Review
Removal of phthalates from aqueous solution by different adsorbents: A short review

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Abstract

This work presents a short review of adsorptive materials proposed and tested for removing phthalates from an aqueous environment. The objective is not to present an exhaustive review of all the types of adsorbents used, but to focus on selected types of “innovative” materials. Examples include modified activated carbon, chitosan and its modifications, β-cyclodextrin, and specific types of biomass, such as activated sludge from a wastewater treatment plant, seaweed and microbial cultures. Data from the literature do not confirm the existence of a broad-spectral adsorbent with high sorption efficiency, low production costs and environmentally friendly manufacture. According to the coefficients of Freundlich’s isotherm, the most promising adsorbent of those mentioned in this work appears to be the biomass of activated sludge, or extracellular polysaccharides extracted from it. This material benefits from steady production, is cheap and readily available. Nevertheless, before putting it in practice, the treatment and adaptation of this raw material has to be taken into consideration.

Highlights

► The widespread use of phthalates causes a serious environmental problem. ► Adsorption on various sorbents is suitable method of PAEs removal from environment. ► New sorbents based on nanomaterial and modified usual sorbents should be developed. ► The importance of low economic costs and biodegradable sorbents will increase.

Introduction

Synthetically prepared chemical substances are eventually bound to become part of the ecosystem, within which they behave quite differently and with varied impacts on the environment. They may be found in the atmosphere, in fog and precipitation, and subsequently in surface water, groundwater and soil. With regard to the fact that toxic substances are spread through the food chain, living organisms and the entire environment of the Earth are contaminated with these substances. In a number of cases, the outcome is serious damage of natural equilibriums (Siglová et al., 2007).

Ranked amongst the most hazardous and, concurrently, the best-known substances harmful to human health is a group of organic compounds that include persistent organic pollutants (POPs). Other substances that have been considered such for years are DDT, polychlorinated biphenyls (PCB), polycyclic aromatic hydrocarbons (PAH) and a wide range of organic pesticides. Incorporated in the Stockholm convention list, these have been monitored for some time; consequently their production and distribution is prohibited. At present, no further doubts linger about their danger. Nevertheless, substances do exist that undergo continuous manufacture and which are used in enormous quantities, despite the negative impacts of the same becoming ever more apparent (Siglová et al., 2007). For example, flame retardants, pharmaceutical and personal health care products, and detergents feature amongst these potentially harmful substances. An important group of organic substances of this type is formed by phthalates (phthalic acid esters – PAEs).

Phthalates are non-halogenated esters of phthalic acid that find widespread utilization in various industrial and consumer-orientated applications. For the vast majority of their usage – 90% – they are plasticizers for polymers, mainly as polyvinyl chloride (PVC). Plasticized PVC is utilized in a number of consumer-orientated products – ranging from packing and building materials (flooring, hoses, cables and other items), through to toys and medical implements. The various forms of PAEs are not bound chemically in a polymeric matrix and are gradually released during usage. Less significant phthalate applications include components of inks, adhesive materials, lacquers, sealing and packing materials, materials for treating surfaces, solvents and fixing agents in fragrances, as well as additives in other kinds of cosmetics (Roy, 2004, Serodio and Nogueira, 2006, Yuan et al., 2008, Šuta, 2007).

Some PAEs are supplied in the form of pure chemical products, e.g. di(2-ethylhexyl) phthalate (DEHP), while others represent a complex blend of isomers consisting of many compounds of similar chemical structure; for example, di-isononyl phthalate (DINP) and di-isodecyl phthalate (DIDP).

The most commercially applied phthalate is DEHP, which not only is controlled under the strictest supervision but is also regarded as the most problematic from the viewpoint of undesirable effects on health. Other frequently used phthalates are di-n-butyl phthalate (DBP), diethyl phthalate (DEP), di-isononyl phthalate (DINP), butyl benzyl phthalate (BBP), di-isodecyl phthalate (DIDP) and di-n-octyl phthalate (DNOP).

