Halloysite nanotubes
(HNT) and ball-milled biochar (BC) incorporated
biocompatible mesoporous adsorbents (HNT-BC@Alg) were synthesized
for adsorption of aqueous heavy-metal ions. HNT-BC@Alg outperformed
the BC, HNT, and BC@Alg in removing cadmium (Cd), copper (Cu), nickel
(Ni), and lead (Pb). Mesoporous structure (∼7.19 to 7.56 nm)
of HNT-BC@Alg was developed containing an abundance of functional
groups induced from encapsulated BC and tubular HNT, which allowed
heavy metals to infiltrate and interact with the adsorbents. Siloxane
groups from HNT, oxygen-containing functional groups from BC, and
hydroxyl and carboxyl groups from alginate polymer play a significant
role in the adsorption of heavy-metal ions. The removal percentage
of heavy metals was recorded as Pb (∼99.97 to 99.05%) >
Cu
(∼95.01 to 90.53%) > Cd (∼92.5 to 55.25%) > Ni
(∼80.85
to 50.6%), even in the presence of 0.01/0.001 M of CaCl
2
and Na
2
SO
4
as background electrolytes and
charged organic molecule under an environmentally relevant concentration
(200 μg/L). The maximum adsorption capacities of Ni, Cd, Cu,
and Pb were calculated as 2.85 ± 0.08, 6.96 ± 0.31, 16.87
± 1.50, and 26.49 ± 2.04 mg/g, respectively. HNT-BC@Alg
has fast sorption kinetics and maximum adsorption capacity within
a short contact time (∼2 h). Energy-dispersive X-ray spectroscopy
(EDS) elemental mapping exhibited that adsorbed heavy metals co-distributed
with Ca, Si, and Al. The reduction of surface area, pore volume, and
pore area of HNT-BC@Alg (after sorption of heavy metals) confirms
that mesoporous surface (2–18 nm) supports diffusion, infiltration,
and interaction. However, a lower range of mesoporous diameter of
the adsorbent is more suitable for the adsorption of heavy-metal ions.
The adsorption isotherm and kinetics fitted well with the Langmuir
isotherm and the pseudo-second-order kinetic models, demonstrating
the monolayer formation of heavy-metal ions through both the physical
sorption and chemical sorption, including pore filling, ion exchange,
and electrostatic interaction.
