Fig. 1. The coordination complexation of iron ions with the four hydroxyl groups of FA^[10]

Fig. 2. FTIR and 3DEEM spectra of 9 kinds of FA before and after complexing Cu^{2+ [22]}

Fig. 3. Differential optical absorption spectroscopyof FA^[25]

Fig. 4. The logarithmic fitting parameters of the distribution of carboxyl and phenolic hydroxyl groups based on the average CAS value at different pH values (logk'_Cu is a parameter related to the affinity of sites for copper under different copper ion concentrations, p(logk'_Cu) is a parameter related to the distribution of the site on FA)^[18]
Q₁: Carboxyl group; Q₂: Phenol hydroxyl group

Fig. 5. The effect of pH on the complexation process of FA and Cu^{2+ [22]}

Fig. 6. FTIR peak area of FA before and after complexing with Cu^2+[22]

Fig. 7. (a) The value of (k_m,jc_m)nm,j of different metal ions combined with FA phenolic hydroxyl site^[29]; (b) The value of (k_m,jc_m)nm,j of different metal ions combined with FA carboxyl site ((k_m,jc_m)nm,j is a parameter related to the concentration of competitive ions. The higher the value, the stronger the competitive effect of this ion)^[29]

Fig. 8. FTIR spectra before and after complexation of 9 kinds of FA with Cu²⁺, Pb²⁺, Cd^2+[38]

Fig. 9. The effect of ionic strength on the complexation of FA and metal ions^[39]