**4. Phosphorus-based flame retardants: halogenated compounds**

Organophosphorus FRs can be divided into nonhalogenated and halogenated. Halogenated organophosphates have chlorinated forms, which are mainly used as FRs in furniture, building materials, textiles, and electronics [118]. According to the 2012 Chemical Data Reporting from the U.S. Environmental Protection Agency (EPA), about 22,700 tons/year of TDCPP, one of the most used organophosphorus FRs, were manufactured or imported by the U.S.A. in 2010 and 2011 [118, 119]. Tris- (chloropropyl)phosphate (TCPP), tris-(2-chloroethyl) phosphate (TCEP), tris-(1,3 dichloro-2-propyl) phosphate (TDCPP), and tetrakis-(2-chlorethyl) dichloroisopentyldiphosphate (V6) are the main halogenated organophosphates.

### **4.1 Physicochemical characteristics**

*4.1.1 Tris-(1,3-dichloro-2-propyl) phosphate (TDCPP)*

Tris-(1,3-dichloro-2-propyl) phosphate is commonly abbreviated in the literature as TDCPP, TDCP, or TDCIPP. It a viscous colorless liquid with boiling of 457°C, water solubility of 1.5 mg/l, and log *K*ow of 3.8 [6, 120, 121].

#### *4.1.2 Tris-(chloropropyl)phosphate (TCPP)*

TCPP is a clear, colorless liquid. Its commercial formulation consists of a mixture of tris-(chloroiso-propyl)phosphate (75%) and bis-(1-chloro-2-propyl)- 2-chloropropyl-phosphate (15–30%). It has a boiling point of 342°C, the water solubility of 1.6 g/l, and log *K*ow of 2.59 [6, 121].

#### *4.1.3 Tris-(2-chloroethyl) phosphate (TCEP)*

TCEP has boiling point of 351°C; however, above 220°C, it rapidly decomposes to carbon monoxide, hydrogen chloride, 2-chloroethane, and dichloroethane. It has a water solubility of 7.0 g/l and log *K*ow of 1.44 [6, 121].

#### *4.1.4 Tetrakis-(2-chlorethyl) dichloroisopentyldiphosphate (V6)*

V6 has a boiling point of 620°C, the water solubility of 2.1 mg/l, and log *K*ow of 1.9. In addition, V6 can be used in conjunction with TDCPP and TCPP, but with specific criteria [6, 121, 122].

**Table 5** summarizes the chemical structure and physicochemical properties of Halogenated Organophosphates.

#### **4.2 Environmental occurrence and ecotoxicological effects**

Because organophosphate FRs are not chemically bound to the original material, they are slowly released into the environment by abrasion and volatilization [123, 124]. Consequently, they are widely distributed in indoor and outdoor environments [124]. In addition, dust and air are important means of human exposure to these compounds via skin and breathing [4]. Furthermore, dust may settle into water bodies, contaminate the water environment and affect aquatic organisms [4].

A study investigated organophosphate FR concentrations in air and dust in 63 homes in Canada, the Czech Republic, and the U.S [125]. The highest concentration of halogenated compounds was found in the U.S.—an average of 1440 ng/g TCEP to 4530 ng/g tris-(2,3-dibromopropyl) phosphate (TDBPP), followed by Canada, and the Czech Republic [11]. Regarding air, TCIPP was detected at the highest average concentration: 73.6 ng/m<sup>3</sup> in Canada, followed 26.3 ng/m<sup>3</sup> in the U.S. and 16.4 ng/m<sup>3</sup> in the Czech Republic [125].

A study carried out in Brazil investigated indoor dust concentrations in different places in Araraquara (Brazil) [22]. The authors observed that TDCIPP (up to 61,200 ng/g) was the second most abundant compound in homes and apartments, [12] and the most abundant compound in cars (from 1050 to 1,600,000 ng/g) [22].

Regarding halogenated compounds in outdoor dust, a study investigated FR concentrations in outdoor dust from urban and rural areas in Nanjing (China) [126]. The authors identified halogenated compounds as the most abundant in both the rural (median: 45.9%) and urban (median: 56.8%) areas [126] and TCPP as the most abundant FR in both studied areas [126].

Assessing sediments is important for monitoring aquatic ecosystems and the aquatic environments quality [127, 128]. Halogenated FRs may accumulate in sediments because they have low solubility in water and a relatively high octanolwater partition coefficient (*K*ow) [129].
