In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 8

Table 1.1. Some commercially available arsenic sorbents.

Name of the
product Company Material type

Adsorpas Technical University of Berlin Granular iron hydroxide - α -FeOOH

ARM 300 BASF Iron oxides (hematite, α -Fe 2 O 3 )

G2 ADI International Modified iron; diatomites covered with iron

hydroxides

SMI III SMI Iron/sulfur

GEH Wasserchemie GmbH Granular iron hydroxide; Fe(OH) 3 and FeOOH

(akaganeite, β -FeOOH)

Bayoxide E33 Bayer AG Iron oxide; 90% goethite ( α -FeOOH)

1.3.2.3 Barriers with mixtures of iron hydroxides and activated alumina
GFH (granular ferric hydroxide) and GFO (granular ferric oxide) are excellent As sorbents. GFH
is prepared from a solution of ferric chloride and precipitation with sodium hydroxide; the material
is washed, centrifuged and granulated at low pressure (Driehaus et al ., 1998). Aqueous silicates
interfere and reduce the removal capacity of As(V) by GFH.
Table 1.1 shows some other iron-based sorbents (IBS) available in the market. The sorption
is of chemical origin and thus it is irreversible. IBS may be used either in fixed bed columns
identical to those used with activated alumina (AA) or in PRBs.
The affinity of these sorbents for As under natural pH conditions is much higher than that
of AA. This fact allows IBS to treat a much higher total bed volume without pH adjustment.
Nevertheless, just as with AA, the best behavior of IBS is attained at low pH. In columns, the
recommended operational conditions imply a residence time of five minutes and a hydraulic
charge of 0.2 (m 3 min − 1 )m − 2 . Phosphates compete with As(V) for the sorption sites, and
each increment of 0.5 mg L − 1 above the threshold of 0.2 mg L − 1 reduces the sorption capacity

by 30%.
Ipsen et al . (2005a), in a research on the identification and testing of materials that could
be used in PRBs, mention that the best sorbents are materials based on akaganeite/ferrhydrite
( β -FeOOH/Fe 2 O 3 · 5H 2 O). The commercial product GEH presented a loading capacity of 36 g
As kg − 1 , showing a better behavior than other tested materials. The minerals of the goethite type
( α -FeOOH), improved by addition of titanium, showed clearly a less loading capacity.
Jang et al . (2007) showed that amorphous hydrous iron oxides embedded in natural diatomite
(aluminum silicates) were more efficient than elemental iron for As removal.
Silva et al . (2009) studied hydrous iron oxides loaded on activated carbon (HFO/AC) and the
commercial sorbent ARM 300 as possible As sorbent materials for application in PRBs. The last
material presented the largest loading capacity, 49 ± 20gkg − 1 , followed by HFO, 38 ± 2gkg − 1 ,

0.5gkg − 1 .
No barriers based on AA or IBS have yet been constructed so far.

and finally by HFO/AC, only 5.5 ±

1.3.2.4 Composite barriers
The fourth type of conception for a PRB is the composite or multiple functional barrier. Using this
design, a first barrier is located upstream the source of As contamination; the reactive material
is organic matter, usually compost or wood chips, promoting the biological reduction of sulfates,
with subsequent precipitation of metal sulfides. Downstream to the contamination source, a
second conventional barrier is built, for instance, with Fe(0). As first step, the sulfides released
by the organic matter can start a reductive dissolution of the As loaded into the Fe and Mn oxides
and hydroxides. This causes an increase in the As concentration over a short period of the As
elution and reduces the remediation time. On the other hand, the precipitation of As sulfides
reduces the As emissions, thus increasing the lifetime of the downstream barrier. Precipitations