appear high in DPA, they experience relatively low mobility when encapsulated in PCM matrices as indicated by their calculated leachability index. The results presented in this paper indicate that DPA poses no environmental risk to human health when used as cement replacement in PCM.
1. Introduction Beneficial use is the recycling of waste materials as ingredients for novel products or for use in manufacturing processes. Research has shown how waste material produced in large quantities such as bio solids, coal ash, or water treatment sludge is repurposed for profitable use in a variety of applications across the United States of America [1]. The environmentally friendly reuse of waste in this manner reduces landfilling and eliminates harmful manufacturing footprints. A variety of industrial waste materials are suitably explored in a number of applications around the world [2]. For example, Waste to Energy (WTE) bottom ash is beneficially used across the Eastern Hemisphere, perhaps most notably in the Netherlands and Denmark, where over 90% of WTE bottom ash is recycled [3]. Much of this ash is utilized in road construction applications as a coarse base material which provides structural support under roadways [4]. A potential target for such productive use is in the Saudi Arabian date palm industry which generates more than 200,000 tons of waste annually [5]. The waste is generated in palm oil mills and includes date palm (DP) ash, a by-product of burning extracted palm oil fibers and shells. Currently, date palm ash is not being commercially utilized in any reuse applications but it’s potential for reuse has received some recent attention [6]. Research on the chemical properties of DPA, as observed in beneficial use opportunities, is limited. Additionally, with the exception of work done by [6], no research studies have examined the factors
E-mail address: [email protected]. https://doi.org/10.1016/j.jhazmat.2018.06.013 Received 30 December 2017; Received in revised form 3 June 2018; Accepted 4 June 2018 Available online 05 June 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
affecting DPA’s performance in reuse scenarios [7]. This lack of information coupled with uncertainty in the DPA market, due to fluctuating volumes of DPA, results in a lower than nominal amount of recycling for this waste material [6]. A key to alleviating the scientific uncertainty in DPA’s reuse is dependent upon its physical properties and how well it performs in environmental tests. As citizens across the globe grow increasingly more aware of the human impact on the environment governments have responded with initiatives, as is the case in Saudi Arabia, to investigate the effective utilization of solid waste materials in an effort to decrease landfilling [8]. A potential application of DPA that has been examined in the past is its utilization as partial cement replacement, in different construction applications [9,10]. For example, Khellou et al. [11] investigated the mechanical properties of construction pavement containing DPA (4–12%) mixed gypsum-calcareous material. The author concluded from the findings that 8% DPA replacement substantially increase the compressive strength and bearing index. Additionally, W. Al-Kutti et al [7] evaluated the compressive strength development in concrete and mortar using (10 to 30) % DPA Type I cement replacement at different curing period. The author reported that 10% DPA in concrete and mortar showed significant improvement in compressive strength development in 28 to 360 days as compared to pure PCM and reached to maximum value of 85.5 MPa in 360-days. Recently, Gunarani and Chakkravarthy [12] investigated the effect of using date palm seed ash (DPSA) (4–12) % as partial replacement of conventional cement on strength, water absorption,
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cost effective use of DPA in Portland cement.
