The use of solid phosphate absorbers to reduce phos-
phate concentration has been a big step forward for the
reef aquarium because it prevents the various harmful
and undesirable effects of high phosphate, which are
often observed in older aquariums. In a 2009 issue of
CORAL, Daniel Knop talked about using a lanthanum
solution in very large systems and demonstrated con-
vincingly that it is a very efficient, simple, and cost-
effective method of controlling phosphate in the reef
aquarium. However, in that article he said, “I strongly
advise hobby aquarists against doing experiments with
lanthanum chloride; I recommend that they wait until
the aquarium industry has developed appropriate meth-
ods.” I must confess that, rather than stopping me from
trying a new strategy, a statement like this piques my
curiosity and triggers my desire to test the method and
see if it can become standard in the hobby.
TOXICITY
Lanthanum has a very low toxicity. As Knop reported
in his article, lanthanum carbonate is used in human
medicine under the trade name Fosrenol to reduce phos-
phate levels in the blood of dialysis patients. Patients
who respond to therapy with lanthanum usually achieve
an acceptable phosphate serum level when taking 1,500
to 3,000 mg of lanthanum per day. However, it is impor-
tant to determine whether a given substance accumu-
lates in the body of the recipient. Animal experiments
have shown this to be the case for lanthanum; enrich-
ment was observed in the gastrointestinal tract, the liver,
and the bones. Lifelong studies in healthy ani-
mals, however, did not indicate any danger to
humans from the use of Fosrenol. In addition,
lanthanum is eliminated from the tissues af-
ter discontinuation of the drug.
To find out more about the toxicity of lan-
thanum, I investigated the lethal dose, or LD 50
(the concentration at which 50 percent of the
test animals die), an important parameter for
assessing the toxicity of a substance. The acute
toxicity of lanthanum chloride when taken
orally by rats is very low, with an LD 50 of 2.370
to 4.184 mg per kilogram of body weight. If
lanthanum chloride is injected subcutaneous-
ly, the LD 50 is >1,000 mg/kg body weight for
frogs, 3.500 mg/kg for mice, and >500 mg/kg
for rats (Afsar & Groves 2009).
For us aquarists, it was crucial to know
the toxicity of lanthanum to aquatic organ-
isms. Such studies were published by Yasseri
(2008). The parameter used in this case was
an EC 50 value, which describes the concentra-
tion in which a half-maximal effect is observed in phar-
macology and toxicology. If the maximum effect were
the death of certain organisms, a dose-response curve
would have to be determined experimentally with a
group of these organisms to define the half-maximal ef-
fect. In the Yasseri study, the following EC 50 values were
given: 150 mg/L for fish eggs, 103 mg/L for water fleas,
and 37 mg/L for bacteria.
Another study investigated the toxicity of lanthanum
to the embryos of the sea urchin Paracentrotus lividus
(Oral et al. 2012). A concentration of 1.38 mg/L in the
experimental tank did not lead to the death of the sea
urchin embryos, but there were developmental defects
in all cases. When the concentration of lanthanum
was reduced to 0.138 mg/L, only 10 percent of the em-
bryos examined had defects. The same authors reported
a bioaccumulation study of Chlamys varia scallops by
Bustamante & Miramand (2005). A 26-fold enrichment
of lanthanum in the digestive glands occurred in rare-
earth–polluted waters.CLAY EARTH WITH LANTHANUM IONS
For some time, lanthanum has been used on a large
scale in a commercial product called Phoslock to pu-
rify eutrophic waters. Phoslock consists of benton-
ite clay in which the sodium and/or calcium ions are
exchanged for lanthanum, allowing the lanthanum
ions to bind the reactive phosphate in eutrophic wa-
ters. According to the manufacturers, when used as
a suspension, it binds up to 95 percent of the reac- DR. DIETER BROCKMANTypical degeneration of Favites and Fungia corals
accompanied by nuisance algae growth due to high
levels of inorganic nutrients.