Metal(loid)s immobilization in soils of Lebanon using municipal solid waste compost: Microbial and biochemical impact

Organic based amendments can be used to reduce metal(loid)s mobility in polluted soils and alleviate possible
toxicity phenomena towards soil microbiota. Therefore, the objectives of this study were to evaluate the influence
of municipal solid waste compost (MSWC) on the mobility of metal(loid)s in different contaminated soils
and to determine the amendment effect on selected soil biochemical and microbial features. To this aim, MSWC
was added at two rates (1 and 3% w/w) to different soils sampled in distinct regions of Lebanon (Halba and
Terbol, i.e. HS and TS respectively) polluted with metal(loid)s such as As (total concentrations between 31 and
54 mg·kg−1) and Sb (between 93 and 120 mg·kg−1), and trace metals such as Ni, Cr and Zn. After two months of
soil-amendment contact, compost addition at both rates significantly increased the non-extractable fractions of
As, Sb, Ni, Cr and Zn (e.g. +25% of residual Sb in TS-MSWC 3% with respect to untreated TS), while decreased
the environmentally-relevant water soluble and exchangeable fractions of all the metal(loid)s. This effect was
more relevant for both soils when 3% MSWC was applied. The amendment addition similarly affected the
community level physiological profile (Biolog CLPP) of the respective soil culturable microbial communities,
which however showed an opposite trend in the two soils. In particular, compost addition increased the metabolic
potential and catabolic versatility of the HS microbial community (compared to the untreated control),
while the same proxies were significantly reduced in the amended TS soil. Differently, soil dehydrogenase
(DHG), β-glucosidase (β-GLU) and urease (URE) activities were significantly enhanced in both soils treated with
compost (e.g. + 17, 45 and 27% for DHG, URE and β-GLU in TS-MSWC 3% with respect to untreated TS). The
results obtained showed that the addition of MSWC, particularly at the 3% rate, was effective at fixing the metal
(loid)s present in both soils, had a substantial influence on the structure of the soil microbial communities and a
positive effect on soil functionality. Taken together the results presented suggest that MSWC can be used as an
alternative and environmental friendly amendment for the in situ remediation of metal(loid)s-polluted soils.