Slags, ferrous metal, blast furnace

Adsorption / desorption

Description of key information

High Kd values for trace elements indicate that the distribution of trace elements between slags (solid phase) and their leachates (liquid pahse) is almost exclusively on the side of the solid phase, and very low mobility of trace elements is predicted. 

Additional information

K_d values of trace elements in soils

A literature review was done on soil−liquid partitioning coefficients (K_d) of several trace elements of toxicological relevance. Although the reported values scattered, mobility and bioavailability of the elements could be predicted. The kd values especially for the metals cadmium, copper, lead, nickel, and zinc exhibited a relationships to variations in the pH of the soil solution, the content of metals and organic matter of the soil (SOM). Multiple linear regressions showed highest correlations to linear regression with pH or with pH and log of one of the other parameters. Solubility of trace metals was interpreted as competitive adsorption of metal cations and hydrogen ions.

Arithmetic Means of K_d values (L/kg) of elements in soil/water

As 13119

B 160

Ba 3434

Cd 2869

Cr 14920

Cu 4799

Hg 8946

Mo 36

Ni 16761

Pb 171214

Se 43937

Sr 137

Zn 11615 (Sauvé et al. 2000)

K_d values of trace elements of slags

Soil-to-water partition coefficients K_d of individual trace elements occurring in slag were calculated from their concentration in the solid slag, divided by their concentrations in leachate. In total, slag from 58 steel works with blast furnaces, basic oxygen furnaces (converters) and/or electric arc furnaces, were examined for leachability at acidic and neutral pH values.

The calculated K_d values are higher than 1000 L/kg with few exceptions due to calculation procedure using half the limit of detection as an estimate for concentration in leachates. These calculated results comprise arsenic and tin in blast furnace slag (K_d= 300 and 500 L/kg, for arsenic and tin, respectively) and mercury in the EAF slag (K_d= 900 L/kg). With very low levels both in the slag and in the leachate, the following K_d's were calculated: 700 L/kg for molybdenum in BOS, 535 L/kg for chromium-VI in EAF, and 819 L/kg for arsenic in EAF.

The K_d values of different slags were similar. Their differences between the slags did not exceed one order of magnitude.

The high K_d values suggest that the trace elements are very tightly bound into the matrix. Leaching of trace elements is extremely low and transport in soil will be very slow (Proctor et al. 2000).

These results were confirmed by data on total trace element levels in slags and the corresponding leaching data taken from a report of the German Oekoinstitut. The K_d values (L/Kg) calculated for ABS are

As 2500

Pb 3500

Cd 1667

Cr(total) 10000

Cu 4000

Ni 7143

Hg 3500

Zn 7500

cyanide(total) 250

The Kd values (L/kg) for GBS are

As 3214

Pb 5250

Cd 2000

Cr(total) 14400

Cu 6000

Ni 10000

Hg 4200

cyanide(total) 300

There are no relevant differences in the adsorption/desorption properties of the 2 subtypes of blast furnace slag in regard to the K_d of the trace elements tested (data taken from Oekoinstitut 2007).

To determine the kd values of trace elements of toxicological relevance in the slags ABS, BOS, and EAF C, existing concentration data of the BfG of trace elements in solid slag and natural rocks were compared with their respective concentrations in leachates of slags and natural rocks.

The kd values of Hg and Tl could not be determined, because no data had been measured in leachates. The K_d values of As and Cd are low (e.g. >20 L/Kg) because these elements (and Hg and Tl) are not present in slags and basalt at relevant levels. The same is most likely correct for Pb. For Ni (< 700, >4000 L/kg), Cu (> 9400, > 6000 L/kg), Zn (> 40000, >6600 L/kg) and Cr (> 800000, > 10000 L/kg) high K_d values have been obtained, suggesting a very tight binding of these elements into slag and basalt, respectively (data taken from BfG 2008).

The environmental behaviour of slags was evaluated from the total content and solubility of analytes in the slags. A simple re-evaluation of the existing data was done to determine the retention potential of slags for trace metals and other analytes. All K_d values obtained were very high suggesting that the analytes examined are effectively retained by slags. The solid/liquid adsorption coefficients Kd of both ABS and GBS were high (L/kg): ABS: Ca 1312, Mg 97500, Fe 66110, Mn 329220, SiO₂ 1684; GBS: Ca 10978, Mg 19500, Mn 109750, SiO₂ 7424 (data taken from LECES 1991).

Kd values of ABS and GBS in comparison with natural rocks

To elucidate the risk of the use of slags in the environment in comparison to natural rock, the 2 types of leaching experiments were performed: Ordinary availability tests consisting of two leachings at neutral and acidic pH (NT ENVIR 003) and oxidised availability tests at fully oxidising conditions by addition of hydrogen peroxide (NT ENVIR 006, 1999). 9 different types of natural rock material were used for comparison.

The leaching behaviour of natural rocks and blast furnace slags was very similar. Most of the metals exmined were less soluble from slags than from natural rock. As both total content of the trace elements of toxicological relevance, as well as their solubility under extreme environmental conditions, were supplied, K_d values can be derived (in the report inverse K_d values are given as the percentage of the total released into leachate). In general, these Kd were lower than typical Kd values of these elements in slags. The environmental relevance of the findings was clarified by comparison with natural rocks. The slags released less trace metals than the natural rocks, suggesting that their use in the environment poses no hazard (Tossavainen 2002).