Layer-by-Layer Deposition of Titanium Dioxide Nanoparticles on

Article pubs.acs.org/IECR

Layer-by-Layer Deposition of Titanium Dioxide Nanoparticles on Polymeric Membranes: A Life Cycle Assessment Study Stefano Zuin,*,† Petra Scanferla,† Andrea Brunelli,‡ Antonio Marcomini,†,‡ John E. Wong,§ Wilco Wennekes,∥ and Inge Genné⊥ †

Venice Research Consortium, Via della Libertà 12, c/o PST VEGA, 30175 Venice, Italy University Cà Foscari Venice, Dorsoduro 2137, 30121 Venice, Italy § RWTH AAchen University, Turmstrasse 46, 52064 Aachen, Germany ∥ X-FLOW BV, Marssteden 50, 7547 TC Enschede, The Netherlands ⊥ VITO-Flemish Institute for Technological Research, Boeretang 200, 2400 Mol, Belgium ‡

S Supporting Information *

ABSTRACT: Membrane processes are widely used in wastewater treatment and for removal of contaminants from drinking water. Engineered nanomaterials (ENMs) can be integrated into membranes structure to enhance their performance (e.g., fouling mitigation and improvement of permeate quality). However, in order to ensure a sustainable use of nanoactivated membrane, the potential environmental impacts should be evaluated in an early stage of their development. In this study, we performed a cradle to gate life cycle assessment (LCA) to evaluate the environmental impacts due to the integration of titanium dioxide (TiO2) engineered nanoparticles (ENPs) in polyethersulfone (PES) membrane using the layer-by-layer (LbL) technology. The PES membrane manufacturing and electrostatic deposition of TiO2 ENPs on PES membrane were investigated in this case study. The results show that the LbL deposition stage of TiO2 ENPs on membrane has an insignificant effect on all selected impact categories, in comparison to PES membrane manufacturing stages investigated. The electricity use during the membrane production as well as the solvents and polymers needed for making PES membrane are the main contributions to the overall environmental impact. their application.8,9 Evidence for the toxic effect of ENPs is increasing,10 and uncontrolled release of ENPs from products containing them has been recently demonstrated in textile11 and façade coating.12,13 To date, the widely accepted view is that there are many unanswered questions, although numerous reports have been published discussing the potential environmental impacts and human health risks due to the manufacture, use, distribution, and disposal of products containing ENPs.14 One approach that may improve our understanding on the possible impacts of every nanobased product is the life cycle assessment (LCA).8,10 This methodology can be used to evaluate how a product (or service), from the extraction of raw materials through to end-of-life, affects ecosystems and human health by compiling an inventory of relevant inputs and outputs, evaluating the potential environmental impacts associated with those inputs and outputs, and interpreting the results of the inventory and impacts in relation to the objectives of the study. LCA has been already applied to ENMs or nanobased products because it provides a more comprehensive overview of

1. INTRODUCTION The membrane separation technologies are used in different applications: from biotech separations and biomedical applications to large-scale water and wastewater treatment as well as in the food and beverage industries. The improvement of separation efficiency, e.g., by developing low-fouling membranes, is the focus of many research efforts.1 In this context, nanotechnologies (NTs) may play an important role in water and wastewater treatment.2 In detail, NTs can play a role in water treatment through the functionalization of microfiltration (MF) and ultrafiltration (UF) membranes by engineered nanoparticles (ENPs) deposited on their surfaces or embedded into the matrixes and the use of engineered nanomaterials (ENMs) to bind specific contaminants or catalyze degradation reactions.2,3 For example, the incorporation of silver (Ag) ENPs in membranes may improve the antifouling and antimicrobial properties of membranes,4 while titanium dioxide (TiO2) or alumina (Al2O3) ENPs are used to enhance membrane flux (hydrophilic membrane).5−7 At the European level, several projects grouped into the nano4water cluster (http://nano4water.eu/) are founded by the EC to support research and technological development in the field of water treatment by applying developed or adapted ENMs. However, in order to ensure that the nanoactivated membrane (NAM) developments take place in a safe and sustainable manner, the potential benefits due to the use of ENPs should be weighted against any potential effect on the environment and human health, especially in an early stage of © XXXX American Chemical Society

Special Issue: Recent Advances in Nanotechnology-based Water Purification Methods Received: November 6, 2012 Revised: February 13, 2013 Accepted: February 13, 2013

A

dx.doi.org/10.1021/ie302979d | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Figure 1. Boundaries of system analyzed: (a) life cycle phases considered and (b) ENPs activated membrane; dashed boxes: processes investigated are membrane manufacturing, layer-by-layer of polyelectrolyte (LbL PE), and TiO2 ENPs deposition. Legend: NMP = N-methyl-2-pyrrolidone; PES = polyethersulfone; PVP = polyvinylpyrrolidone; PVC = polyvinylchloride; PU = polyurethane; PE = polyelectrolyte.

polyethersulfone (PES) membrane surface, process tested within the EU-funded project NAMETECH - Development of intensified water treatment concepts by integrating nano and membrane technologies (FP7, Grant Agreement No. 226791; www.nametech.eu), are investigated in this cradle-to-gate LCA study. The analysis also includes an assessment of the PES membrane manufacturing process. The LCA study was carried out according to ISO standards 1404X series (2006).30

the potential environmental impacts of nanobased products, including all other substances as ENPs, used during manufacturing of such products. In addition, applying LCA may avoid the unintended shifting of environmental burdens.8 To date, however, only a few studies have been reported for nanobased products, and only some of them have been applied to nanotechnological production methods.15−22 In detail, LCA has been used to compare a product that includes ENMs with similar products without ENMs and thus to assess the relative environmental performance of nanobased products in comparison with their conventional equivalents.15,17,21,23,24 The production of ENMs through different technologies has been also evaluated in some cradle-to-gate LCA studies in terms of energy requirements and potential impacts on human health and the ecosystem.16,18,25−27 Recently, some commercialized products containing Ag ENPs have been analyzed according to the LCA methodology, like socks and T-shirts.28,29 Furthermore, although aspects relating to (eco)toxicity are usually assessed in LCA, the specific potential toxicological effects of ENMs have not been included in the studies done so far due to a lack of knowledge in relation to risk assessment.8,15 The general aim of this article is to evaluate the potential environmental impacts attributable to the integration phase of ENPs in polymeric membrane. In detail, the impacts posed by the layer-by-layer (LbL) deposition of TiO2 ENPs on a