At present, global production of phthalates is in the order of millions of tonnes per year. In the USA, a decline in the volume of production of diethyl phthalate meant a drop from approximately 9500 tonnes in 1980 to 8000 tonnes in 1987. However, production increased again in 1988 to 11,800 tonnes. Meanwhile, the figure for European Union countries is around 10,000 tonnes, based on data from 1999. In Japan, 700 tonnes were produced in 1999. A survey of fragrance manufacturers, conducted in 1995–1996 by the Research Institute for Fragrance Materials, reported an annual use of approximately 4000 tonnes in the preparation of fragrance mixtures (Sekizawa et al., 2003). In Germany alone, in 1994–1995, the production of phthalates was calculated at 400,000 tonnes (of which DEHP equalled 250,000 t, DBP 21,000 t and BBP 9000 t) (Fromme et al., 2002).

Due to this enormous consumption of phthalates, at least 23,000 tonnes of them entered the environment in 1984. The greatest contamination of the environment by phthalates may usually be expected in the surroundings of industrial estates (e.g. where plasticized PVC is manufactured) and landfill sites. A paper (by Fromme et al., 2002) mentions that DEHP concentrations in surface water can amount to 98 μg/L, with the figure for outlets from wastewater treatment plants being up to 182 μg/L. Concentrations found in bed sediment were up to 8.44 mg/kg, and 154 mg/kg in wastewater treatment sludge (Fromme et al., 2002).

Commonly produced phthalates are colorless or yellowish oil-like liquids and almost odourless. Their melting point is predominantly below −25 °C, whereas the boiling point ranges between 230 °C and 486 °C. The most commonly used phthalates are relatively hydrophobic, which greatly influences how they behave in the environment. If relatively long alcohol moieties are present in the molecule, solubility can be less than 1 mg/L (Roy, 2004) Table 1 shows the physico-chemical data for selected phthalates (Serodio and Nogueira, 2006, Thomsen et al., 1999) as octanol-water partition coefficients (Log Ko/w), solubility (−Log Cwsat) and sorption (Log Koc).

Even though phthalates are quite rapidly excreted from an organism, some of these – owing to their lipophilic properties – get deposited in fatty tissue. PAEs are not acutely toxic but exposure over a reasonable period could probably result in potential carcinogenic effects and an unfavorable influence on the hormonal and reproduction system (via estrogenic effects). However, these largely depend on the kind of ester (Pitter, 2009).

For example, DEHP and DBP, according to the guideline 67/548/EHS of the European Union on classifying and designating hazardous substances, are ranked as reprotoxic substances. Recent research suggests reproduction toxicity occurs even with other currently used phthalates, e.g. BBP and DINP. In the case of DINP and DIDP, their nefrotoxicity and influence on liver function is regarded as the most serious. The latest research findings have associated DEP with a potential influence on the development of sperm in men, although DEP has been regarded as a phthalate of rather low toxicological significance and is thus currently used in perfumes. A correlation has also been described between the incidence of asthma and the occurrence of materials containing phthalates in homes, this in addition to a statistically significant higher occurrence of allergies and asthma correlating with the concentration of phthalates in household dust (Šuta, 2007).

The European Union (EU) has published a list of substances linked with potential endocrine-disrupting action, which includes di-n-butyl phthalate, butylbenzyl phthalate and di-2-ethylhexyl phthalate (DEHP). Since DEHP is the most widespread phthalate to be produced and used, it is included on the list of priority substances in the sphere of water policy as established by the EU and the World Health Organization (WHO), with a guideline value of 8.0 μg/L in fresh and drinking water (Serodio and Nogueira, 2006).