alkalinity and soprtivity. They found that using 4% DPSA replacement to PCC showed higher compressive strength while 2% and 8% replacement level can be effectively applied for construction requiring higher acid resistance and bond strength, respectively. Similar behavior was observed in our previous study [6] at 10% DPA as cement replacement in mortar. Palm tree signifies an important national status. Production of palm waste and ash deserves its usage as a raw supplement material in building industry. An advocate for usage of the material demands such kind of environmental assessment study to reassure its health safety issues. Therefore, in order to justify the use of DPA in PCM, multiple environmental impact assessments have to be conducted. The environmental risk a beneficially used waste material poses is assessed by examining two exposure pathways: direct human exposure and leaching to water supplies [13]. Direct human exposure is commonly evaluated by comparing total element concentrations (mg/kg) to risk thresholds based on toxicity data and exposure assumptions [14]. Leaching risk is assessed using standardized tests that are commonly used for both hazardous waste determinations and beneficial use determinations [13]. Hazardous waste determination is assessed in the United States using the Environmental Protection Agency’s (EPA) Toxicity Characteristic Leaching Procedure (TCLP), a method designed to simulate leaching conditions resulting from disposal with municipal solid waste [15]. Leaching test results are then compared to toxicity characteristic (TC) limits to evaluate whether a material should be designated as hazardous waste. Batch and monolithic leaching tests assess the pollution potential of leachate originating from size-reduced waste materials used as aggregate replacements in semi-impermeable monoliths [1]. These tests measure the mobilization of trace elements as a function of time and allow for the calculation of observed elemental diffusivity [16]. When waste materials are incorporated into media, such as PCC, or when treated with solidification and stabilization (S/S) for disposal purposes, the mobility of pollutants out of contaminated media is governed by diffusion [18]. Further background information on the protocols used during these leaching tests was outlined by [17] in a review of the methods as presented in the EPA’s Leaching Environmental Assessment Framework (LEAF). Current research on DPA used as a cement replacement in profitable application, has evaluated its civil engineering properties. However little other work has examined its pollution potential in either un-encapsulated or encapsulated forms [18]. While some research has evaluated pollutant leaching from un-encapsulated DPA no work has attempted to evaluate the concentration of trace elements originating from beneficially used DPA in its encapsulated form. This information is necessary to assess the environmental risk recycled DPA poses and to serve as a basis by which applicable waste management practices can be recommended in any future uses of the waste as a cement replacement in PCM. The work presented in this paper evaluates the environmental risk associated with pollutants leaching from DPA in its un-encapsulated and encapsulated forms. Laboratory-scale leaching experiments, using standardized EPA methodologies, quantified the pollution potential of DPA used as replacement material in PCM. Results obtained from leaching tests were compared to Florida’s Soil Cleanup Target Levels (SCTL) to evaluate whether ash amended samples should be characterized as posing a hazardous risk to humans. Batch test results from the DPA evaluated relative leaching risk of ash amended samples and were used, subsequently, to determine proper disposal methods of the waste if it was not suited for reuse. Monolithic leaching tests, conducted under diffusion control scenarios, evaluated the mobility of trace elements in encapsulated, DPA amended monoliths which simulate mortar blocks and sidewalks. Leachability index (LI) and observed elemental diffusivity were then calculated for the monoliths to quantify trace element mobility. These tests provide insight into the environmental impact of DPA reuse and can be used by stakeholders to make informed decisions regarding the productive and
2. Methods 2.1. Facility description and sample collection DPA samples were collected from a date palm recycling facility located in Saudi Arabia’s Eastern Province. The facility utilizes desiccated palm fronds in the production of coal and firewood with a maximum capacity of 2 tons per day. The facility first grinds and then thermally processes the desiccated material which produces DPA. Sampling trips consisted of collecting 40 kg sub-samples in sealed containers. Samples were then homogenized, and stored in sealed polypropylene plastic containers until analyzed. In order to preserve sample consistency particle size requirements, as outlined in EPA method 1311, were achieved by passing DPA through a 425 μm US sieve. Particle distribution of the DPA and mix design is referenced in [6]. Mortar specimens for Method 1315 test were produced using a mix design with three replacements (control, 10%, and 100% replacement) and aged for 28 days and mix design composition is listed in Section S1. The structural data such as workability, density, compressive strength, approx. setting time and chemical composition of OPC and DPA is demonstrated in section S2 (A–E). 