2. METHOD 2.1. Introduction to LCA Methodology. LCA is a widely and recognized tool used to assess the environmental impacts of a product, process, or activity throughout its life cycle, i.e., from the extraction of raw materials through processing, transport, use, and disposal. The general categories of environmental impacts to be considered include resource use, human health, and ecological consequences. The result of a LCA is an environmental profile that expresses the performance of the total system life cycle and single life cycle stages. According to the ISO standard 14040 (2006),30 the LCA methodology encompasses four phases: (i) goal and scope definition, where the purpose and analyzed product (object) of the study are defined and the functional unit and system boundaries (i.e., what is and is not included in the study) are specified; (ii) the life cycle inventory (LCI) analysis, such as a B



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afterward through a spinneret. A bore fluid, mostly a nonsolvent (i.e., water), is transported through the inner tube of the spinneret by a pump in order to shape the hollow fiber. Then the membrane passes a controlled atmosphere, a chimney with steam. In the chimney, water vapor enters the fiber and moves the composition closer to the direction of the binodal. When a solution passes the binodal, it is not stable anymore and phase separation takes place. For the following coagulation, the fiber is immersed in a bath containing water and collected in a container with water to wash out the rest of the solvent and nonsolvent. The coagulation bath has a temperature of approximately 80 °C and the container is maintained at 60 °C. In the post-treatment phase, membranes are transported to a vessel containing water. Membranes are rinsed several times with fresh water and chlorine to remove PVP. After all the washing steps, the membranes are preserved with glycerine to prevent pore collapse. After the glycerine treatment, the membranes are dried in a hot air oven. Polyvinylchloride (PVC) and polyurethane (PU) resins needed for module preparation are also included. Input/output data of membrane manufacturing are not reported here, as this information is confidential (proprietary data of Pentair X-Flow). PES membrane is then modified by applying the LbL technique (Figure 1b), i.e., an alternating adsorption of oppositely charged polyions on a charged membrane surface to enable attachment of TiO2 on it by electrostatic forces.31−33 In this case study, the PES membrane was modified with a single layer of PE. In detail, a 0.05 M solution of poly(ethylenimine) (PEI; molecular weight = 750 000 g/mol, purchased from SigmaAldrich) was prepared in deionized water. One layer of polycation PEI was adsorbed on the membrane surface by dipping the PES membrane into PEI solution at room temperature (20 °C). Membrane was washed many times with deionized water to remove any loosely bound PE before the TiO2 ENPs deposition. PEI was chosen as a polycation due to its high degree of branching and high surface charge density to promote anchoring of the first layer to the PES membrane.33 Finally, the PEI-modified membrane was immersed in TiO2 ENPs dispersion in order to anchor TiO2 ENP on the membrane surface. The membrane is then repeatedly rinsed and washed with water and finally stored in a water bath. TiO2 ENPs aqueous dispersion was prepared by adding approximately 0.5 g of TiO2 ENPs (Evonik; Aeroxide P25) in 200 mL of deionized water and sonicated in a water bath. 2.2.4. Data Source. The main data sources are summed up in Table 1. Data were collected from different sources. Primary data were collected from interviews of NAMETECH partners as well as laboratory visits. Detailed LbL and TiO2 ENPs deposition process data were provided by the Chemical Process Engineering of RWTH Aachen University (RWTH; Germany). Membrane manufacturer Pentair X-Flow provided data of PES membrane production. Survey data included quantities and composition of relevant material input (i.e., raw materials and auxiliary), water consumption, energy flows (i.e., energy carrier, fuels, etc.), and material output, such as emissions to air and water, and waste generated (includes the kind of waste treatment) within all stages of processes investigated. To ensure the most reliable data quality, the following data quality indicators (DQI) were used for primary data collection: data source (site-specific, literature, or others), data development (measured, calculated, or estimated), and type of data (single value, mean value, others). Finding all the required quality data criteria is difficult

systematic collection of all energy and material flows and emissions connected to the object under investigation during its entire life cycle; (iii) the life cycle impact assessment (LCIA), where inventory data are grouped and assigned to specific impact categories (e.g., climate change, acidification potential, etc.), characterized using suitable LCIA models into common equivalence units (estimates) and finally summed to provide an overall impact; (iv) the life cycle interpretation, where conclusions and recommendations are set and opportunities to reduce energy, material inputs, or environmental impacts at each stage of the product life-cycle are suggested. 2.2. LCA Case Study. 2.2.1. Goal and Scope Definition. The goal of this LCA case study was to analyze potential impact due to the integration of TiO2 ENPs on the PES membrane structure and to detect whether the LbL deposition phase of TiO2 ENPs is a critical stage during the membrane production from an environmental point of view. The analysis was performed on conventional membrane manufacturing process, i.e., spinning, and on electrostatic deposition (ED) process of TiO2 ENPs on manufactured membrane. The aim is to quantify the contribution of the TiO2 ENPs deposition phase to the total environmental load, with respect to other stages needed. This study focuses on polymeric membranes for water treatment, more specifically on PES flat sheet membranes activated with TiO2 ENPs. 2.2.2. Functional Unit. The functional unit is defined as “the quantified performance of a product system for use as a reference unit in a life cycle assessment study”.30 It is a measure of the function of the studied system and provides a reference to which the inputs and outputs can be related in order to compare different systems. For our study, we defined the functional unit as “the production of 1 m2 of PES membrane activated with TiO2 ENPs needed for treating 1 m3 of feed per hour (h)”. Consequently all inventory data collected (and impacts calculated) are based on the production of 1 m2 of PES membrane. 2.2.3. System Boundaries. The systems investigated include the following phases: from raw material extraction to the membrane production and its activation with ENPs, i.e., cradle to gate analysis, as shown in Figure 1a. In detail, the system includes (Figure 1b): (i) PES membrane manufacturing by spinneret process, where materials used for the fabrication of membrane module are shown, (ii) activation of the PES membrane surface through LbL assembly of polyelectrolyte (PE), and (iii) TiO2 ENPs deposition on the membrane surface by electrostatic deposition (ed). With regard to PES membrane production, the Pentair XFlow process was considered (X-Flow BV; division of Pentair Inc.). It consists of wet spinning, in which a solution is extruded into a coagulating bath. Generally, the wet spinning involves the formation of continuous filament strands by forcing the material (i.e., polymers) through circular dies, with a subsequently removal of solvent to form the solid filament. The polymer solution is then immersed in a nonsolvent bath for the polymer, where a mass transfer process involving interchange of solvent/non-solvent occurs. In detail, the PES membrane manufacturing considered in our study consist of (i) preparation of the polymeric solution containing three components, such as PES and polyvinylpyrrolidone (PVP) polymers/N-methyl-2-pyrrolidone (NMP) solvent/and glycerine non-solvent. The solution is then degassed by heating at 60 °C and at −0.5 bar; (ii) spinning process, in which a viscous, air-free polymer solution is pumped through a filter and C



