Coagulation–flocculation processes play an
important role in the treatment of water using inorganic and
synthetic materials. Presently, cactus
Opuntia ficus indica
is explored as a cheap, abundant and eco-friendly
bioflocculant of Pb(II) ions from aqueous solutions. The
cactus powder was characterized by Fourier transform infrared
spectrometer (FTIR), and the coagulation–flocculation
process was optimized for pH, initial concentration of
Pb(II) ions, particle size, dosage, ionic strength, contact
time and temperature before its application to Mukuvisi
River water. FTIR confirmed the involvement of –C–O–C–,
–OH, –C=O, –N–H and –C–H groups of the polysaccharides,
carboxylic acids and amino acids in the removal of
Pb(II) ions. The variation of percentage removal and zeta
potential with pH and flocculant dosage suggested the involvement
of adsorption–charge neutralization and adsorption–
bridging mechanisms in the coagulation–
flocculation process. The process was optimum at pH 5,
natural ionic strength, dosage of 8 mg/L, corresponding to
zero zeta potential, with particle size <75 µm, at 35 ºC for
a floc settling time of 180 min. The coagulation–flocculation
process was spontaneous and endothermic with a
positive change in entropy of the system. Total heavy metal
removal of 100 % (Pb), 85.74 % (Zn), 84.16 % (Cd) and
93.02 % (Cu) were achieved, under optimum conditions,
when
Opuntia ficus indica powder was used to remove
metals from water sampled from Mukuvisi River, Harare,
Zimbabwe. The cactus
Opuntia ficus indica powder was
found to be an efficient, eco-friendly flocculant that can
effectively remediate Pb(II) ions from wastewaters.