The mechanisms of heavy metal removal with biochar
amendment might be attributed to electrostatic interactions,
precipitation and other reactions according to current research
(46e49). With the incorporation of biochar, there are more
negative charge on soil surface due to the decreasing zeta potential
and increasing CEC (46). Therefore, the electrostatic
attraction between heavy metals with positive charge and soil will
be enhanced. In relation to precipitation, the markedly increased
soil pH arising from biochar amendment may lead to decreased
mobilization of heavy metal. Various oxidates, phosphate or carbonate
would be formed in different conditions, for instance, a
new precipitate was solely observed on Pb-loaded SDBC (sludge
derived biochar) as 5PbO$P2O5$SiO2 (lead phosphate silicate) at
initial pH 5 (47). Some other more complex mechanisms may also
come into play during biochar and heavy metal interactions.
Because there are many functional groups (carboxylic, alcohol and
hydroxyl group etc.) in the surface of biochar, it is easy to form
complexes between heavy metals and these groups. Jiang et al.
(48) used batch methods to investigate the effect of biochar on soil
surface charge and Pb(II) adsorption. Although both electrostatic
and non-electrostatic mechanisms contributed to the increased
Pb(II) adsorption, non-electrostatic mechanism in which the formation
of complexes between Pb2þ and functional groups predominated.
These above mechanisms did not act separately when
biochar is used to remove heavy metals, they worked together to
achieve the remediation effect. Lu et al. (47) showed that the
combination with organic hydroxyl and carboxyl functional
groups accounted for 38.2e42.3% of the total sorbed Pb varying
with pH, while co-precipitation or combining on mineral surface
accounted for 57.7e61.8%. Removal of Cr(VI) in the presence of
biochar is also due to the joint action of electrostatic attraction,
participation and combination