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finally milled in wet conditions to produce approximately 40 nm anatase phase TiO2 ENPs. With regard to investigated processes, although the LbL technique and TiO2 ENPs deposition were tested for flat sheet membranes, these modification methods can be principally applied for hollow fibers as well, without any substantial requirements. In the manufacturing phase of PES hollow fiber membrane, production of machineries and plants were neglected, and the capital energy, i.e., the energy needed for the industrial plants building, was not included. In addition, the quantity of raw materials and auxiliaries used to produce PES membrane was provided in a combined form due to confidentially aspects. The main background data (e.g., fuel extraction, refinery and combustion, energy supply, etc.) were selected from the Ecoinvent database v. 2.0 that usually refers to studies developed in Europe before 2007; for example, the emissions due to the use of electricity were considered relative to the European electric grid, according to the UCTE production data of 2004/2005. This is a mix of coal, gas, oil, nuclear, hydro, biomass, and wind energy. 2.2.6. Impact Assessment and Categories. The impact assessment of the manufacturing of PES membrane activated with TiO2 ENPs was performed by applying the CML- and Eco-Indicator ’99 methods,36,37 as these LCIA methods are used to represent the midpoint (CML-method) and end point approaches (Eco-Indicator ’99). The term “midpoint” expresses that this point is located on the impact pathway at an intermediate position between the LCI results and the ultimate environmental damage, often referred to as end points.38 Then midpoints are considered to be a point in the cause-effect chain of a particular impact category, prior to the end point, at which characterization factors can be calculated to reflect the relative importance of an emission or extraction in a LCI phase (e.g., global warming potentials defined in terms of radiative forcing and atmospheric half-life differences). In this case study we applied the CML 2 baseline 2000 characterization method,39 a midpoint method widely used and recommended for simplified studies. The impact categories selected in this study were global warming, abiotic resource depletion, acidification, eutrophication, human toxicity, ecotoxicity, stratospheric ozone depletion, and photooxidation. End points, on the other hand, are those elements inside the impact pathway that consist of an independent value for society. The term end point refers to the category indicator for each impact category located at the end of the impact pathway.38 End point indicators modeling the cause-effect chain up to the environmental damages, the damages to human health, to the natural environment, and to natural resources. Here we applied the Eco-Indicator ’99 damage oriented method,36 which collectively takes into account all damages related to human health, ecosystem quality, and resource depletion into one single indicator on the end point level expressed as Eco-indicator points (Eco-point). The EcoIndicator ‘99 methodology selected here is based on the Hierarchist (H) perspective that is considered the default perspective, and the average weighting set (A) was selected, as recommended by Eco-Indicator ’99.36 This allows for an overall assessment of the environmental impacts related to the system under investigation. 2.2.7. Interpretation. The interpretation includes an investigation of the environmental impacts per functional unit and an analysis with regard to the main contributing stages and

Table 1. Main Data Sources and Data Set Selected process production and acquisition of raw materials

water use energetic processes

waste disposal and wastewater treatment

data source Ecoinvent

Plastic Europe PE International Grubb and Bakshi, 2011 Ecoinvent Ecoinvent Industry data 2.0 ETH-ESU 96 Ecoinvent

data set N-methyl-2-pyrrolidone, at plant/RER ethylenediamine, at plant/RER glycerine, from epichlorohydrin, at plant sodium hypochlorite, 15% in H2O, at plant polyurethane, flexible foam, at plant PVC resin PES (polymerization of 4,4′ dichlorodiphenylsulfone) TiO2 ENPs production water, deionized, at user/CH electricity, production mix UCTE on site steam average E natural gas HP user in Europe treatment, sewage, to wastewater treatment, class 3; Disposal, plastics, mixtures, 15.3% water, to sanitary landfill

