Reaction mass of aluminium chloride, iron dichloride, magnesium chloride and manganese dichloride

Adsorption / desorption

Description of key information

Iron is considered Immobile or non-mobile, log Kp sed 5.08 L/kg and log Kp susp 2.34 L/kg are used.

Manganese mobility is considered low , it is assumed slightly mobile, log Kp sed 3.19 L/kg and log Kp susp 1.45 L/kg are used.

Aluminium mobility is considered in the medium range, it is moderately mobile, log Kp susp 2.67 is used

Additional information

It is considered inappropriate to use the Kow and Koc concept for inorganic compounds as outlined in the waiving argumentation. Adsorption/desorption as a partitioning process associated with organic carbon is not a relevant endpoint for these inorganic salts. There is no existence of single values to fulfil this endpoint. However, in water, there is a rapid formation and deposition of iron and manganese oxides and other salts in sediments. Iron species are rapidly removed from solution as insoluble precipitate, and any direct impact of dissolved iron ion on the aquatic environment will be reduced. The bioavailability of aluminium depends more on the geochemical situation but is characterized by equilibration between the dissolved fraction a large soil and/or sediment reservoirs making aluminium the most frequent metal in the biosphere.

Some information derived from monitoring of water and corresponding sediment or suspended matter is available. The reported Kd reflects the total metal concentration ratio in equilibrium for all species under environmental conditions, where 12 °C are assumed. It is likely that the data reflect the environmental speciation behaviour.

Iron

Roychoudhury & Starke (2006) examined the partitioning of trace metals between surface water and sediments and their fate within the sediments of a river in South Africa in which mine water was drained. A mean log Kp (solids-water in sediment) value of 5.08 L/kg for 20 locations, range: 4.38 to 5.81 is reported. Li (2000) lists a world average reference value of 6.08 L/kg

Veselý et al (2001) studied partitioning between solids (suspended matter) and water by filtration and dialysis in situ in Czech freshwaters. Field-based distribution (partition) coefficients, Kd, between suspended particulate matter and filtrate (‘dissolved’ fraction) were derived. The median Kd for samples from 54 rivers in 119 localities was 2.34 L/kg.

Environmental fate is dominated by abiotic and physico-chemical processes, including precipitation and settling. Iron reactions in soil comprise precipitation, hydrolysis, complexation, and redox processes. The iron released precipitates readily as oxides and hydroxides. It substitutes for Mg and Al in other minerals and often form complexes with inorganic ligands. Iron occurs mainly in the forms of oxides and hydroxides either as small particles or in association with the surfaces of other minerals. Soil iron shows a great affinity to form mobile organic complexes and chelates which are important compounds responsible for the migration of iron between soil horizons and the leaching of iron from soil profiles and also supply of iron to root of plants (Kerndorff & Schnitzer 1980).

- Precipitation of iron oxides during dredging operations has been reported to decrease dissolved concentrations of Cd, Cu, Pb and Zn.

- Iron (and manganese) are used as scavengers in wastewater treatment and may be very effective in retarding the migration of pollutants in the subsurface.

- Deep-sea nodules, concretions of manganese and iron oxides growing on the ocean floor, are known to concentrate trace elements, such as Co, Ni, Zn and Pb from seawater.

- In soils, various trace elements are concentrated by iron oxides, including Zn, Pb, Mn, Ni, Cu, Co, V, Mo and Cr.

- The most widely observed sorption capacities of iron oxides are those for phosphates, molybdates and selenites.

- Iron oxides also reacts with carbonates in soil systems.

- Fe (III) can be incorporated in hydrated phosphates and Fe (II) reacts with sulphur to form stable minerals pyrite and jarosite.

(Windom HL 1973, Drever 1982, Khalid et al 1977, Kabata-Pendias A, Pendias 1984)

Concerning data interpretation of the measured Kd (and the resulting Koc) in sediments, the available data clearly indicate that the iron species have to be considered as “immobile” (Kd or Koc > 5000) or “non-mobile” (Kd or Koc > 4000) in soils or sediments according to the scales of McCall et al (1981) or Hollis (1991) respectively. Nonetheless the sorption to suspended matter is less strong.

