Performance of granular ferric hydroxide process for removal of humic acid substances from aqueous solution based on experimental design and response surface methodology

Graphical abstract


Protocol data
The result of this study show that the adsorption capacity of granular ferric hydroxide is high and hence, is a capable adsorbent for removing humic substance (HS) from aqueous solution.
The result of response surface model shows that the experiments are highly accurate and the model is significant. Statistical analysis of data shows that small variations in the values of the selected variables alter the HA removal efficiency. It can be concluded that granular ferric hydroxide (GFH) have desirable quality, cost-effective, and high efficiency to the problem of humic substance (HS) from surface water

Description of protocol
Preparation of granular ferric hydroxide Granular ferric hydroxide (GFH) is an adsorbent, developed at the environmental engineering especially for selective removal of natural organic matter from aqueous solution. Fig. 1 shows the absorbent used in the study. The characteristics of GFH used in this study is reported in Table1. In order to removal of the moisture, GFH was dehydrated in the oven (105 C) for 90 min and also, has been place in the desiccator for cooling [3][4][5].

Adsorption process studies
After preparation of the GFH, required solutions of a certain concentration were also prepared. Distilled water was used for preparation of all these solutions. The experimentations were carried out in the Chemistry laboratory of Environmental Health Engineering Department in Tehran University of Medical Sciences. Then, the factors affecting the performance of the studied processes, like the initial concentration of the humic acid, pH contact time and adsorbent dose were investigated. Isotherm and kinetics were also studied.
For working in a discontinuous system, a 250 ml Erlenmeyer flask was used. In each step, a certain volume of humic acid solution and a specific dose of GFH were added to Erlenmeyer flask. Also the desired conditions in experiment were adjusted. In the following, mixing process (250 rpm) was carried out by use of a shaker and after passing through the filter, considering the required concentrations, the desired volume of the storage solution was removed and the humic acid was analyzed. The humic acid of removal efficiency was determined by adsorbent of granular ferric hydroxide using the following equation: Where, C i and C t are the initial concentrations and concentrations at time t, respectively.

Preparation humic acid solutions and measurement of humic acid concentration
In order to running the adsorption experiment, in the first step, the required solutions of humic acid was prepared. The humic acid was purchased from Acros Co. which had some impurities and in order to eliminate this impurity, 1.82 g of humic acid with a purity of 57% was weighed. Purification was performed by the method of Zakoni et al., as follows: after filtration, the humic acid was mixed with potassium hydroxide 0.2 M and potassium chloride 0.3 M for 4-5 h. Afterwards, centrifugation was done to remove dissolved material such as humic acid. In order to coagulate the humic acid and centrifuging, the floated liquid reached pH = 0.5 by the Hydrochloric acid. Humic acid, which is relatively pure, was washed again with deionized water and Hydrochloric acid and re-centrifuged. Then, deionized water was added to deposited materials and they reached a volume of 1 L in a flask. The solution was then poured into a beaker and agitated for three hours. The solution obtained after  three hours of agitation was purified humic acid with concentration 1 g/L, which was stored at 4 C [1,2,6-10]. Measurement of humic acid concentration was carried out by the DR 5000 spectrophotometer of HACH model with UV/vis detector at the wavelength of 254 nm [1,2].

Experimental design
The central composite design was used to determine the interaction of parameters pH, contact time, adsorbent dose and initial concentration of humic acid on the removal process and optimization of the removal process. Also, in the different stages of determining the number of empirical tests, the RSM was used for modeling and optimization. Data analysis was performed using the R software, version 3-3-2 (2016-10-31) and Microsoft Excel version 2016 for plotting calibration curves and basic mathematical calculations. The Solver plugin in Microsoft Excel software was used to determine the optimal values for the study variables. Table 2 shows the experimental ranges used in central composite design (CCD) design for humic acid adsorption in the study. For humic acid, our preliminary guess in order to optimize the same central points of 16.25, 4, 93.75 and 5 was respectively, for the concentration of humic caid, the adsorbent dose, contact time and pH.
Contour and 3D plots for the interaction effect of variables on the humic acid removal are shown in Figs. 2 and 3 also, regression analysis and analysis of variance (ANOVA) on the humic acid removal are shown in Tables 3 and 4.

Determination of isotherms
The tests required to determine the isotherms of adsorption were performed after choosing the basic conditions of 4 g/L of adsorbent, the initial concentration of humic acid 5-30 mg/L at pH = 5. The linear form of the Langmuir equation used to investigate the adsorption phenomena is as follows [6,11,12,10]: Where qm is the maximum amount of humic acid where qm is the maximum of humic acid per a layer of humic acid in mg/g, KL is the constant of the absorption energy (l/mg) [13][14][15]. Freundlich equation does not predict maximum adsorption and the linear form of the equation is as follows. By illustration of the logarithmic curve qe as a function of the ce logarithm, the n, KF values can be calculated, which are constants of the Freundlich equation. K represents the amount of adsorption of humic acid per unit of equilibrium concentration and n indicates the distribution of particles of adsorbed materials which are bound to the surface of the adsorbent. Various values of 1/n between 0 and 1 denote the surface heterogeneity, and as n approaches zero, the surface heterogeneity increases [8][9][10]6,11,12]. Using the following equation: Isotherm and their parameters for humic acid adsorption onto the GFH are shown in Table 5.

Reaction kinetics
The adsorption process in kinetic studies is linear. The chemical reaction rate is expressed by the chemical kinetics. Most kinetic models for adsorption are of zero, 1 and 2 orders. Table 6 was shown the equations for adsorption kinetics, in which, k is the velocity coefficient and qe, qt are the    Table 6 Kinetic equations.
Pseudo-first-order kinetic ln 1 À q t q e ¼ k 1 t Pseudo-second-order kinetic q e t adsorption capacity at equilibrium and at time t [1][2][3][4][5]. Kinetic and their parameters for humic acid adsorption onto the GFH are shown in Table 7.

Conflict of interest
The authors of this article declare that they have no conflict of interests. Table 7 Kinetic model and their parameters for humic acid adsorption onto the GFH.