The United States Environmental Protection Agency (US EPA), under the Safe Drinking Water Act, regulates DEHP and di(2-ethylhexyl) adipate through its National Primary Drinking Water Regulations. Maximum contaminant levels have been set at 6.0 μg/L and 400 μg/L respectively. Furthermore, the US EPA advises close screening of phthalates in drinking water at concentrations above 0.6 μg/L (Serodio and Nogueira, 2006).

The presence of phthalates in the environment has been discussed for dozens of years, but in the 1980s only studies indicating the possible carcinogenic effects of DEHP led to greater interest in these compounds. In the mid 1990s, the situation resulted in efforts to map the real levels of phthalates in all components of both the environment and food, and subsequently in an endeavour to minimize their penetration into the environment.

Literature describes a number of methods for removing phthalates from an aqueous environment. One of the most frequently applied methods is microbial degradation. In the study of PAE biodegradation under aerobic and anaerobic conditions, pure (Chang et al., 2004, Chao et al., 2006) and heterogeneous microbial cultures (Marttinen et al., 2003) have been used. Subsequently, a wide range of bacterial cultures capable of decomposing certain phthalates, including DEHP, have been isolated. The properties of these isolated cultures are quite varied, and these equally, within the phthalates, consequently degraded in accordance with their efficiency; i.e. especially the degradation rate of certain concentrations and degradation mechanisms (Houser et al., 2008). Virtually all authors consider microbial degradation of phthalates a realistic means of removing these pollutants from a contaminated environment. However, these methods require a long time to render the phthalates harmless, and micro-organisms do not manage to degrade them entirely or completely remove them from soil or aqueous solution (Roy, 2004, Zhang et al., 2007).

Other methods of PAE removal concern advanced oxidation processes (AOP). Their principle is the addition or production of highly reactive particles capable of oxidizing even highly stable molecules. The literature describes, for example, photochemical degradation of diethyl phthalate or di-(2-ethylhexyl) phthalate via UV/H2O2 (Xu et al., 2007, Chen, 2010); photochemical degradation of dimethyl phthalate by Fenton’s reagent (Zhao et al., 2004); photochemical mineralization of di-n-butyl phthalate with H2O2/Fe3+ (Chiou et al., 2006); photocatalytic ozonation of dibutyl phthalate or di-(2-ethylhelxyl) phthalate over TiO2 film (Li et al., 2005) or powder (Chung and Chen, 2009); photocatalytic degradation of 1,2-diethyl phthalate mediated via TiO2 (Muneer et al., 2001); sonolytic degradation of phthalic acid esters in aqueous solutions (Yim et al., 2002); oxidative degradation of diethyl phthalate by a photochemically enhanced Fenton reaction (Yang et al., 2005); aqueous oxidation of dimethyl phthalate in an Fe(VI)–TiO2–UV reaction system (Yuan et al., 2008), degradation of dimethyl phthalate in aqueous solution by UV/Si–FeOOH/H2O2 (Yuan et al., 2011) and photocatalytic ozonation of dimethyl phthalate with TiO2 prepared by a hydrothermal method (Jing et al., 2011).

Removal may also take place in coagulation during the process of turning surface water into drinking water. Although coagulation, including flocculation, is useful for removing organic micropollutants and its removal mechanism has been reported (Thebault et al., 1981), coagulation by ferric chloride proved ineffective at degrading phthalate (Zhang et al., 2007).

Adsorption appears to be an efficient tool for processing greater quantities of wastewater containing phthalates. A whole range of commercial adsorptive materials (active carbon, active coke, carbon molecular sieves, carbon fabrics, ion exchange resins, magnesium hydroxide and others) may be utilized for the adsorption of organic substances, the efficiency of which is relatively high, but it is burdened by the disadvantage of a higher price. Another option is the use of natural sorbents (brown and bituminous coal, humates, humic acids, seaweed and others), whose price is incomparably lower but their capacity for sorption is limited.

