Article pubs.acs.org/IECR
Arsenic Removal from Natural Waters by Adsorption or Ion Exchange: An Environmental Sustainability Assessment Antonio Dominguez-Ramos,† Karan Chavan,‡ Verónica García,† Guillermo Jimeno,⊥ Jonathan Albo,∥ Kumudini V. Marathe,‡ Ganapati D. Yadav,‡ and Angel Irabien*,† †
Departmento de Ingenierías Quimica y Biomolecular, Universidad de Cantabria, Avda de los Castros, s.n., 39005 Santander, Spain Department of Chemical Engineering, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai 400019, India ∥ Departmento de Ingeniería Química, Universidad del País Vasco, Apdo. 644, 48080 Bilbao, Spain ⊥ Centre for Oscillatory Baffled Reactor Applications, School of Engineering and Physical Science, Chemical Engineering, Heriot-Watt University, Edinburgh, Riccarton EH14 4AS, United Kingdom ‡
S Supporting Information *
ABSTRACT: The Environmental Sustainability Assessment of the adsorption and ion-exchange processes for arsenic removal was the focus of this work. The pursued goals were to determine the impact of regenerating the activated alumina used as adsorbent and the comparison of the environmental performance of two ion-exchange resins. Additional goals were the comparison between the environmental performance of adsorption and ion-exchange processes and the evaluation of the effect of integrating the proposed techniques on a water purification facility. The Life Cycle Inventory was obtained by means of simplified models and simulation. In this work it was concluded that the removal of As(V) by adsorption consumed between 2 and 13 times more primary resources and created 3−17 times more environmental burdens than the ion-exchange process. The integration of adsorption or ion-exchange technology in the drinking water plant would raise the primary consumption of energy, materials, and water by 27−155%, 7−94%, and 0.48−5.3%, respectively. The increase in the environmental burdens was mainly because of the generation of hazardous spent materials.
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INTRODUCTION Excessive quantities of arsenic (As) in drinking water1,2 in Spain3−5 and India6−9 are well documented. The problem is more acute in India, where as many as 60 million people are at risk of chronic As poisoning.10 The World Health Organization has established 10 μg As·L−1 as the safety standard for As concentrations in drinking water.11 This value was endorsed by the European Union12 and by the Bureau of Indian Standards.13 In order to remove As from water for drinking purposes, enhanced treatment processes are needed14 due to the limitations of conventional treatments.15 A broad range of enhanced As removal techniques are reported in the literature: oxidation/precipitation,16,17 coagulation/electrocoagulation/ coprecipitation,18−20 membrane technologies,21−23 adsorption,15,24−26 photocatalysis,27 biosand filters,28 and ion-exchange.29 Among the mentioned technologies, adsorption is considered the less expensive procedure and safer to handle than precipitation, ion-exchange, and membrane filtration.15 Adsorption is simple in operation, and it is used at different scales, ranging from household modules to community plants.30−32 However, the adsorbent can be regenerated few times. On the other hand, ion-exchange is another effective technique for As removal, which tolerates a larger number of regeneration cycles.30 Research regarding the technical performance of adsorption and ion-exchange for As removal is abundant, and many reviews are being published.14,33−38 However, the environmental perspective is not usually considered.39 The life cycle approach can be adopted to evaluate the Environmental Sustainability Assessment (ESA) of products, processes, or services.40 © XXXX American Chemical Society
The present study focuses on the ESA of adsorption and ionexchange processes for As removal within a drinking water treatment plant using Life Cycle Assessment. The ESA of the adsorption process was conducted considering activated alumina (AA) as it is the most common industrial adsorbent.15,30 The ESA of the ion-exchange process considered the use of commercial and under-development selective ion-exchange resins. The used resources and the environmental burdens generated by the two processes during the Cradle-to-Gate and Gate-to-Gate steps were accounted. The effect of the regeneration of the AA and the type of resins were considered. Additionally both technologies were compared.