2.2. Total concentration Total elemental analysis was conducted using the total environmentally available procedure outlined in EPA method 3050b at a constant temperature of 95 ± 5 °C [19]. Prior to conducting the procedure, DPA was ground to a powder using a ceramic ball mill. Throughout the course of this study, samples were analyzed in triplicate. Quality control protocols were used for all laboratory analysis in this research. 2.3. Leaching tests All leachate generated from leaching experiments were digested in accordance to EPA Method 3010 A. Batch leaching test Toxicity Characteristic Leaching Procedure (TCLP) was conducted on DPA and control samples as outlined by EPA Method 1311 [20]. TCLP extraction fluid was determined using the final pH of DPA samples after preliminary testing was completed (EPA method 1311). Extraction Fluid #2 was used because pH was > 5.0. TCLP test Fluid #2 is an acetic acid, sodium hydroxide solution with a pH of 4.93 ± 0.05 [20]. The TCLP test utilizes a liquid to solid (L/S) ratio of 20:1 mL reagent water/ g-dry sample and an extraction period of 18 ± 2 h at 28 rpm. Following extraction, samples were filtered using borosilicate glass fiber filter. Some samples were centrifuged at 4000 ± 100 rpm for 10 min when high suspended solid content was observed. Triplicate monolith testing was conducted on PAA (100% DPA replacement), PAAO (10% DPA replacement) and PCM (100% Portland cement) samples as outlined by EPA method 1315. Cylindrical samples were submerged in reagent water and measured 10.1 cm in diameter and ranged in height from 10.3 to 10.6 cm. Reagent water was renewed after 0.08, 1.0, 2.0, 7.0, 14, 28 42, 49, and 63 days [21] using a liquid to exposed surface area ratio of 9 ± 1 mL reagent water/cm2 of sample area. Equations for calculating cumulative mass release from monolith testing are included by [14]. Leachability Index is calculated by taking the –log10 (Diobs) and has units of cm2/s. High mobility waste has an LI < 6.5 while moderate and limited mobility is understood as 6.5 < LI < 8.0 and LI > 8.0, respectively [22]. 2.4. Trace elemental analysis Total elemental analysis for arsenic, cadmium, iron, lead, selenium, silver, and vanadium was evaluated using Inductively Coupled Plasma176
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Mass Spectrometry (ICP-MS) (Agilent Technologies 7700X) as outlined by EPA Method 6020 A [23]. Total elemental analysis for aluminum, barium, copper and chromium was evaluated using Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) (Horiba Scientific, ACTIVA S) as outlined in EPA Method 6010D [24]. Leachate samples were analyzed for aluminum, arsenic, barium, beryllium, cadmium, chromium, copper, iron, lead, selenium, silver, and zinc using ICP-OES.
Table 2 TCLP trace elemental concentrations on DPA (values in mg/L).
2.5. Risk assessment approach to assessing reuse Risk thresholds, such as Florida’s SCTL, were developed to protect human health from direct contact with harmful pollutants and are often used to determine the suitability of drinking water (Solo-Gabriele et al. [25]). Their use in the context of evaluating both total and leachable concentrations of beneficially used waste is well documented (Townsend et al. [13]). Florida’s SCTL are classified as being either industrial or residential; both risk threshold categories were used in this study [26]. An upper confidence limit (UCL) is used to account for uncertainties in data when comparing experimental results to published risk thresholds.
Material characterization of DPA produced a solids content of 99.8%; a particle size distribution of the “as used” sample can be found in [7]. Total elemental concentration of the DPA is shown in Table 1. These values were compared to concentrations of another thermal treatment residual found in the literature: wood ash [13]. Trace elemental concentrations for DPA were obtained and the resultant data are shown in Table 1 alongside the elemental composition data of wood ash, a comparable and conventionally used material. Aluminum and iron were found in DPA in the highest abundance. The concentration of these trace elements in DPA is found to be 4300 and 9530 mg/kg, respectively. Other trace elements content in DPA including arsenic, barium, copper, lead, silver and zinc exits in low level i.e. < 80 mg/kg. Their order of concentration level was appeared as Cd > Ba > Zn > Cu > Cr > V > Ni > Pb > As > Ag (Table 1). DPA’s elemental composition is dependent on the occurrence of those elements existing naturally in source material such as date palm shells, its combustion temperature, and whether or not fertilizers used in the growing process included heavy metals such as iron [27,28]. Moreover, the varied range of concentration of arsenic, zinc, and lead in DPA as
Florida Residential SCTLs
Florida Industrial SCTLs
Ag Alc
< 0.1 4300
410 80,000
8,200 –
As Ba Cd Cr (total) Cu Fec
2.0 32.0 76.0 20.5 22.5 9530
2.1 120 82 210 150 53000
12 130,000 1,700 470 89,000 –
Ni Pb V Zn
16.0 14.0 17.5 25.0
0.2-0.4 40004500 42-53 220-300 5.5-6.1 12-14 41-46 59006100 6-8 29-35
340 400 67 26,000
35,000 1,400 10,000 630,000
a b c
380-420
Ag Al As Ba Cd Cr Cu Ni Pb V
< 0.1 350 (35.0) 0.7 (0.050) 17.0 (1.5) < 0.1 3.50 (0.95) 3.0 (0.85) 3.80 (1.10) 0.5 (0.0085) 15.0 (1.6)
5 – 5 100 1 5 – – 5 –
Regulatory TC limits are not provided for Al,Cu,Ni, and V.