The adsorbents tested for sorption of phthalates were of organic and inorganic origin. Examples include active carbon (Ayranci and Bayram, 2005, Mohan et al., 2007, Fang and Huang, 2009, Fang and Huang, 2010), active carbon modified with cupric nitrate or tetrabutylammonium iodide, cellulose triacetate (Kosobutskaya and Phthal, 1982), nitrocellulose fibres (Lewis and Roberts, 1982), biomass of activated sludge or seaweed, chitosan (Salim et al., 2010) and modified chitosan with α-cyclodextrin or molybdenate, β-cyclodextrin (Murai et al., 1998), manganese oxides (Matocha and Sparks, 1997), kaolinite, boehmite – Al2O3·H2O, goethite – Fe2O3·nH2O (hydrated oxide), montmorillonite, Ca-montmorillonite and calcite (Sullivan et al., 1982), surfactant-coated nano/micro-sized alumina (Li et al., 2008) and carbon nanotubes (Wang et al., 2010) or multi-walled carbon nanotubes (Den et al., 2006). Specific types of polymeric adsorbents were also tested, e.g. hydrophilic hyper-cross-linked polymer resin NDA-702 (Zhang et al., 2007, Xu et al., 2011), aminated polysterne resin NDA-101 (Zhang et al., 2008) homogenous polystyrene resin XAD-4 (Zhang et al., 2008, Xu et al., 2011) and acrylic ester resin Amberlite XAD-7 (Zhang et al., 2008).

As can be seen from the overview of adsorbents above, the problems associated with phthalate adsorption via various types of sorbents are extensive, and it is due to this that the paper presented here primarily focuses on unusual adsorptive materials.

Section snippets

Adsorption of phthalates

The adsorption of organic substances depends on the size of their molecules, chemical structure and polarity. Usually, the larger the molecule, the better its sorption. The influence of chemical structure is particularly apparent through the presence of various functional groups that undergo, or not, dissociation. Therefore, they may specifically adsorb onto different surfaces depending on the pH value. The adsorption of organic substances is also significantly affected by the latter’s

Adsorption of phthalates by active carbon and modified active carbon

A type of sorbent long considered capable of adsorbing phthalates is active carbon. For example:

Mohan et al. (2007) focused in their paper on the study of DEP sorption on activated carbon from the aqueous phase, using activated carbon of the geometric mean size of 70 μm (surface area 500 m2/g) as adsorbent. Principally, they studied the influence of various parameters such as contact time (sorption kinetics), initial concentration, carbon dosage and operational pH.

Their work first studied the

Adsorption of phthalates from aqueous solution by chitosan beads

Chen and Chung (2006) selected chitosan as the biosorption material, which is produced by N-acetylation of chitin, and it was employed to remove inorganic and organic pollutants, for example, chlorophenols and nitrophenols. The phthalates under study were dimethyl phthalate (DMP), diethyl phthalate (DEP), di-n-propyl phthalate (DPP), di-n-butyl phthalate (DBP), diheptyl phthalate (DHpP) and di-2-ethylhexyl phthalate (DEHP). For studying adsorption, the authors selected a static arrangement of

Adsorption of phthalates from aqueous solution by β-cyclodextrin

Murai et al., 1998, studied the removal of PAEs from aqueous solutions through forming inclusion complex compounds during adsorption onto β-cyclodextrin (β-CD) or to its cross-linked polymer (β-CDP) by means of epichlorhydrin. It was discovered via NMR spectra measurement that β-CD forms hydrophobic cavities, inside of which PAEs may become adsorbed along with the formation of equimolar complexes. The stability of complex compounds of PAEs with β-CD was studied by measuring fluorescence

Adsorption of phthalates (DEP, DBP) by activated sludge and extracellular polymeric substance extracted from activated sludge

The type of biomass employed in a study (by Fang and Zheng, 2004) was activated sludge from a municipal wastewater treatment plant, containing 4.265 g/L of suspended solids (SS) and 3.505 g/L of volatile suspended solids (VSS). The second type of adsorbent was extracellular polymeric substance (EPS), including carbohydrate, protein, humic substance, uronic acid, DNA and unidentified substances, most of which were likely to be lipids or phenols extracted from activated sludge. Owing to good

