vermicomposting of biosolids experimentally contaminated with tetracycline
revealed that the concentration of 100 mg/kg had a stimulating effect on earthworm
growth and organic matter decomposition, whereas that higher concentrations
(500 and 1,000 mg/kg) led to a significant decrease of the decomposition process
and to the emergence of antibiotic-resistant genes, thus compromising the quality
and environmental safety of the final vermicompost [127]. Similarly, degradation of
oxytetracycline and its main metabolites in chicken manure mixed with shredded
paper waste was monitored in a co-composting system, which consisted in a first
thermophilic composting phase followed by vermicomposting. Results from that
study revealed that the additional phase of vermicomposting increased the degradation of oxytetracycline and its metabolite 4-epi-oxytetracycline in the feedstock
containing a C:N ratio of 40 [128]. Despite these studies, there are still many
unknowns on the efficiency of vermicomposting in reducing the concentration and
toxicity of APIs and their metabolites. Furthermore there is no data available on the
microorganisms and enzyme activities implied in API biodegradation, so that the
vermicomposting process can be externally modified to facilitate removal of APIs.
Earthworms can accumulate biosolids-bound APIs. For example, E. fetida accumulated around 20% of ciprofloxacin and 40% of azithromycin present in soils
amended with anaerobically digested biosolids which were contaminated with these
antibiotics [26]. Although the study suggests ecological implications of the moderate
bioaccumulation of APIs by earthworms, as these organisms may introduce APIs in
food webs, their bioaccumulation capacity can be also regarded as an opportunity for
removing APIs during biosolids vermicomposting.
6 Pharmaceutical-Contaminated Soils (In Situ
Vermiremediation)
Soil bioturbation by earthworms has been exploited as a bioremediation strategy
[129]. Earthworms are able to facilitate biodegradation of organic contaminants via
three processes: (1) stimulating soil microorganisms, which may co-metabolise
pollutants; (2) mobilising contaminants entrapped in soil organomineral complexes,
thus rendering them bioaccessible to microbial biodegradation; and (3) altering the
soil physicochemical properties (e.g. pH), which may contribute to contaminant
degradation. Besides these external degrading processes, the gastrointestinal tract of
earthworms contributes to contaminant degradation by the action of the gut symbionts and digestive enzyme secretion [47, 130]. Many studies have shown that
earthworm activity in soils contaminated by environmental pollutants such as
pesticides, polycyclic aromatic hydrocarbons (PAHs) or polychlorinated biphenyls
(PCBs) reduces the initial concentration of these organic pollutants [47]. However,
most of these studies have been performed under controlled conditions of laboratory
(microcosm), and the real impact of earthworms in soil persistence of contaminants
requires field validation [131]. Nevertheless, earthworm activity may also have no
354
J. C. Sanchez-Hernandez
revealed that the concentration of 100 mg/kg had a stimulating effect on earthworm
growth and organic matter decomposition, whereas that higher concentrations
(500 and 1,000 mg/kg) led to a significant decrease of the decomposition process
and to the emergence of antibiotic-resistant genes, thus compromising the quality
and environmental safety of the final vermicompost [127]. Similarly, degradation of
oxytetracycline and its main metabolites in chicken manure mixed with shredded
paper waste was monitored in a co-composting system, which consisted in a first
thermophilic composting phase followed by vermicomposting. Results from that
study revealed that the additional phase of vermicomposting increased the degradation of oxytetracycline and its metabolite 4-epi-oxytetracycline in the feedstock
containing a C:N ratio of 40 [128]. Despite these studies, there are still many
unknowns on the efficiency of vermicomposting in reducing the concentration and
toxicity of APIs and their metabolites. Furthermore there is no data available on the
microorganisms and enzyme activities implied in API biodegradation, so that the
vermicomposting process can be externally modified to facilitate removal of APIs.
Earthworms can accumulate biosolids-bound APIs. For example, E. fetida accumulated around 20% of ciprofloxacin and 40% of azithromycin present in soils
amended with anaerobically digested biosolids which were contaminated with these
antibiotics [26]. Although the study suggests ecological implications of the moderate
bioaccumulation of APIs by earthworms, as these organisms may introduce APIs in
food webs, their bioaccumulation capacity can be also regarded as an opportunity for
removing APIs during biosolids vermicomposting.
6 Pharmaceutical-Contaminated Soils (In Situ
Vermiremediation)
Soil bioturbation by earthworms has been exploited as a bioremediation strategy
[129]. Earthworms are able to facilitate biodegradation of organic contaminants via
three processes: (1) stimulating soil microorganisms, which may co-metabolise
pollutants; (2) mobilising contaminants entrapped in soil organomineral complexes,
thus rendering them bioaccessible to microbial biodegradation; and (3) altering the
soil physicochemical properties (e.g. pH), which may contribute to contaminant
degradation. Besides these external degrading processes, the gastrointestinal tract of
earthworms contributes to contaminant degradation by the action of the gut symbionts and digestive enzyme secretion [47, 130]. Many studies have shown that
earthworm activity in soils contaminated by environmental pollutants such as
pesticides, polycyclic aromatic hydrocarbons (PAHs) or polychlorinated biphenyls
(PCBs) reduces the initial concentration of these organic pollutants [47]. However,
most of these studies have been performed under controlled conditions of laboratory
(microcosm), and the real impact of earthworms in soil persistence of contaminants
requires field validation [131]. Nevertheless, earthworm activity may also have no
354
J. C. Sanchez-Hernandez