3.2. Leaching test Leachable concentrations (mg/L) were determined using the TCLP (Fluid #2) test (mean initial pH of DPA was 9.05). TCLP testing data for DPA was found below the U.S. toxicity characteristic (TC) criteria intended for hazardous waste materials. Fluid #2 was used for the TCLP test because initial mean pH of DPA was 9.05. DPA did not meet the criteria for classification as hazardous as defined by the US EPA’s Resource Conservation and Recovery Act (RCRA). For example, average TCLP lead concentrations (0.5 mg/L) in DPA samples were lower than RCRA and CCR hazardous waste regulatory limits (5.0 mg/L). TCLP is designed to mimic co-disposal with MSW and does not provide adequate information for a regulatory agency to use in a risk assessment approach. Future research that attempts to further assess the beneficial use of DPA should do so by replicating scenario specific conditions and conduct additional monolithic testing to mimic reuse applications that utilize DPA in its encapsulated form. EPA method 1315 was carried out to investigate Al release from the mortar to: (a) differentiate the quantity release between DPA amended mortar and PCM (b) compare the cumulative mass release [MCumu(mg−COPC/kg-dry-pavement)] between DPA amended mortar and PCM, (c) determine the leachability index (LI) to measure the release rates of diffusion controlled elements. This semi-dynamic leaching procedure was conducted under diffusion-controlled release conditions on ash-amended PAAO, ordinary PCM and PAA samples (Table 3). During testing ordinary PCM (control) had a mean initial pH of 10.0 and a final pH of 8.50. A similar trend was observed for the PAAO and PAA samples, with initial pH values of 10.5 and 10.4 and final pH values of 8.51 and 8.65 respectively (Table 3). The analogous pH response of DPA mortar can be associated to the components within the PCM drove the elevated pH seen for the DPAamended and PCM (control) and not due to the incorporation of the DPA. Similar results was also reported in previous study using bottom ash amended mortar [16]. Of all elements evaluated only the Aluminum concentrations (∼0.2 mg/L) in PAA samples were elevated and compared to GCTL across all time periods and samples. The elevated Al concentrations occurred despite the similarities in pH for each sample.
Table 1 Trace elemental composition of DPA (values in mg/kg-dry). Wood Ashb
TC (mg/L)a
compared to wood ash (Table 1) may also associated to the elemental volatilization during thermal processing supported by hypothesis made by Moustakas et al. [29]. Therefore, speciation during thermal processing of DPA may explain why certain elements are more volatiles than others, however analyzing the mechanisms that induce speciation was not the intent of this study [30]. Lastly, the concentration of available elements found in DPA were compared to Florida’s industrial and residential SCTLs. No element, except for arsenic, exceeded Florida’s SCTL. Environmental availability of Arsenic (2.2 mg/kg) was marginally higher than residential SCTL (2.1 mg/kg). The data in Table 1 demonstrates that using DPA in commercial or industrial reuse applications poses little to no concern with regard to elevated human exposure risk (Table 2).
