Indium trihydroxide

Endpoint summary

Description of key information

For metals, the transport and distribution over the different environmental compartments e.g., the water (dissolved fraction, fraction bound to suspended matter), soil (fraction bound or complexed to the soil particles, fraction in the soil pore water, etc.) is described and quantified by the metal partition coefficients between these different fractions.

Kd values for indium are:

-log Kd suspended matter/water: 5.9

-log Kd sediment/pore water: 5.1

-log Kd soil/soil pore water: 4.23

At environmental conditions in water (pH 5-9) indium rapidly forms insoluble hydroxide precipitates, predominantly as the neutral solid complex – In(OH)₃. Due to limitations on the relevancy of measuring dissolved concentrations, the total measured indium concentration is used for all ecotoxicological assessments.

There are strong physicochemical indications that indium will be sequestered into non-bioavailable complexes within both sediment and soil compartments.

Additional information

Environmental distribution in water

At environmental conditions in water (pH 5-9) indium rapidly forms insoluble hydroxide precipitates (see Figures below, from Chrysikopoulos and Kruger, 1986), predominantly as the neutral solid complex – In(OH)3. However, as observed in synthetic laboratory waters used for ecotoxicity testing, the hydroxide complexes do not form larger polymers and remain as small agglomerates/particles suspended in solution. As a result, particles variably pass through synthetic filters used to operationally define dissolved chemical speciation (0.45-micrometer pore diameter). In solutions with elevated dissolved organic carbon present (> 2 mg/L), weak acids present at the organic matter surface scavenge indium ions from suspended particles whereby increasing the measureable dissolved fraction of indium (organic carbon bound indium). However, the presence of dissolved organic carbon in exposures also decreases the toxicity of indium.

Following from the principles of bioavailability demonstrated for many metals, the interaction with components of the water and biota were briefly investigated for indium (section 7). This resulted in a preliminary understanding of indium bioavailability as a function of water hardness (Ca and Mg competition) and complexation with dissolved organic carbon (see section 7.1.2.), similar to concepts established for divalent metals. However, the formation of insoluble indium hydroxide precipitates suggested that particle interactions with biota may occur and the combined effects with In3+ions cannot be unambiguously determined. As such, and due to dissolved measurement limitations described above, the total measured indium concentration is used for all ecotoxicological interpretations.

The speciation of indium in the aquatic compartment is of high complexity and depends on abiotic factors, such as pH, organic matter content (see further discussion below), water hardness, etc. It is assumed that speciation is very relevant for the migration of indium through the water column, distribution among its truly dissolved and non-dissolved forms, and for the uptake of indium by some aquatic organisms.

Figure 1. Distribution diagram of indium hydrolysis: see Chrysikopoulos and Kruger, 1986

 

Figure 2. Precipitation region of In3 +: see Chrysikopoulos and Kruger, 1986

 

Environmental distribution in soil and sediment

Although information on indium partitioning in benthic and terrestrial environments is sparse, some inferences can be made concerning preferential binding to mineral and organic surfaces. First, metal-sulfide complexation has been shown to influence sediment bioavailability by forming sparingly insoluble compounds that do not contribute to toxicity (DiToro, 1990).  Again, indium has a much higher affinity than most other metals, suggesting removal from porewater compartments in both soil and benthic matrices (Table 1). When solubility constants for metal-sulfide complexes are compared, it can be assumed that indium will be preferential bound in anoxic sediments, by many orders of magnitude, relative to other metals (Table 1). Second, comparison among metals for binding with common organic acids (e.g., EDTA or NTA; carboxylic acids) can provide a relative approximation for preferential binding to organic carbon. Again, indium has a much higher affinity than most other metals, suggesting removal from porewater compartments in both soil and benthic matrices (Table 1).

The average calculated partitioning constant (Log Kd) from two standard terrestrial bioassays (see section 7.2) was estimated at 4.23, providing confirmation of this predicted behaviour in soil. This is nearly ten times stronger than affinities reported for other metals (Table 1). Furthermore, the systematic relationship between soil and sediment (or suspended matter) partitioning constants (Kd) suggests indium would have strong affinity for other mineral/organic surfaces. Although several factors remain to be investigated for indium (range of sediment/soil quality, ageing, leaching, and chemical source), there are strong physicochemical indications that indium will be sequestered into non-bioavailable complexes within both sediment and soil compartments.

Table 1. Metal partitioning coefficients among (surrogate) sediment and soil constituents.