without resorting to some approximations, especially in the case of ENMs where data regarding the elementary flows are missing or very limited, while data on potential impacts on humans and the environment are uncertain.8 Foreground data were then integrated with the literature with regard to TiO2 ENPs production,27 and the database included in the LCA software used (SimaPro 7.1). In detail, inventory data of TiO2 ENPs synthesis by the Altair chloride process27 was selected and used in this case study. The Altair process involves ilmenite ore chlorination, distillation of titanium tetrachloride (TiCl4), and oxidation of the TiCl4 vapor at high temperatures. Ecoinvent v2.0 has been used as the database for background data.34 The data set of PE International was used with regard to PES production (polymerization of 4,4′-dichlorodiphenylsulfone monomers). Energy consumption values regarding electricity have been considered taking into account the European Union for the Coordination of Transmission of Electricity (UCTE) energy mix. Infrastructure facilities and engaged equipment (e.g., service and maintenance of used machineries) from the project partner during membrane manufacturing were excluded in this case study. 2.2.5. Assumptions and Limitations. The main assumptions concern data collection and calculation, particularly for processes such as extraction and manufacturing of raw materials. Inventory data relative to some chemicals, such as PEI and N-ethyl pyrrolidone (NEP) solvent, were not available. Nevertheless, we selected chemicals originating from a similar extractive and refining process and with comparable function. In detail, we selected the amine ethylenediamine instead of PEI and the N-methyl pyrrolidone (NMP) solvent instead of NEP, both from the Ecoinvent database. The NMP solvent data set was also used for membrane manufacturing. We assumed that TiO2 ENPs manufactured by Altair Nanomaterial Inc. are deposited on the membrane surface, even if TiO2 ENPs Aeroxide P25 (Degussa, Evonik) were tested in the project. The Altair data are available in the literature.35 This technology is based on the “chloride” process, which involves digestion of ilmenite (FeTiO3) ore by an excess of hydrochloride acid (HCl), the reduction of mixture feed with the addition of iron powder, and the pyrohydrolysis of formed FeCl2 crystals after all the iron and chloride ions are removed. The spray hydrolysis reaction produces high-purity TiO2 in the form of hollow spheres a few micrometers in diameter. These last particles are D



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Table 2. Inventory of Selected Raw Materials (Cut Off Value of 1%) for Manufacturing of PES Membrane Activated with TiO2 ENPsa substance gas, natural, in ground gravel, in ground oil, crude, in ground potassium oxide renewable fuels (renewable energy resources) sodium chloride, in ground water, unspecified natural origin/kg air inert rock water, cooling, unspecified natural origin water, process, surface water, process, well, in ground

unit

total

g g g g g g g kg kg kg kg kg

× × × × × × × × × × × ×

3.25 6.69 4.24 3.58 3.31 5.40 4.06 9.63 2.30 8.47 3.22 3.49

PES mem 2

3.25 3.62 3.80 3.58 3.31 5.00 4.06 9.63 2.30 8.47 3.22 3.49

10 102 102 102 102 102 102 100 100 100 100 100

× × × × × × × × × × × ×

LbL PE

TiO2 ed

1.45 × 102 1.30 × 101

1.62 × 102 3.17 × 101

2.63 × 101

1.34 × 101

2

10 102 102 102 102 102 102 100 100 100 100 100

a

Legend: PES mem = PES membrane manufacturing (Spinneret process); LbL PE = layer-by-layer of polyelectrolyte; TiO2 ed = electrostatic deposition of TiO2 ENPs.

Table 3. Inventory of Selected Air Emissions (Cut Off Value of 1%) for Manufacturing of PES Membrane Activated with TiO2 ENPsa unit 1,4-butanediol carbon dioxide carbon monoxide methane nitrogen nitrogen oxides NMVOC particulates > 10 μm sulfur dioxide sulfur oxides used air exhaust [other emissions to air] steam

g kg g g g g g g g g g kg kg

total 2.09 1.66 3.49 1.09 5.56 1.22 4.20 2.24 1.30 2.42 2.75 6.02 1.01

× × × × × × × × × × × × ×

PES mem

100 100 100 101 101 101 100 100 101 100 102 100 101

2.09 1.66 2.91 9.65 5.56 1.09 4.02 1.89 1.11 2.42 2.75 6.02 1.01

× × × × × × × × × × × × ×

100 100 100 100 101 101 100 100 101 100 102 100 101

LbL PE

TiO2 ed

2.24 × 10−1 2.40 × 10−1

3.57 × 10−1 1.06 × 100

3.04 4.80 7.51 3.50

× × × ×

10−1 10−2 10−2 10−1

9.92 1.30 2.72 1.57

× × × ×

10−1 10−1 10−1 100

a

Legend: PES mem = PES membrane manufacturing (Spinneret process); LbL PE = layer-by-layer of polyelectrolyte; TiO2 ed = electrostatic deposition of TiO2 ENPs; NMVOC = nonmethane volatile organic compounds.

Results of the LCI are subdivided for each stage considered: PES membrane manufacturing (PES mem), activation of PES membrane by polyelectrolyte (LbL PE), and electrostatic deposition of TiO2 ENPs on activated PES membrane (TiO2 ed). Concerning raw material, the PES mem stage required a larger amount of substances than the other two stages (Table 2). In detail, with regard to fossil resources, 0.325 kg of gas and 0.380 kg of oil crude are needed for the PES mem stage, resources needed for the polymerization of 4,4′-dichlorodiphenylsulfone. Extraction of oil crude is less relevant than the LbL PE and TiO2 ed stages. Water is another important resource for PES mem, especially cooling water (8.47 kg). Gravel is a substance consumed in all stages. Especially, 362 g of gravel inventoried are needed to produce NMP and glycerine, both used during polymer solution preparation and post-treatment of PES mem stage. Gravel used for the LbL and TiO2 ed stages (145 and 162 g, respectively) are associated with the treatment plant of wastewater generated during these stages. In total, 0.500 kg of sodium chloride is needed for the production of PES polymer used for the membrane manufacturing stage. The PES mem is thus the stage that requires a large amount of fossil fuels, cooling and process waters, and substances. The LbL PE and TiO2 ed stages require substantially lower amounts of

materials needed. The results of the impact assessment were interpreted for each impact category. Analysis was performed with the LCA software SimaPro 7.1. This tool includes several inventory databases with different types of materials and processes as well as the most important impact assessment methods.37 2.2.8. Review Process. The entire LCA case study in all its subsequent sections was subject to internal review performed by partners of the NAMETECH project by analyzing inventory data and assumptions done and by verifying if the performed LCA satisfies the ISO 1404X:2006 series.

