Digestion

Enumerating microplastics in biota, excised tissues or environmental samples can be challenging because the plastic may be masked by biological material, microbial biofilms, algae and detritus [9]. To isolate microplastics, organic matter can be digested, leaving only recalcitrant materials (Table 7.1). Traditionally, digestion is conducted using strong oxidizing agents. Synthetic polymers, however, can be degraded or damaged by these chemical treatments, particularly at higher temperatures. Environmentally exposed plastics, which may have been subject to weathering, abrasion and photodegradation, may have reduced structural integrity and resistance to chemicals compared to that of virgin plastics used in these stress tests [96]. As such, data ascertained using caustic digestive agents should be interpreted with caution, and the likely loss of plastics from the digestive treatment carefully considered.

TABLE 7.1 Optimized Protocols for Digesting Biota or Biogenic Material to Isolate Microplastics

Treatment

Exposure

Organism

Reference

HNO, (22.5 M)

20°C (12 h) + 100°C (2 h)

Blue mussels

78

HNO, (22.5 M)

20°C (12 h) + 100°C (2 h)

Blue mussels oysters

75

HNO, (22.5 M)

20°C (12 h) + 100°C (2 h)

Blue mussels lugworms

76

HNO, (100%)

20°C (30 min)

Euphausids copepods

11

HNO, (69%—71 %)

90 °C (4 h)

Manilla clams

74

HNO, (70%)

2 h

Zebrafish

38

HNO, (22.5 M)

20°C (12 h) + 100°C (15 min)

Brown mussels

104

HNO, (65%)

20°C (12 h) + 100°C (10 min)

Blue mussels

77

HC104 (68%) (4:1) HNO, (65%)

20°C (12 h) + 100°C (10 min)

Brown shrimp

73

HC104 (68%) (4:1) CH,0, (3%)

72 h

Corals

99

KOH(10%)

2-3 weeks

Fish

54

KOH(10%)

60°C (12 h)

Fish

21

KOH(10%)

2-3 weeks

Fish

33

H,0, (30%)

60°C

Blue mussels

92

H,0, (30%)

20°C (7 d)

Biogenic matter

100

H,0, (15%)

55°C (3 d)

Fish

72

H,0, (30%)

65°C (24 h) + 20°C (<48 h)

Bivalves

98

NaCIO (3%)

20°C (12 h)

Fish

101

NaClOj (10:1) Proteinase К

20°C (5 min) 50°C (2 h)

Zooplankton copepods

9

Assumptions: “Overnight” Given as 12 h; "Room Temperature” Given as 20°C.

Nitric Acid

Nitric acid (HNO-) is a strong oxidizing mineral acid, capable of molecular cleavage and rapid dissolution of biogenic material. When tested against hydrochloric acid (HC1), hydrogen peroxide (H,0,) and sodium hydroxide (NaOH), HNO; resulted in the highest digestion efficacies, with >98% weight loss of biological tissue [78]. The optimized protocol involved digesting excised mussel tissue in 69% HN03 at room temperature overnight, followed by 2 h at 100°C. Desforges et al. [11] also tested HN03, HC1 and H,0, in digesting zooplankton and similarly identified nitric acid as the most effective digestion agent based on visual observations; here, the optimized digestion protocol consisted of exposing individual euphausiids to 100% HN03 at 80°C for 30 minutes. Adaptations of nitric acid protocols have been successfully used to isolate fibers, films and fragments from a range of organisms [38-39,73-76]. While largely efficacious in digesting organic material, a number of studies observed that oily residue and/or tissue remnants remained postdigestion [18,51,63] which have the potential to obscure microplastics. Consequently, De Witte et al. [77] proposed using a mixture of 65% HN03 and 68% perchloric acid (HC104) in a 4:1 v/v ratio (500 mL acid to 100 g tissue) to digest mussel tissue overnight at room temperature followed by 10 minutes boiling, resulting in the removal of the oily residue. Recovery rates for 10 and 30 mm PS microspheres spiked into mussel tissue and subsequently digested with a nitric acid range between 93.6% and 97.9% [78]. However, the high concentrations of acid and temperatures applied resulted in the destruction of 30-200 mm nylon fibers and melding of 10 mm polystyrene microbeads following direct exposure. Researchers have found that polymeric particles, including polyethylene (PE) and polystyrene (PS), dissolved following overnight exposure and 30 minutes boiling with 22.5 M HN03 [72,97]. Polyamide (PA, nylon), polyester (PET) and polycarbonate have low resistance to acids, even at low concentrations. High concentrations of nitric, hydrofluoric, perchloric and sulfuric acid are likely to destroy or severely damage the majority of polymers tested, particularly at higher temperatures. The absence of synthetic fibers in biota digested using HNO; is likely a reflection of the destructive power of the acid [98].

 
Source
< Prev   CONTENTS   Source   Next >