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METHODOLOGY
Goal and Scope. The study compares the removal of As from natural waters using adsorption or ion-exchange processes within a drinking water treatment plant. The main objective was to quantify the ESA of the adsorption and ion-exchange processes and to compare both options. Additional goals were to determine the impact of regenerating the used AA as well as the utilization of two types of resins. Special Issue: Ganapati D. Yadav Festschrift Received: December 30, 2013 Revised: February 6, 2014 Accepted: February 15, 2014
A
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adsorption or the ion-exchange process. The environmental burdens originated from the release of the removed As to water bodies and spent adsorbent/resin to land. There were no emissions to air. The treatment of the liquid and solid wastes from the As removal step, i.e., the Gate-to-Grave step, was out of the scope of this work. This was due to the large uncertainty of the potential waste management options. Land usage was also excluded in this study, as the alternatives are expected to provide similar values for this resource. Therefore, the two steps within the Cradle-to-Gate analysis shown in Figure 1 (raw material acquisition and operation) are clearly identified with the steps depicted in Figure 2 as C-G and G-G. Description of Systems under Study. Two different As(V) removal processes were considered, leading to four scenarios as shown in Figure 3: the first two included the use of AA adsorption for As removal, and the last two regarded the utilization of ion-exchange. In order to weigh the resources and burdens generated by the removal processes within a conventional drinking water treatment plant, a fifth reference scenario was added. A brief description of each scenario is provided below, specific features of each scenario are provided in the Supporting Information (SI): • Scenario ADS: the removal of As using AA. No regeneration step was considered, and the spent AA containing the retained As became hazardous waste. • Scenario ADS-R: Drinking water treatment by AA including its regeneration utilizing chemicals. The AA became hazardous waste after a certain number of regenerations steps. Each regeneration cycle caused a loss in the removal capacity of AA. The cleaning solution was released to the water compartment. • Scenario IEX-R: the use of commercial resin Lewatit FO36 for drinking water treatment. This scenario considered the resin regeneration without any efficiency loss. The cleaning solution was released to the water compartment. Once the resin reached its lifetime, it was discarded as hazardous waste. • Scenario SIEX-R: the removal of As using a selective lab resin based on an iron(III) cover which included the regeneration step without any efficiency loss. Similar to scenario IEX-R, the selective resin was discarded as hazardous waste after use and the cleaning solution released to seawater. The regeneration procedure was different than the regeneration used in scenario IEX-R. • Scenario DW: this scenario regarded the pretreatment of the water prior to the As removal. It was based on a conventional water purification facility. It is assumed that no As is removed in this step. The adsorption was modeled assuming that the solid−liquid equilibrium was attained in a fixed bed configuration; the removal of As(V) was in continuous mode until the bed was fully saturated. Life Cycle Inventory. In order to compare the environmental sustainability of the processes, the adsorption and the ion-exchange stages were simulated. As a result of the simulation, the Life Cycle Inventory (LCI) was estimated for each scenario by means of the amount of final resources and pollutants for the calculation of the Natural Resources Sustainability (NRS) and of the Environmental Burdens Sustainability (EBS), respectively. The same procedure was used for the analysis, modeling and simulation of the four
The scope of the assessment was based on the integration of the As removal process into a medium-sized drinking water treatment plant of continuous flow rate of 1.38 × 106 m3·year−1. Regarding the source, i.e., surface water or groundwater, different specific pretreatments could be required. However, this work was focused on the concentration of As in the raw water and not on the source of the drinking water. Consequently a general intensive physical and chemical pretreatment process was considered: precipitation/oxidation, coagulation/flocculation/sedimentation, filtration, and disinfection. Figure 1 depicts the raw materials acquisition and the
Figure 1. Block diagram of the C-G analysis of the system under study.