3. RESULTS 3.1. Life Cycle Inventory Analysis. For each of the processes investigated, inventories of material and energy flows were produced. Over 800 inputs and outputs were tracked in the manufacturing of PES membrane activated with TiO2 ENPs (from cradle to gate) as result of the inventory phase. Results from the LCI analysis are shown in Tables 2−5. An inventory of resources is provided in Table 2, while inventoried emissions to air, water, and soil are also displayed in Tables 3, 4, and 5, respectively. Only resources and emissions contributing the highest 99% by mass are presented in the tables (i.e., a cutoff value of 1%) in order to highlight more relevant contributions. E



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Table 4. Inventory of Selected Water Emissions (Cut Off Value of 1%) for Manufacturing of PES Membrane Activated with TiO2 ENPsa aluminum ammonium, ion BOD5, biological oxygen demand calcium, ion chloride COD, chemical oxygen demand DOC, dissolved organic carbon iron, ion nitrate silicon sodium, ion solids, inorganic sulfate suspended solids, unspecified TOC, total organic carbon

unit

total

g g g g g g g g g g g g g g g

× × × × × × × × × × × × × × ×

4.77 4.74 1.46 5.51 1.87 2.56 8.77 5.80 1.89 2.83 5.66 3.53 7.40 7.39 1.28

PES mem 0

3.33 4.44 1.09 3.07 1.53 1.49 4.86 3.49 5.87 2.42 4.41 3.03 3.20 7.29 8.98

10 101 101 101 102 101 100 100 101 101 101 100 101 100 101

× × × × × × × × × × × × × × ×

0

10 101 101 101 102 101 100 100 100 101 101 100 101 100 100

LbL PE 5.20 1.57 1.72 1.14 2.01 5.20 1.91 9.96 6.62 1.20 9.47 8.90 2.02 1.00 1.88

× × × × × × × × × × × × × × ×

TiO2 ed −1

10 100 100 101 101 100 100 10−1 100 100 100 10−2 101 10−2 100

9.20 1.47 1.97 1.30 1.42 5.46 1.99 1.32 6.39 2.89 2.99 4.10 2.17 9.00 1.96

× × × × × × × × × × × × × × ×

10−1 100 100 101 101 100 100 100 100 100 100 10−1 101 10−2 100

a


Table 5. Inventory of Selected Soil Emissions (Cut Off Value of 1%) for Manufacturing of PES Membrane Activated with TiO2 ENPsa unit aluminum calcium chloride iron magnesium silicon sulfur carbon oils, unspecified

mg mg mg g mg mg mg g mg

total 2.81 9.82 1.08 2.45 1.07 5.19 2.82 3.07 7.50

× × × × × × × × ×

PES mem

102 102 102 100 102 102 102 100 102

9.64 3.54 9.07 7.95 3.58 1.53 9.03 9.27 5.92

× × × × × × × × ×

101 102 101 10−1 101 102 101 10−1 102

LbL 9.14 3.10 6.98 8.26 3.48 1.82 9.54 1.07 2.43

× × × × × × × × ×

101 102 100 10−1 101 102 101 100 101

TiO2 ed 9.27 3.17 1.04 8.28 3.60 1.83 9.61 1.07 1.33

× × × × × × × × ×

101 102 101 10−1 101 102 101 100 102

a


The most important emissions to soil (Table 5) are carbon (3.07 g) in the LbL PE and TiO2 ed stages and unspecified oils (0.75 g), due to the PES mem and TiO2 ed stages. 3.2. Life Cycle Impact Assessment with the CML Method. With regard to LCIA, Figure 2 shows the environmental impact assessment of manufacturing of PES membrane activated with TiO2 ENPs, as calculated by the CML 2 baseline 2000 method. The contribution of 10 impact categories is considered for each of the stages considered. Absolute values (i.e., characterized indicator results) of each impact category are reported in the Supporting Information (Table S1). For almost all CML categories, the PES mem stage has the greatest contribution (Figure 2). The resulting environmental impacts for the LbL and TiO2 ed stage are significantly lower than the respective environmental impacts for the PES mem stage. For example, for the abiotic depletion potential (ADP) category, the PES mem stage contributes with 0.060 kg of Sb eq, while the TiO2 ed stage contributes with 0.0044 kg of Sb. The most relevant contribution in the ADP category are the electricity and gas consumed during the PES mem stage as well as the use of PES and NMP. Similarly, for the global warming potential (GWP) category, the PES mem stage contributes with 5.68 kg of CO2 eq, while the TiO2 ed and LbL PE stages contribute with 0.63 kg of CO2 and 0.14 kg of CO2 eq, respectively. This is mainly due to use of

substances with respect to polymerization of dichlorodiphenylsulfone. Considering the release to the air (Table 3), steam is the greatest quantity (10.1 kg) released to the air during the PES mem stage. In total, 1.66 kg of carbon dioxide is released during the PES mem stage, due to almost exclusively the PES polymerization (0.125 kg). Methane released to air during the PES mem stage is due to PES polymerization but also to gas consumed during the spinneret process. The PES mem contributes with 11.1 g of sulfur dioxide, 55.6 g of nitrogen, and 2.42 g of sulfur oxides. The PES mem is the most relevant stage also for emissions to water, as shown in Table 4. The main releases to water are chloride (0.187 kg), sodium (0.056 kg), ammonium (0.047 kg), and calcium ions (0.055 kg) (Table 4). Sulfate (0.074 kg) and silicon (0.028 kg) are also relevant water emissions as well as BOD5 (0.014 kg) and COD (0.025 kg). Chlorides and sodium release are mainly due to PES polymerization, while ammonium release is due to PVC use during PES mem. The NMP use in PES mem stage significantly contributes in terms of BOD5 (6.9 g). The LbL PE and TiO2 ed stages are relevant for release of sulfate to water (20.2 and 21.7 g, respectively) as well as for release of nitrate (6.62 and 6.39 g, respectively). F



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Figure 2. Results of the characterization of environmental impacts for PES membrane activated with TiO2 ENPs, according to the CML method, without normalization. The contribution of 10 category indicators is considered: abiotic depletion potential (ADP); acidification potential (AP); eutrophication potential (EP); global warming potential for the time horizon of 100 years (GWP100); ozone layer depletion potential (ODP); human toxicity potential (HTP); marine aquatic, freshwater aquatic, and terrestrial ecotoxicity potential (MAETP, FAETP, and TETP); photochemical ozone creation potential (POCP).