Other adsorbents

A study by Den et al. (2006), focuses on DMP and DEP adsorption onto multi-walled carbon nanotubes (MWCNTs). For a description of the adsorption isotherms, the authors employed both Langmuir’s and Freundlich’s model. On the basis of the data acquired, the authors then evaluated Langmuir’s model as the more appropriate. In both cases, however, the time needed to achieve the equilibrium decreased with the concentrations of the DMP and DEP in the aqueous phase. In MWCNT, with an outer diameter of

Conclusion

The brief summary of reports published so far on the removal of phthalates via adsorption shows that, despite the extensive knowledge assembled by scientific teams around the world over recent decades, research in this area should continue.

As follows from the knowledge acquired (Table 2), the use of different materials for adsorption is dependent on many factors determined by the physico-chemical properties of not only phthalates (solubility in water), but also the matrix (pH, HA content,

Acknowledgements

This article was created with the support of Operational Program Research and Development for Innovations, co-funded by the European Regional Development Fund (ERDF) and national budget of the Czech Republic, within the framework of a project of the Centre of Polymer Systems (reg. number: CZ.1.05/2.1.00/03.0111). Gratitude is also extended to J. Kupec for his thoughtful review of the manuscript.

References (51)

  • L.S. Li

    Photocatalytic ozonation of dibutyl phthalate over TiO2 film

    J. Photochem. Photobiol. A-Chem.

    (2005)
  • S.K. Marttinen

    Removal of bis(2-ethylhexyl) phthalate at a sewage treatment plant

    Water Res.

    (2003)
  • C. Moreno-Castilla

    Adsorption of organic molecules from aqueous solutions on carbon materials

    Carbon

    (2004)
  • C.J. Salim et al.

    Comparative study of the adsorption on chitosan beads of phthalate esters and their degradation products

    Carbohydr. Polym.

    (2010)
  • P. Serodio et al.

    Considerations on ultra-trace analysis of phthalates in drinking water

    Water Res.

    (2006)
  • P. Thebault et al.

    Mechanism underlying the removal of organic micropollutants during flocculation by an aluminium or iron salt

    Water Res.

    (1981)
  • M. Thomsen et al.

    SAR/QSAR approaches to solubility, partitioning and sorption of phthalates

    Chemosphere

    (1999)
  • X.L. Wang et al.

    Comparison of biosorption isotherms for di-n-butyl phthalate by live and dead bacteria

    Water Res.

    (1994)
  • B. Xu

    Photochemical degradation of diethyl phthalate with UV/H2O2

    J. Hazard. Mater.

    (2007)
  • G.P. Yang

    Oxidative degradation of diethyl phthalate by photochemically-enhanced Fenton reaction

    J. Hazard. Mater.

    (2005)
  • B.L. Yuan et al.

    Aqueous oxidation of dimethyl phthalate in a Fe(VI)–TiO2–UV reaction system

    Water Res.

    (2008)
  • B.L. Yuan

    Degradation of dimethyl phthalate (DMP) in aqueous solution by UV/Si–FeOOH/H2O2

    Colloids Surf. A

    (2011)
  • W.M. Zhang

    Assessment on the removal of dimethyl phthalate from aqueous phase using a hydrophilic hyper-cross-linked polymer resin NDA-702

    J. Colloid Interface Sci.

    (2007)
  • W.M. Zhang

    Equilibrium and heat of adsorption of diethyl phthalate on heterogeneous adsorbents

    J. Colloid Interface Sci.

    (2008)
  • X.K. Zhao

    Photochemical degradation of dimethyl phthalate by Fenton reagent

    J. Photochem. Photobiol. A-Chem.

    (2004)
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