operation process for the removal of As in a drinking water treatment plant. The raw water could be directly diverted to the plant or to intermediate processing as a natural resource. The inlet concentration for the As removal process was set up at 100 ppb: the maximum level of As that raw water could contain in order to be treated for drinking purposes under the Spanish regulation.41 The composition of the inlet water was modeled as exclusively of As(V); thus, the presence of other competing ions was not considered for the sake of simplicity. The safety value of 10 ppb was considered as the outlet concentration level for the As removal process. In this work, the focus was made on the removal of As; thus, the pretreatment or further disinfection was not modeled. Consequently, the adsorption and the ionexchange steps were analyzed, modeled, and simulated in detail. The functional unit in this work was 1 m3 of treated water. Within the scope of the work, the study considered the Cradleto-Gate approach (Figure 2): Cradle-to-Gate (C-G): The environmental burdens generated by the transformation of natural/primary resources into usable forms of resources. This step included all individual transformation processes such as raw materials extraction, manufacturing, transportation, etc. The natural resources included the primary form of energy, materials, water, and land, whereas the final resources were sorbent or resins, electricity, water, and regeneration reagents. Gate to Gate (G-G): the environmental burdens generated by the transformation of final resources into a product, a process, or a service. In this study, G-G step referred to the B
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Figure 2. Block diagram of the Life Cycle Environmental Sustainability Assessment. NRS: Natural Resources Sustainability, EBS: Environmental Burdens Sustainability, LCESA: Life Cycle Environmental Sustainability Assessment.
Figure 3. Block diagram of the removal of As by the adsorption or ion-exchange processes in a drinking water treatment plant. F: flow rate incoming to the As removal step, C0: inlet concentration of As, Ce: outlet concentration of As. ADS: adsorption of AA without regeneration; ADS-R: adsorption of AA with regeneration; DW: pretreatment of water prior to the As removal; IEX-R: ion-exchange using commercial resin and conducting a regeneration step; SIEX-R: ion-exchange using lab resin and conducting a regeneration step.
This liquid waste was produced as a result of each regeneration step. Full details of the simulation carried out are provided in the SI along with the selected parameters. The LCI referred to the results of this simulation as shown in Figure 3. It is important to note that the results presented in the LCI depended on the used parameters, and the selected
simulated scenarios in order to obtain the resources used in the G-G step: the amount of adsorbent or resin, the electricity needed for overcoming the pressure drop, the rinsing water, and the products and water used in the regeneration process (Figure 2). Further, two wastes were generated: the spent sorbent or resin and a liquid effluent that contained As(V). C
dx.doi.org/10.1021/ie4044345 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Table 1. Results of the Scenarios under Study Related to the Functional Unit (G-G step)a scenarios final resources (related to the functional unit) NRS
energy
electricity adsorbent/resin materials NaOH NaCl H2SO4 water water emissions, effluent and wastes (related to the functional unit)
EBS
wastes (land) effluents (seawater)
spent adsorbent As(V) Na+ SO42‑ Cl−
units
ADS
ADS-R
IEX-R
SIEX-R
kW·h·m kg adsorbent or resin·m−3 kg NaOH·m−3 kg NaCl·m−3 kg H2SO4·m−3 m3·m−3
0.124 0.119 − − − −
0.124 0.026 0.292 − 0.268 0.008
0.124 0.004 0.011 0.016 − 0.002
0.124 0.004 0.003 − − 0.003
−3
units kg kg kg kg kg
adsorbent·m−3 As(V)·m−3 Na+·m−3 SO42‑·m−3 Cl−·m−3
ADS
ADS-R
IEX-R
SIEX-R
0.119 − − − −
0.026 9 × 10−5 0.16 0.263 −
0.004 9 × 10−5 0.002 − 0.010
0.004 9 × 10−5 0.013 − −
a
ADS: adsorption of AA without regeneration; ADS-R: adsorption of AA with regeneration; IEX-R: ion-exchange using commercial resin and conducting a regeneration step, SIEX-R: ion-exchange using lab resin and conducting a regeneration step; NRS: Natural Resources Sustainability, EBS: Environmental Burdens Sustainability.