1.33 kg of CO2 eq. Electricity use during the PES mem stage is also the most relevant contribution to the acidification potential (AP), with 0.0073 kg of SO2 eq, as a result of sulfur dioxide (SO2) released to air during electricity production; for the

nonrenewable resources like natural gas, coal, and oil crude needed to produce electricity consumed during the PES mem stage and consequent greenhouses gases released to the air. The use of NMP and PES contribute with 1.32 kg of CO2 eq and G



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not an essential step in LCA methodology, it is strongly recommended to understand the relative importance and magnitude of the results for the process. In the normalization phase, the results of the characterization step are related to a reference situation. This reference information may be related to a given community, country, or region over a period of time. In our case, we have chosen a normalization method applicable to the European region. Among the ones included in the CML 2 method, the West-Europe factors for normalization have been chosen as they are the ones that best fit our study.40,41 According to this normalization, the most relevant categories to the total impacts are ADP and MAETP (Figure 3). The electricity use is the prevalent stage within the MAETP category, while for the ADP category also the gas, NMP, and PES usage are the most relevant contributions. 3.3. Life Cycle Impact Assessment with the EcoIndicator ’99 Method. Figure 4 shows the environmental impacts of the manufacturing of membrane activated with TiO2 ENPs as calculated with the Eco-Indicator ’99 (H/A) methodology. The contribution of three Eco-Indicator ’99 damage categories Human Health, Ecosystem Quality, and Resources is considered for each of the stages considered. Absolute values of damage categories are reported in Table S2 (Supporting Information). As depicted in Figure 4, the results for the Human Health (HH) damage category are greatly dominated by the PES mem stage with 4.11 × 10−6 DALY; the remaining damages result are 5.31 × 10−7 DALY for the TiO2 ed stage and 1.99 × 10−7 DALY for the LbL PE stage. Also for the Ecosystem Quality (EQ) and Resources (R) damage category, the PES mem is the most relevant stage (0.168 PDF m2 year and 12.7 MJ). The LbL and deposition of TiO2 ENPs have a lower contribution than the PES mem in the three damage categories of the EcoIndicator ’99. To compare the environmental impacts on the same scale and better understanding of the magnitude of all environmental

marine aquatic, freshwater aquatic, and terrestrial ecotoxicity potential (MAETP, FAETP, and TETP) and human toxicity potential (HTP) categories, the TiO2 ed stage contributes with about 15% in these categories. The normalized results obtained considering the CML West Europe factors37 are reported in Figure 3. Although the normalization of characterized results is

Figure 3. Normalized impact values applying the CML2001 method (West Europe, 1995). The normalization is applied on 10 category indicators selected for the characterization on impacts: abiotic depletion potential (ADP); acidification potential (AP); eutrophication potential (EP); global warming potential for the time horizon of 100 years (GWP100); ozone layer depletion potential (ODP); human toxicity potential (HTP); marine aquatic, freshwater aquatic, and terrestrial ecotoxicity potential (MAETP, FAETP and TETP); photochemical ozone creation potential (POCP).

Figure 4. Results of the characterization of environmental impacts for the PES membrane activated with TiO2 ENPs, according to the Eco-indicator ’99 (H/A) method. The contribution of three Eco-Indicator’99 damage categories is considered: Human Health, Ecosystem Quality, and Resources. H



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Figure 5. Normalized and weighted damage values applying the Eco-Indicator ’99 (H/A) method. The normalization is applied on three damage categories: Human Health, Ecosystem Quality, and Resources.

addition to the electricity use. In detail, the gas use during the PES mem stage is responsible for about 20% of the overall impact in the ADP category, the NMP use for about 19%, while the PES usage for about 16%. These impacts are associated with the extraction of fossil resources needed for gas supply and for making NMP and PES. The use of NMP is also relevant for the HTP (28% of the overall impact) and POCP categories (26% of the overall impact). In the case of the HTP category, this is due to emissions of arsenic and chromium to water as well as emissions of hydrocarbons to air from the NMP production. For the POCP category, this is also due to emissions of sulfur dioxide to the air. The use of NMP is particularly relevant for the EP category (69% of the overall impact), following the release of ammonium to water during NMP synthesis. The highest impact for the ODP category (48.7%) comes from the use of glycerine during the PES mem stage. This is almost due to the release of tetrachloromethane (CFC-10) to air. Glycerine is also relevant for the TETP category (about 23%), due to the release of mercury to water from the production process itself. As depicted in Figure 6, the resulting environmental impacts are mainly associated with manufacturing of the PES membrane (PES mem). More than 70% of the overall impacts comes from the electricity and gas consumed during manufacturing of the membrane and from the use of raw materials needed (i.e., PES and NMP). In detail, PES and NMP are used for making the polymer solution, the first process of membrane manufacturing (Figure 1b). Gas is instead used for degassing the polymer solution before the spinning process. The electricity is essentially consumed during the spinning, the second step of membrane preparation. The contribution of glycerine used during post-treatment of the membrane is marginal, except the ODP category, as well as the use of PVC and PU polymers for module preparation.

