272
B. Gunawardana et al.
The United States Environment Protection Agency has listed some of the CPs
as priority toxic pollutants (USEPA 2019, 2018a, b). Toxicity of CPs increases with
increasing degree of chlorination (Czaplicka 2004). Among all CPs, pentachlorophenol (PCP), a wood preservative and biocide (UNEP 2014), is reported as the most
toxic CP (Tanjore and Viraraghavan 1994). Further, PCP is listed as a human carcinogen (IARC 2019; USEPA 2010), priority pollutant (USEPA 2019, 2018a, b;
EC 2016), and toxic to aquatic organisms (UNEP 2014). The maximum contaminant level of PCP in drinking water has been imposed as 1 ppb (USEPA 2018a,
b). Increased awareness about the toxicity and environmental impacts of CPs has
led to banning or restricting the use of PCP worldwide (UNEP 2014). However,
persistence of CPs leads to the continued ubiquitous presence of CP compounds
as contaminants in surface water, groundwater, soil, and sediments. Dechlorination and removal of PCP have gained attention in contaminated groundwater treatment. However, degradation/removal of tetrachlorophenol (TeCP), trichlorophenol
(TCP), and dichlorophenol (DCP) have received only little attention. Treatment and
removal of CPs has become a difficult task due to their inherent characteristics such
as hydrophobic nature and aromaticity linked stability.
Degradation of various CPs has been reported using different technologies in the
literature. Reductive dechlorination of CPs with zero-valent iron (ZVI) has been
reported by Gunawardana et al. (2018) and Kim and Carraway (2000); photocatalysis process has been tested by Lan et al. (2011), Li et al. (2011) and Ma et al.
(2019); microbial degradation-assisted dechlorination was reported by Xu et al.
(2018), Yang and Chen (2016); use of membrane biofilm reactors was reported by
Long et al. (2018). Among the various treatment techniques available, ZVI is popular
as a reactive medium for dechlorination of chlorinated contaminants because of the
aspects such as economical, relatively highly reactive, and pose minimal environmental impact (Choi et al. 2008; Gunawardana et al. 2011; Kim and Carraway 2000).
Use of permeable reactive barriers (PRBs) is a passive treatment and a financially
feasible method for the treatment of contaminated groundwater (Henderson and
Demond 2007; Fu et al. 2014; Furukawa et al. 2002). ZVI with a variety of particle
sizes can be used as the reactive medium in PRB applications. However, microscale
ZVI has been commonly tested and popular for dechlorination of chlorinated organic
contaminants (Cheng et al. 2007; Choi et al. 2008; Chun et al. 2010; Gillham and
Ohannesin 1994; Feng and Lim 2005; Kim and Carraway 2000; Klausen et al. 2003;
Matheson and Tratnyek 1994; Nardo et al. 2010; Patterson et al. 2016; Phillips et al.
2010).
The reductive dechlorination process of chlorinated organics by ZVI is dependent on a number of variables. The contaminant transformation when in contact
with ZVI in the aqueous medium is led by various species which act as reducing
agents that are simultaneously present in the aqueous system and lead to, (1) direct
contaminant reduction by the electrons released from Fe
0 (primary reductant), and
(2) indirect contaminant reduction by other reductants such as electrons released by
adsorbed/structural Fe
II (secondary reductants), H/H 2 and Fe 3 O 4 and green rust (tertiary/quaternary reductants) (Hu et al. 2018). Previous studies have reported various
factors that influence the reactivity, longevity, and dechlorination potential of ZVI
B. Gunawardana et al.
The United States Environment Protection Agency has listed some of the CPs
as priority toxic pollutants (USEPA 2019, 2018a, b). Toxicity of CPs increases with
increasing degree of chlorination (Czaplicka 2004). Among all CPs, pentachlorophenol (PCP), a wood preservative and biocide (UNEP 2014), is reported as the most
toxic CP (Tanjore and Viraraghavan 1994). Further, PCP is listed as a human carcinogen (IARC 2019; USEPA 2010), priority pollutant (USEPA 2019, 2018a, b;
EC 2016), and toxic to aquatic organisms (UNEP 2014). The maximum contaminant level of PCP in drinking water has been imposed as 1 ppb (USEPA 2018a,
b). Increased awareness about the toxicity and environmental impacts of CPs has
led to banning or restricting the use of PCP worldwide (UNEP 2014). However,
persistence of CPs leads to the continued ubiquitous presence of CP compounds
as contaminants in surface water, groundwater, soil, and sediments. Dechlorination and removal of PCP have gained attention in contaminated groundwater treatment. However, degradation/removal of tetrachlorophenol (TeCP), trichlorophenol
(TCP), and dichlorophenol (DCP) have received only little attention. Treatment and
removal of CPs has become a difficult task due to their inherent characteristics such
as hydrophobic nature and aromaticity linked stability.
Degradation of various CPs has been reported using different technologies in the
literature. Reductive dechlorination of CPs with zero-valent iron (ZVI) has been
reported by Gunawardana et al. (2018) and Kim and Carraway (2000); photocatalysis process has been tested by Lan et al. (2011), Li et al. (2011) and Ma et al.
(2019); microbial degradation-assisted dechlorination was reported by Xu et al.
(2018), Yang and Chen (2016); use of membrane biofilm reactors was reported by
Long et al. (2018). Among the various treatment techniques available, ZVI is popular
as a reactive medium for dechlorination of chlorinated contaminants because of the
aspects such as economical, relatively highly reactive, and pose minimal environmental impact (Choi et al. 2008; Gunawardana et al. 2011; Kim and Carraway 2000).
Use of permeable reactive barriers (PRBs) is a passive treatment and a financially
feasible method for the treatment of contaminated groundwater (Henderson and
Demond 2007; Fu et al. 2014; Furukawa et al. 2002). ZVI with a variety of particle
sizes can be used as the reactive medium in PRB applications. However, microscale
ZVI has been commonly tested and popular for dechlorination of chlorinated organic
contaminants (Cheng et al. 2007; Choi et al. 2008; Chun et al. 2010; Gillham and
Ohannesin 1994; Feng and Lim 2005; Kim and Carraway 2000; Klausen et al. 2003;
Matheson and Tratnyek 1994; Nardo et al. 2010; Patterson et al. 2016; Phillips et al.
2010).
The reductive dechlorination process of chlorinated organics by ZVI is dependent on a number of variables. The contaminant transformation when in contact
with ZVI in the aqueous medium is led by various species which act as reducing
agents that are simultaneously present in the aqueous system and lead to, (1) direct
contaminant reduction by the electrons released from Fe
0 (primary reductant), and
(2) indirect contaminant reduction by other reductants such as electrons released by
adsorbed/structural Fe
II (secondary reductants), H/H 2 and Fe 3 O 4 and green rust (tertiary/quaternary reductants) (Hu et al. 2018). Previous studies have reported various
factors that influence the reactivity, longevity, and dechlorination potential of ZVI