scenario ADS did not need water for the regeneration as opposite to scenario ADS-R. The LCI also indicated that resource usage for the scenarios IEX-R and SIEX-R was similar in terms of energy and water. Concerning the material consumption, Table 1 shows that comparable quantities of the two resins were needed. This was due to the similar values of the qe and the lifetime LT in both scenarios. However, the regeneration procedure affected the amount of final materials used, and almost 3 times more final materials were needed in scenario IEX-R. Consequently, scenario SIEX-R was preferable from the resource usage perspective. The comparison of the material usage among the scenarios ADS, ADS-R, and SIEX-R showed that the removal of As from raw water by means of ion-exchange reduced the amount of adsorbent needed. The observed decrease was from 0.119 kg·m−3 in ADS and 0.026 kg·m−3 in ADS-R to approximately 0.004 kg·m−3 in SIEX-R. The reduction was observed in spite of the fact that the qe value for the used AA was greater than for the ion-exchange resins. This was due to the possibility of performing significantly more regeneration cycles N in the ion-exchange process, N = 47 in SIEX-R, compared to N = 5 in scenario ADS-R. Regarding the release of pollutants, the four scenarios produced a solid waste that consisted of the spent adsorbent or resin. The solid wastes were considered hazardous in all the scenarios. The total amount of the As(V) released as liquid waste is also indicated in Table 1. According to US-EPA, the pH of the resulting regeneration solution should be around 12.42 However, the burdens generated by the basic nature of the effluent were not considered in this work. In this study, we assumed that the burden associated with the liquid effluents generated in scenarios ADS-R, IEX-R and SIEX-R was due to the presence of As(V), and it was represented by the metric named Ecotoxity to Aquatic Life (metals). No burdens were derived from the release of sulphates, sodium, or chloride because the potency factor for seawater was zero for those substances. Life Cycle Environmental Sustainability Assessment (LCESA). The results of the NRS and EBS for the scenarios ADS and ADS-R were compared in order to evaluate the influence of the regeneration step in terms of primary resources and environmental burdens on the adsorption process. Table 2 shows
model. Regarding the technical performance, the scenario ADSR treated a total volume of around 27,000 BV during the five regeneration cycles, which was equivalent to a lifetime of 95 days. This value was lower than the 10,000 BV of AA per cycle reported by US-EPA.42 Therefore the value of the adsorption capacity qe used in this work directly affected the number of BV that could be treated, i.e., changing the number of BV is equivalent to altering the total treated volume of water VT which in turn modifies the life cycle inventory. Around 210,000 BV were treated with the considered resins. Once the simulations of the different scenarios were completed, the emissions to air, effluents, and solid wastes were transformed into EBS values by means of GaBi LCA v4.3 software.43 For the upstream processes, the Ecoinvent database44 was used as reference considering European data (Spanish grid mix for electricity consumption, cationic resin for the resins, and aluminum oxide for the AA). Regarding the selected metrics for conducting the EBS of the five scenarios, the IChemE metrics for Process Industries proposed in “The Sustainability Metrics”45 were considered. These metrics expressed the environmental sustainability of emissions, effluents, and wastes from the treatment scenarios. A weighting procedure was conducted according to the European Pollution Release and Transfer Register to obtain dimensionless normalized environmental burdens as described elsewhere.46
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RESULTS AND DISCUSSION The main LCI results for the removal of As by adsorption are summarized in Table 1. According to the results of the simulation, conducting the regeneration step did not affect the amount of electricity used during the treatment process. This was due to the assumption of equal pressure drop for each scenario and that the electricity used in the regeneration step was negligible compared to the former. Regarding the consumption of materials, the scenario ADS used about 5 times more adsorbent to treat 1 m3 of water than scenario ADS-R due to the lack of regeneration. However, the regeneration process entailed a large consumption of chemical reagents, up to 0.56 kg·m−3. Thus, there was a clear trade-off between the usage of AA and regeneration reagents. Finally, the D