impacts, the results of characterization were normalized and then weighted. The obtained results, expressed as the weighted final score (i.e., Eco-Indicator Point; Pt) for the three damage categories are shown in Figure 5. According to this normalization, the most damaged categories are R (0.326 Pt) and HH (0.126 Pt) (Figure 5). For the R damage category, the main contribution is due to the PES mem stage with 0.303 Pt, much the same as the impact category ADP of the CML method; this confirms the dominance of the resources-intensive process in the membrane production. For the HH damage category, the contribution is also due to the PES mem stage with 0.107 Pt. 3.4. Main Contributors to the Environmental Impacts. Figure 6 presents the contributions (in %) of energy and materials inputs needed for the analyzed system (i.e., PES membrane manufacturing followed by LbL and deposition of the TiO2 ENPs membrane), represented by the total EcoIndicator ’99 (H/A) and CML characterization factors. According to the environmental impact represented by the total Eco-Indicator ’99 (single score), the main contribution for the HH damage category comes from the electricity (53.7%) and NMP use (12.4%). The usage of PES, PVC, PU, and glycerine during the PES mem stage contributes to about 5−8% of the overall environmental impact expressed as the EcoIndicator ’99 single score. Remaining inputs (e.g., gas, etc.) are marginal. The electricity consumed for membrane manufacturing is also the relevant contribution for the EQ damage category (43.2%), followed by NMP (20%) use. The gas usage is the most relevant input for the R damage category (23.7%), followed by electricity (20.1%), NMP (18.9%), and PES (14.7%). With regard to the CML characterization factors, the electricity consumption is the main contributor to the environmental impact for the AP, GWP, FAETP, MAETP, TETP, and POCP categories. For example, for the AP category the highest impact (34.1%) comes from electricity consumed during membrane manufacturing, which is almost exclusively a consequence of the indirect emissions of sulfur oxides, sulfur dioxide, and nitrogen oxides during electricity production. The electricity use is also responsible for about 27% of the overall impact in the GWP category (i.e., 1.77 kg of CO2 eq on the total 6.46 kg of CO2 eq). CO2 fossil released in air is the most important contribution to the GWP category. For the ADP category, the gas, NMP, and PES are other important inputs, in

4. CONCLUSIONS This paper presented a LCA case study focused on membrane activation processes. Raw materials and auxiliaries processed in membrane manufacturing and its activation with TiO2 ENPs by the LbL/electrostatic deposition process were collected in order to provide a first inventory table and to assess the contribution of TiO2 NEPs use stage on the total environmental load. Results showed that the use of TiO2 NEPs has a I



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during the production of PES membrane as well as the NMP and PES use are the main contributions to the overall environmental impact. These resource-intensive processes significantly contribute to the overall impacts. The results of this case study depend in great part on the approximations and assumption made, especially with regard to ENPs use. In fact, although the ISO-framework for LCA is fully suitable to ENPs, data regarding their inventory are uncertain and scarce. The available LCI databases are populated with materials and product flows that do not distinguish between the bulk and corresponding ENPs. Generally, when inventory data from industrial production are unavailable, LCA studies often employ data from laboratory-scale or prototype production and from the literature on similar process products.8 In this study, we used the Altair Nanomaterial process data for TiO2 ENPs production,27 although TiO2 ENPs tested in the project are produced by Evonik Industries.46 Both processes are the flame synthesis of TiO2 ENPs, and both start with the same precursor, i.e., titanium tetrachloride (TiCl4), which is produced during the flame hydrolysis of natural rutile or ilmenite.27,46 Evonik and Altair also employ proprietary technology to obtain unique TiO2 ENPs features. An important strength of deriving available data from Altair27 was that their process data are very comprehensive and inclusive considering the level of detail, material, and energy data input, besides the different process steps. These data were selected as they are based on realistic and documented levels of resource use per kg of TiO2 ENPs produced. In addition, these data sets reflected a similar synthesis to the TiO2 Aeroxide P25 (Evonik) used. In addition, results are limited by the impact assessment methods’ ability to reflect specific physicochemical properties of ENPs (e.g., size, shape, surface functionalization, specific surface area, etc.). Although these properties influence toxicity and environmental exposure of ENPs, the current impact assessment tools do not provide suitable characterization factors for ENPs.15 As a consequence, considerable efforts are required in the future (i) to expand the available LCI database in order to include inventory data sets covering at least the most important ENPs (e.g., Ag,44 TiO2, SiO2, etc.) and (ii) to take into account how ENPs properties affect characterization of ENPs’ toxic impacts within the impact assessment of the LCA framework.15,42 However, although each LCA study on nanomaterials-based products suffers from high uncertainty issues, we simply cannot wait to have near-perfect data to perform the analysis.8 In our work we used available literature data concerning TiO2 ENPs synthesis. Then, the potential impacts due to the use of such additives during membrane manufacturing depend in great part on selected literature data. Similarly, the energy consumption during membrane manufacturing directly influences different impact categories. The EU energy mix has been taken into account here, where the total electricity production is heavily dependent on fossil fuels45 and where the chemicals released to the environment (e.g., SO2, NOx, and heavy metals) are the most important emission contributing to different impact categories. This analysis is not a fully fledged LCA as the use phase and end of life of NAM are not included. Ideally the LCA would extend to the “grave” of the membranes because differences in maintenance, service life, and disposal have environmental implications. However, as confirmed by Hischier and Walser, very few of the reviewed LCA studies of ENMs so far have integrated the use and end of the life phase of the ENMs

Figure 6. Contributions (in %) of different energy and material inputs needed for membrane production, represented by the total EcoIndicator ’99 (H/A) for three damage categories and CML characterization factors. Abbreviations: abiotic depletion potential (ADP); acidification potential (AP); eutrophication potential (EP); global warming potential for the time horizon of 100 years (GWP100); ozone layer depletion potential (ODP); human toxicity potential (HTP); marine aquatic, freshwater aquatic, and terrestrial ecotoxicity potential (MAETP, FAETP, and TETP); photochemical ozone creation potential (POCP).

lower environmental impact compared to the membrane manufacturing stage investigated. The electricity consumption J