	Sulfide Solubilty (20°C)^a	EDTA Stabilty (25°C)^a	Soil/ Pore Water^b	Sediment/ Pore Water^b	TSS/Water^b
Element	-log Ksp	log K	log Kd (l/kg)	log Kd (l/kg)	log Kd (l/kg)
Fe	18 (Fe[II])	25.1 (Fe[III])	-	-	-
Ni^2+	19	18.6	2.9	3.9	4.4
Co^2+	21	16.3	2.1	3.1	4.8
Zn^2+	25	16.5	2.7	4.1	5.0
Cd^2+	27	16.5	2.7	3.3	4.9
Pb^2+	28	18	3.7	4.6	5.7
Cu^2+	36	18.8	2.5	3.5	4.7
Ag^+	50 (2AgS)	7.2	2.6	3.6	5.2
Hg^2+	54	21.8	3.6	4.9	5.3
In^3+	69 (2In3S)	24.9	4.23^c	5.1^d	5.9

TSS = Total Suspended Solids

^aNIST, 2004

^bPartition coefficients for metals other than indium from Allison and Allison (2005). Suspended solids partitioning estimates for indium determined from linear regression and reported partition coefficients for indium and other metals.

^cFrom VITO 2012a,b.

^dGeometric mean of 41 observations from ARCHE (2012) and Tessier et al. (2014).

 

Indium fate in soil and sediment has also been investigated experimentally. In soils, VITO (2012a,b) measured indium concentrations in bulk soil and soil pore water from two standard terrestrial bioassays (see section 6.3).

Two reliable soil-soil water partitioning coefficients were derived; a value of (log Kd) 4.95 was determined using a soil from an earthworm growth study and a value of 3.48 was determined using a soil from an arthropod bioassay. The average of these calculations resulted in a value of log Kd =4.23.

In sediments, nearly 70 samples from monitoring studies on the Rhine River and several Canadian Shield lakes were available for calculating partition coefficients for sediment/pore water. ARCHE (2012) collected water and sediment samples from the Rhine River along the entire watershed reach. Of the 28 co-located samples taken, 20 had measurable indium in river water (ranged 0.01-0.4 µg/L) and only one had measurable indium in sediments (0.2 mg/kg dw). As such, one high quality sediment/water partition coefficients from Rhine River was calculated (log Kd = 4.3).

A study by Tessier et al. (2014) characterized the vertical profile of indium in deep sediments collected from four Canadian Shield lakes. Using highly sensitive techniques, pore water and sediment concentrations were reported from 0.05-50 ng/L and 0.1-5 µg/g dw, respectively. Although water and sediment concentrations differed significantly among sites, partition coefficients were relatively consistent, ranging from (log Kd) 3.7-6.5 and averaging 5.1 (see figure 1 in Van Genderen 2017). When only samples collected at the sediment-water interface (top 2 cm; n=8) were considered, the average partition coefficient increased slightly (log Kd = 5.2). The geometric mean value of 5.1., based on the 41 observations from ARCHE (2012) and Tessier et al. (2014)) is taken forward in the assessment.

Although no experimental evidence exists to calculate an indium partition coefficient for suspended matter (TSS)/water, values can be estimated using the calculated values for soil and sediment partitioning and empirical relationships for other metals. A review of metal partitioning by Allison and Allison (2005) was used to determine relationships between partitioning for suspended matter/water and soil/pore water or sediment/pore water for various metals (see Figure 2 in Van Genderen 2017). Based on extrapolation from empirical relationships, an indium suspended matter/water partition coefficient was estimated (log Kd = 5.9; Figure 2).

Summary:

Despite a limited amount of data in the literature concerning the fate of indium in soils and sediments, several lines of evidence suggest that indium will preferentially partition onto organic and or mineral solid phases, relative to other metals. There are also strong physicochemical indications that indium will be sequestered into non-bioavailable complexes within both sediment and soil compartments. Furthermore, the magnitude and sequence of partition coefficients for indium are consistent with what would be predicted from relative binding to other ligands (Table 1).

Soil -  Two reliable soil-soil water partitioning coefficients were derived from soil ecotoxicity bioassays with earthworms and plants. The average of these calculations resulted in a value of log Kd =4.7.  The calculated value is one to two orders of magnitude greater than the average for any other metal (Table 1); however, several metals have reported soil/pore water partition coefficients as high as log Kd = 5.0 (Allison and Allison, 2005).

Sediment - One reliable sediment-water partition coefficient was derived from the Rhine River (log Kd =4.3). Forty reliable sediment-pore water partition coefficients were derived for samples collected from four Canadian Shield lakes. Although water and sediment concentrations differed significantly among sites, partition coefficients were relatively consistent, ranging from (log Kd) 3.7-6.5 and averaging 5.1. The geometric mean value of 5.1., based on all 41 observations is taken forward in the assessment.

The calculated values are slightly higher than most metals, on average (Table 1; Allison and Allison, 2005).

Suspended Matter ·     A suspended matter (TSS)-water partition coefficient was estimated from a calculated value for soil-pore water (log Kd = 4.7) and empirical relationships for other metals. Based on extrapolation from empirical relationships, an indium suspended matter/water partition coefficient was estimated (log Kd =5.9).