3.1. Material characterization and total elemental composition
DPA 95% UCL
Concentration (Std deviation)
a
3. Results and discussion
DPAa
DPA
Concentrations are given for all analyzed elements in this study. Data obtained from [24]. No SCTLs limits are published for Al and Fe. 177
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Table 3 EPA method 1315 elemental concentrations on DPA. Cumulative leaching time
PAAO
PAA
PCM
Days
pH
Al concentration (mg/L)
pH
Al concentration (mg/L)
pH
Al concentration (mg/L)
0.08 1.04 2 7 14 28 42 49 63
10.50 10.9 10.5 10.4 10.1 9.85 8.86 8.75 8.50
0.38 0.57 0.58 0.69 0.72 0.75 0.72 0.67 0.65
10.4 11.1 10.3 10.1 9.97 9.67 8.77 8.70 8.60
0.41 0.81 0.66 1.13 1.20 1.25 1.21 1.18 1.16
10.0 10.3 10.2 10.15 9.85 9.62 9.54 9.52 9.50
0.31 0.33 0.33 0.35 0.34 0.30 0.28 0.28 0.26
these results in real world circumstances. Future studies may seek to emulate local conditions and take into consideration hydrogeological conditions such as aquifer depth and thickness paired alongside an analysis of the thermal processing used to generate the waste material. Analyzing how the DPA is generated can contribute to scientific work examining the volatilization of elements and thus provide an opportunity to recommend best practices to waste generators. A holistic examination of DPA must be conducted prior to recommending its use in real world developments.
Possible explanations for the greater leachability potential of Al in PAA samples include differing starting concentrations of Al or a higher occurrence of the speciated form of Al. 3.3. Mass release in monolith testing Cumulative mass release of Al for each sample tested with EPA method 1315 are presented in Table 3. Mean MCumu of Al from the PAAO, PCM, and PAA samples was 15.1, 7.30, and 25.3 mg/kg respectively. It was observed that with 10% and 100% DPA replacement as cement (PAAO and PAA), the cumulative Al release is increased from 7.30 mg/kg to (15.1 and 25.3) mg/kg, respectively. The results clearly confirm that the incorporation of DPA as cement replacement showed an increase in cumulative mass release of Al. Similar, result was reported on leaching of bottom ash concrete and determined that Al (6.50 mg/kg) leaching was higher in bottom ash amended samples when compared to control [14].
Acknowledgements The authors would like to thank the College of Engineering at Imam Abdulrahman Bin Faisal University. The author would like to thank Mr. Muhammad Nasir Mr. Iehab Mohamed and Mr. Mukarram Zabair for their exceptional assistance with the experimental aspects of the study. Appendix A. Supplementary data
3.4. Leachability index (LI) Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2018.06.013.
Mean LI values of Al for each of three samples are presented in the SI section. LI values on the low end were 10.3 and steadily increased to 12.4, an indication of Al having low mobility (LI > 8) in its encapsulated form. The LI of Al in ash-amended samples exceeded the LI of Al in PCM supporting the hypothesis that the addition of DPA in Portland cement concrete results in greater Al mobility out of monoliths when compared to controls. However, it should be noted that while there was an increased mobility of Al out of ash amended samples, the calculated concentrations of Al that would theoretically leach into the surrounding environment did not exceed GCTL as similarly observed by [14] who used bottom ash amended concrete samples.
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4. Conclusion Saudi Arabia’s date palm industry generates date palm ash from the thermal processing of palm oil fibers and shells. This waste material has great potential in beneficial use as a cement replacement up to 10%. This study examined the pollution potential of DPA in its encapsulated form as part of a PCM matrix using standardized EPA test methods. The elemental composition of DPA revealed a standard elemental profile compared to similarly used wood ash. Al and Fe were elements with the greatest abundance in DPA but no element exceeded Florida’s industrial or residential SCTL. Concentrations of arsenic in DPA approached but did not exceed residential SCTL. Leaching tests were then completed to better understand the mobility of heavy elements out of DPA amended PCM samples. Leachability testing revealed that while concentrations of elements may appear high in DPA, they experience relatively low mobility when encapsulated in PCM matrices. The preliminary results presented in this paper indicate that DPA poses no environmental risk to human health when used as cement replacement in PCM. However, further analysis is still required to determine the overall applicability of 178
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