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(4) Taurozzi, J. S.; Arul, H.; Bosak, V. Z.; Burban, A. F.; Voice, T. C.; Bruening, M. L.; Tarabara, V. V. Effect of filler incorporation route on the properties of polysulfone−silver nanocomposite membranes of different porosities. J. Membr. Sci. 2008, 325, 58−68. (5) Li, J.-F.; Xu, Z.-L.; Yang, H.; Yu, L.-Y.; Liu, M. Effect of TiO2 nanoparticles on the surface morphology and performance of microporous PES membrane. Appl. Surf. Sci. 2009, 255, 4725−4732. (6) Li, J. H.; Xu, Y. Y.; Zhu, L. P.; Wang, J. H.; Du, C. H. Fabrication and characterization of a novel TiO2 nanoparticle self-assembly membrane with improved fouling resistance. J. Membr. Sci. 2009, 326, 659−666. (7) Yan, L.; Hong, S.; Li, M. L.; Li, Y. S. Application of the Al2O3− PVDF nanocomposite tubular ultrafiltration (UF) membrane for oily wastewater treatment and its antifouling research. Sep. Purif. Technol. 2009, 66, 347−352. (8) Klöpffer, W.; Curran, M. A.; Frankl, P.; Heijungs, R.; Kohler, A.; Olsen, S. I. Nanotechnology and Life Cycle Assessment: A Systems Approach to Nanotechnology and the Environment, Synthesis of Results Obtained at a Workshop; Nanotechnology and Life Assessment, Washington, DC, October 2−3, 2006; published March 2007. (9) Som, C.; Berges, M.; Chaudhry, Q.; Dusinska, M.; Fernandes, T. F.; Olsen, S. I.; Nowack, B. The importance of life cycle concepts for the development of safe nanoproducts. Toxicology 2010, 269, 160− 169. (10) Oberdorster, G.; Stone, V.; Donaldson, K. Toxicology of nanoparticles: a historical perspective. Nanotoxicology 2007, 1, 2−25. (11) Geranio, L.; Heuberger, M.; Nowack, B. The behavior of silver nanotextiles during washing. Environ. Sci. Technol. 2009, 43, 8113− 8118. (12) Kaegi, R.; Sinnet, B.; Zuleeg, S.; Hagendorfer, H.; Mueller, E.; Vonbank, R.; Boller, M.; Burkhardt, M. Release of silver nanoparticles from outdoor facades. Environ. Pollut. 2010, 158, 2900−2905. (13) Kaegi, R.; Ulrich, A.; Sinnet, B.; Vonbank, R.; Wichser, A.; Zuleeg, S.; Simmler, H.; Brunner, S.; Vonmont, H.; Burkhardt, M.; Boller, M. Synthetic TiO2 nanoparticle emission from exterior facades into the aquatic environment. Environ. Pollut. 2008, 156, 233−239. (14) van Zijverden, M.; Sips, A. J. A. M. Nanotechnology in Perspective: Risks to Man and the Environment; RIVM Report 601785003; 2009. (15) Bauer, C.; Buchgeister, J.; Hischier, R.; Poganietz, W. R.; Schebek, L.; Warsen, J. Towards a framework for life cycle thinking in the assessment of nanotechnology. J. Clean. Prod. 2008, 16, 910−926. (16) Healy, M. L.; Dahlben, L. J.; Isaacs, J. A. Environmental assessment of Single-Walled Carbon Nanotube Processes. J. Ind. Ecol. 2008, 12, 376−393. (17) Joshi, S. Can Nanotechnology improve the Sustainability of Biobased Products? The Case of Layered Silicate Biopolymer Nanocomposites. J. Ind. Ecol. 2008, 12, 474−489. (18) Khanna, V.; Bakshi, B. R.; Lee, L. J. Carbon Nanofiber Production: Life Cycle Energy Consumption and Environmental Impact. J. Ind. Ecol. 2008, 12, 394−410. (19) Krishnan, N.; Boyd, S.; Somani, A.; Raoux, S.; Clark, D.; Dornfeld, D. A Hybrid Life Cycle Inventory of Nano-Scale Semiconductor Manufacturing. Environ. Sci. Technol. 2008, 42, 3069−3075. (20) Kushnir, D.; Sandén, B. A. Energy Requirements of Carbon Nanoparticle Production. J. Ind. Ecol. 2008, 12, 360−375. (21) Lloyd, S. M.; Lave, L. B. Life Cycle Economic and Environmental Implications of Using Nanocomposites in Automobiles. Environ. Sci. Technol. 2003, 37, 3458−3466. (22) Lloyd, S. M.; Lave, L. B.; Matthews, H. S. Life Cycle Benefits of Using Nanotechnology To Stabilize Platinum-Group Metal Particles in Automotive Catalysts. Environ. Sci. Technol. 2005, 39, 1384−1392. (23) Steinfeldt, M.; Petschow, U.; Haum, R.; von Gleich, A. Nanotechnology and Sustainability; Discussion Paper of the IOEW 65/ 04; Berlin, 2004. (24) Roes, A. L.; Marsili, E.; Nieuwlaar, E.; Patel, M. K. Environmental and Cost Assessment of a Polypropylene Nanocomposite. J. Polym. Environ. 2007, 15, 212−226.

containing products, as there is not available information about this important life cycle stage.47 Consequently, additional studies will be necessary to extend this research to include inservice and end-of-life impacts, especially with regard to ENPs release from the membrane, as the use phase of NAM could be a potential source of ENPs to the environment.9 Regarding ENPs emissions to the water, environmental concentrations will depend on how additives are used in the synthesis procedure, the type and amount applied, but also on the way the membrane is used. Furthermore, the aquatic toxicity of potentially released ENPs shows large uncertainties because current LCIA methods are not able to adequately distinguish the behavior of ENPs in water (e.g., aggregation, dissolution, bioavailability, etc.). To increase our knowledge on the potential impacts posed by the use of ENPs in the membrane, a second step of the study will ensue, which will consist in estimating possible ENPs release to water during the use of membranes in plants. The study could then be integrated by including the use phase of the membrane, considering different operating scenario of membrane plant (e.g., flux, maximum transmembrane pressures, lifetime of the membrane, membrane cleaning step, etc.) as well as input of materials and energy needed to run the membrane plant. In conclusion, the combination of nanotechnology with membrane science is a new scientific challenge that should be integrated with life cycle concepts to mitigate the uncertainty about the effects of ENPs on human health and the environment and to promote a safe and sustainable development of the nanoenhanced membrane.

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ASSOCIATED CONTENT

S Supporting Information *

Values of the characterization of environmental impacts according to CML methods (Table S1) and Eco-Indicator ’99 methods (Table S2) for PES membrane activated with TiO2 ENPs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by the European Commission within the Seventh Framework Program (NAMETECH project; contract no. 226791). The authors acknowledge the contribution of NAMETECH partners, especially Jozef Kochan (RWTH, Germany), for providing relevant data on the LbL and electrostatic deposition section.

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REFERENCES

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