dx.doi.org/10.1021/ie4044345 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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The weight of the adsorption or ion-exchange step in the water treatment process was also evaluated through the integration of the scenarios in scenario DW. The results for the four scenarios showed that the consumption of primary energy, materials, and water of the treatment plant can increase between 27 and 155%, 7−94%, and 0.48−5.3%, respectively. The integration of scenario ADS-R reported the greatest impact while the incorporation of the ion-exchange reported a slight increase. From the NRS perspective, the scenarios IEX-R and SIEX-R were the most adequate alternative. Table 3 indicates the EBS generated in the selected scenarios. The main burden created by the removal of As(V) by adsorption or ion-exchange processes was allocated in the land compartment in the G-G step. In the scenarios ADS, ADSR, IEX-R, and SIEX-R, the total dimensionless normalized index to land contributed to the total index 99.8, 94.3, 57.1, and 57.4%, respectively. It was checked that the use of a more specific Spanish grid mix47 for the electricity has a very low influence in the total EBS values of the overall processes. These results indicated that the main environmental problem caused by the adsorption or ion-exchange processes was the generation of the hazardous solid wastes. The resulted EBS also showed that the regeneration of the AA affected the burdens caused by the adsorption process. The scenario ADS had a total index of 84 compared to the total index of 20 corresponding to the scenario ADS-R. These values were mainly due to the contribution of the generation of hazardous solid waste in the G-G step. The higher value of scenario ADS was due to the G-G burdens derived from the more significant generation of solid hazardous wastes per unit of volume treated. As observed in Table 3, the use of water, NaOH, and H2SO4 in the regeneration procedure resulted in the increase of the environmental contribution to air and water compartment 4 and 44 times higher, respectively. On the other hand, the burden to land in the scenario ADS-R decreased 78%. The comparison among both scenarios showed clearly the trade-off between usage of chemicals, which contributed to a higher value of the total index to air and the total index to water and the generation of spent adsorbents, which contributed to a higher value of the total index to land. Table 3 also shows that no release of As(V) to the water compartment took place in the scenario ADS, and a minor As(V) contribution occurred in the scenario ADS-R. Further, the total index to air in the scenario ADS-R was higher than in the scenario ADS: 0.58 vs 0.14, and both were exclusively due to the C-G step as no emissions to air were given by the adsorption process itself in the G-G step. The emissions were given by the POF metric as a consequence of the emissions of SF6 during the life cycle production of H2SO4. Finally, the majority of the burdens caused by the scenario ADS and ADS-R to the water compartment occurred during the C-G step and G-G, respectively. The burdens produced by the scenario ADS-R to the water compartment were due to the release of As(V) to seawater. Therefore, scenario ADS-R provided the lowest EBS value thanks to the lower production of hazardous waste. Regarding the two scenarios related to the ion-exchange process, similar total indexes, around 4, were presented. The similarity in the total indexes for the two resins was based on the inventories. This total index was much lower than the value obtained for the two AA related scenarios and the contributions of total index to air and total index to water to the total index were around 31% and 11−12%, respectively.
Table 2. Natural Resource Sustainability Values Obtained for the Scenarios under Study Related to the Functional Unita scenarios primary resources
units
ADS
ADS-R
IEX-R
primary energy MJ·m−3 3.69 9.75 1.95 non-renewable 3.60 9.39 1.87 renewable 0.09 0.36 0.08 primary materials kg·m−3 0.22 0.64 0.09 energy resources 0.09 0.27 0.05 nonrenewable 0.07 0.03 0.00 elements nonrenewable 0.05 0.33 0.04 resources renewable 0.01 0.02 0.01 resources primary water m3·m−3 0.01 0.06 0.01 final resources units AA ADS-R IEX-R energy materials water
MJ·m−3 kg·m−3 m3·m−3
0.44 0.12 −
0.44 0.59 0.01
0.44 0.03