Manganese

Roychoudhury & Starke (2006) examined the partitioning of trace metals between surface water and sediments and their fate within the sediments of a river in South Africa in which mine water was drained. A mean log Kp (solids-water in sediment) value of 3.19 L/kg for 20 locations, range: 1.63 to 4.55 is reported. Li (2000) lists a world average reference value of 5.11 L/kg

Veselý et al (2001) studied partitioning between solids (suspended matter) and water by filtration and dialysis in situ in Czech freshwaters. Field-based distribution (partition) coefficients, Kd, between suspended particulate matter and filtrate (‘dissolved’ fraction) were derived. The median Kd for samples from 54 rivers in 119 localities was 1.45 L/kg.

Based on the Kd of 1.45 L/kg reported from Veselý et al (2001) it is assumed that manganese is transported adsorbed to suspended particular matter or suspended sediments in rivers. The finding is supported by Malm et al (1988) who found most of the manganese from metallurgical and chemical plants along the Paraiba do Sul-Guandu River, Rio de Janeiro, Brazil, bound to suspended particles.

Like iron manganese is used as scavenger in wastewater treatment and may be very effective in retarding the migration of pollutants in the subsurface. WHO (2004 & 2005) reviewed the adsorption information for manganese with the following result: The tendency of soluble manganese compounds to adsorb to soils and sediments can be highly variable, depending mainly on the kation exchange capacity and the organic composition of the soil (Hemstock & Low 1953, Schnitzer 1969, McBride 1979, Curtin et al 1980, Baes & Sharp 1983, Kabata-Pendias & Pendias 1984). Laxen et al (1984) and Neal et al (1998, 2000) observed a positive correlation between dissolved manganese concentrations and suspended sediment levels for a number of rivers in the United Kingdom.

Concerning data interpretation of the measured Kd in sediments, the available data indicate that the manganese species mobility has to be considered as “low” (500 = Kd = 2000) or “Slightly mobile” (4000 = Kd = 500) in soils or sediments according to the scales of McCall et al (1981) or Hollis (1991) respectively. Nonetheless the sorption to suspended matter is less strong.

• Baes CFI, Sharp RD (1983). A proposal for estimation of soil leaching and leaching constants for use in assessment models. Journal of Environmental Quality 12:17–28.
• Curtin D, Ryan J, Chaudhary RA (1980). Manganese adsorption and desorption in calcareous Lebanese soils. Journal of the Soil Science Society of America 44:947–50.
• Hemstock GA, Low PF (1953). Mechanisms responsible for retention of manganese in the colloidal fraction of soil. Soil Science 76:331–43.
• Kabata-Pendias A, Pendias H (1984). Trace elements in soils and plants. Boca Raton, FL, CRC Press.
• Laxen DPH, Davison W, Woof C (1984). Manganese chemistry in rivers and streams. Geochimica et Cosmochimica Acta 48:2107–2111.
• Malm O, Pfeiffer WC, Fiszman M, Azcue JM (1988). Transport and availability of heavy metals in the Paraiba do Sul-Guandu River system, Rio de Janeiro State, Brazil. Science of the Total Environment 75:201–9.
• McBride MB (1979). Chemisorption and precipitation of Mn2+ at CaCO3 surfaces. Journal of the Soil Science Society of America 43:693–8.
• Neal C, Robson AJ, Wass P, Wade AJ, Ryland GP, Leach DV, Leeks GJL (1998). Major, minor, trace element and suspended sediment variations in the River Derwent. Science of the Total Environment 210–211:163–172.
• Neal C, Jarvie HP, Whitton BA, Gemmell J (2000). The water quality of the River Wear, north-east England. PMID: 10847159 Science of the Total Environment 251–252:153–172.
• Schnitzer M (1969). Reactions between fulvic acid, a soil humic compound and inorganic soil constituents. Soil Science Society of America Proceedings 33:75–81.
• WHO World Health Organization (2004 and 2005). Manganese and its Compounds: Environmental Aspects. Concise International Chemical Assessment Document 63, Corrigenda published by 12 April 2005 have been incorporated. Self-published, Geneva, Switzerland

