2. Determination of the optimum adsorption conditions
3.2.1. Influence of dosage
Fig. 2 showed the experimental results of the determination of the
optimal dosage, contact time and pH for copper removal, respectively.
Fig. 2(a) indicated that with an increasing of the amount of adsorbent,
the removal efficiency of Cu2+ increased rapidly, which due to that
the active sites enhanced with an increase amount of MgHAp/Fe3O4.
After the dosage reaching 8 mg, the removal efficiency was basically
stable. The removal efficiency of Cu2+ approached equilibrium (96.8%)
at the dosage of 8 mg of MgHAp/Fe3O4. Considering the practical application,
8 mg was selected as the optimum dosage.
3.2.2. Influence of reaction time
The effect of contact time on removal of Cu2+ was also studied and
the results were shown in Fig. 2(b). It seemed that the adsorption
consisted of two phases: a primary rapid phase and a second slow
phase. The first rapid phase lasted approximately 70 min and accounted
for the major part in the total Cu2+ adsorption. Adsorption reached a
plateau value in approximately 90 min, which showed saturation of
the active points. So, 90 min was used as the optimal contact time.
3.2.3. Influence of initial pH
Fig. 2(c) showed that Cu2+ adsorption strongly depended on the solution
pH and increased with increasing pH. The adsorption of Cu2+
clearly increased from 19.4% to 97.6% with the increasing pH of the solution
from 1.0 to 5.9 and then slightly decreased to 95.8% at pH 6.9.
The cause of this phenomenon was associated with two possible reactions
capable of removing Cu2+ from the solutions. The first mainly
possible reaction was the adsorption of Cu2+ on the surfaces followed
by the ion exchange reaction between adsorbed Cu2+ and Mg2+ of
MgHAp/Fe3O4. The ion exchange interactions can be presented as
follows [45]:
Cu2þ
ð Þs þ ≡ Mg2þ
ð Þ H → ≡ Cu2þ
ð Þ H þ Mg2þ
ð Þs
Subscripts (s) and (H) denote solution and MgHAp/Fe3O4 phase, respectively.
The metal in the solution (Cu2+(s)) replaces a surface Mg2+
of the adsorbent (≡Mg2+(H)), and the second mechanism is surface