Aluminium

A number of chemical factors can alter the speciation of aluminium, thereby affecting the extent of adsorption and desorption of aluminium on suspended particles, as a result aluminium speciation is complex and changes significantly with changes in pH. In the absence of organic matter, Al3+is the predominant aluminium species at low pH (less than 5.5). As pH increases above 5.5, aluminium-hydroxide complexes formed by hydrolysis become increasingly important and dominate aqueous aluminium speciation. The presence of a moderate amount of organic matter in soft water (2 mg/L as dissolved organic carbon or DOC is used here) results in organically complexed aluminium being the dominant aluminium form when the pH is between 4 and 7. Above pH 7, anionic aluminium hydroxide predominates, although organically complexed aluminium remains the second most important form of dissolved aluminium.

Aluminium speciation can also include the formation of insoluble polymeric aluminium-hydroxide species. Polymeric aluminium hydroxides tend to exist as amorphous colloids and solid phases. The kinetics of this transformation to polymeric species, including aqueous colloids and amorphous precipitates, depends on many factors but typically occurs over a time scale of minutes to hours. Subsequent formation of more crystalline solid phases may take additional time, as much as a few days. As a result of these relatively slow transformations from dissolved to crystalline forms of aluminium, there is a considerable range of solubilities that have been reported for aluminium hydroxide solid phases (Lindsay and Walthall 1996).

As a result of this dynamic chemistry, the amount of aluminium associated with suspended particles is dependent on the chemical conditions. Factors that are known to affect aluminium speciation, such as pH and dissolved organic carbon (DOC), are also known to affect adsorption and desorption from particle surfaces. To illustrate this further, the amount of aluminium associated with suspended particles was estimated by chemical simulation that included aqueous aluminium speciation (inorganic and organic), aluminium solubility, and complexation by natural organic matter (NOM). For these simulations a NOM concentration of 4 mg/L (2 mg/L as DOC) and a total suspended solids (TSS) concentration of 1 mg/L were chosen to represent a reasonable lower bound for the range of values of these substances that would be expected in the environment. Suspended particles were assumed to be composed primarily of silica (80%) with a small amount of clay (10%) and particulate organic matter (10%). Aluminium concentrations were set to the maximum allowable by solubility with amorphous gibbsite at a temperature of 20 °C. Under these conditions, the amount of aluminium bound to particles as a result of surface complexation (i.e. adsorption) was pH dependent, but was typically less than 8% of the total aluminium at pH 6, and was further reduced to below 1% at pH values above 7. This distribution was similar in both soft and hard waters. The corresponding Log Kd values for this distribution range between 3 and 5. Very similar results were obtained with higher DOC concentrations of 4 mg/L. Veselý et al (2001) studied partitioning between solids (suspended matter) and water by filtration and dialysis in situ in Czech freshwaters. Field-based distribution (partition) coefficients, Kd, between suspended particulate matter and filtrate (‘dissolved’ fraction) were derived. The median Kd for samples from 54 rivers in 119 localities was 2.67 L/kg.

Concerning data interpretation of the Kd as a measure for sorption to suspended matter the mobility of aluminium has to be considered as “Medium” (500 = Kd = 150) or “Moderately mobile” (499 = Kd = 75) according to the scales of McCall et al (1981) or Hollis (1991) respectively.

Magnesium and Chlorine

Magnesium is considered very mobile as it was detectable in the water phase only (Veselý et al 2001). The Chlorine shows approximately no retention in soils (Bohn et al 1979). Concerning data interpretation mobility of Magnesium and Chlorine has to be considered as Very high (50 = Kd = 0) or “Very mobile” (Kd < 15) according to the scales of McCall et al (1981) or Hollis (1991) respectively.