Introduction

Metal ions are used in various industries due to their technological importance: metal processing, electroplating, electronics and a wide range of chemical processing industries. Hence, treatment of contaminated waste should be considered a priority, before being discharged into the environment (Rio and Patrick 2012). Pollution of ground and surface waters with toxic metals ions leachate from industries and urban dumpsites is a major problem since most heavy metals like copper, cobalt, cadmium, chromium, lead, etc. are not biodegradable and hence they tend to accumulate in living organisms with an enduring toxic and carcinogenic effect on the body (Pfeiffer 1997; Desai and Kaler 2008; Araya et al. 2005; Johnson et al. 1992; Moradi et al. 2012; Kim et al. 2006; Mendy et al. 2012). Water pollution and its remediation are principal health issues, and various procedures have been proposed including chemical precipitation, membrane filtration, ion exchange, coagulation, electrocoagulation, and adsorption. In the meantime, adsorption is considered a reliable process that can be used to remediate a mixture of contaminants with low concentrations (Chermisinoff and Ellerbusch 1978). Naturally occurring and synthesized materials have been employed as adsorbent (Mittal et al. 2015a, b; Naushad et al. 2014; Mittal and Kurup 2006). Synthetic materials appear to be particularly competitive and effective for the removal of heavy metals at trace level (Shawabkeh 1998). Besides, the use of synthetic adsorbents is considered inapt for developing countries because of its high manufacturing cost. Recently, attempts have been made to increase the sorption capacity of natural adsorbents towards heavy metals by chemical modification of their surface (Knoerr et al. 2013; Şan et al. 2009; Zhu et al. 2012; Liu et al. 2013; Han et al. 2006, 2009; Jeon et al. 2009; Zou et al. 2009; Choi et al. 2012). The common and highest concentration for heavy metal adsorption in aqueous solution in most recent investigations has been between 200 and 300 mg/l. Nonetheless, the concentration is often higher in situations involving the release of heavy metal to the environment from manufacturing plants (Apostoli et al. 2012). Hence, increasing the capability of adsorbents to remove heavy metals at high concentrations (Ajenifuja et al. 2016) is emphatically significant for the survival of organisms in the environment. With the current trend, if high concentrations of toxic pollution occur in emergency as effluents from factories or mining sites, trace and low concentration adsorbents may not be suitable to save the situation. There are needs for adsorbents for high concentrations pollutants (100–1000 mg/l). The foregoing presents necessity for continual studies and investigations on the preparation of an efficient, portable and affordable water remediation materials and gadgets for domestic applications.

With the additional value of being inexpensive and readily available, natural clay aluminosilicate possesses unique combination of physical and chemical properties which makes it suitable for a wide range of industrial applications (Wu et al. 2005; De Castro Dantas et al. 2001; Irani et al. 2011; Khan et al. 2015; Jiang et al. 2010; Bhattacharyya and Gupta 2006, 2008a, b; Gupta and Bhattacharyya 2008; Karaoğlu et al. 2010). However, it is important to increase its adsorptive capacity of natural adsorbent to meet wider range of challenges; this is usually achieved by increasing the number of chemically active sites or enlarging its surface area. The removal and addition of cations using aluminosilicates are dependent on ion exchange and adsorption mechanisms. Most recent works on the use of modified diatomaceous aluminosilicate as adsorbents are based on the removal of toxic metals from aqueous solutions at relatively low concentrations which may not always be the case in real situations (Linghu and Cui 2014; Knoerr et al. 2013; Zhu et al. 2012; Liu et al. 2013; Irani et al. 2011; Danil de Namor et al. 2012; Yusan et al. 2012; Sheng et al. 2008, 2012; Sprynskyy et al. 2010; Pookmanee et al. 2011). Also natural diatomites are contaminated with impurities, which impact negatively on their removal of heavy ions (Zhu et al. 2012). Hence, activation of the diatomite surface becomes necessary for removal process optimization. Photocatalytic nature of TiO2 has been utilized in the environmental remediation studies (Hanaor et al. 2011; Hanaor and Sorrell 2014; Yang et al. 2012; Lasek et al. 2012; Kujawa et al. 2013; Kamegawa et al. 2013). Wu et al. (2009) reported visible light-induced photocatalytic inactivation of bacteria with nitrogen-doped TiO2.

Toxic metal contamination is a global challenge, and an inexpensive and simple solution is required. Utilizing pure photocatalysts (titanium oxide) may not be feasible due to cost and availability for other applications. Modification of raw diatomite with photocatalyst Ag–TiO2 enhances its removal efficiency by photo-degradation of the pollutant even at high concentrations as demonstrated. In this work, TiO2 was doped with Ag nanoparticles in order to extend the photo-activation of the catalyst to the visible range, which will make the use of the ceramic adsorbent to be possible everywhere. This study investigated the use of photocatalytically modified local diatomite as adsorbents for the removal of Co (II) and Cu (II) ions from high concentration aqueous solutions.

Materials and methods

Raw materials

The abundantly available diatomaceous aluminosilicate materials were sourced in northern Nigeria. Diatomaceous earth usually occurs in two types of geological environments, marine and lake. This raw material sample used in this work was obtained in North-East Nigeria, which is close to the Lake Chad basin; therefore, it was originated most probably in the lacustrine environment. The obtained dried sample is a loosely cemented porous and lightweight rock of sedimentary origin, mainly formed from skeleton of diatomea. The modifying materials are anatase TiO2 nanoparticles and AgNO3 salt, which was used as the silver ion source. Sodium carbonate (99.0%, Na2CO3) was used to reduce the silver ions.

Diatomaceous powder preparations (ZEO)

The raw material was pulverized and the fine powder obtained was mixed with distilled water in a beaker to form a colloid. The colloidal solution was agitated and left for a few minutes for heavy unwanted particles and other impurities to settle at the base of the beaker; thereupon the top suspension was separated into another beaker, and the process was repeated until a homogeneous gray gelatinous separate was obtained and then centrifuged. The collected diatomaceous gel was transferred into an oven for drying for 18 h at 100 °C; and a soft white lightweight cake was formed. To enhance the surface properties of the aluminosilicate material, activation was carried out on the well-dried diatomaceous powder using 30% dilute HNO3. The acid treatment was intended for dissolution of some amorphous materials that might block the natural pores of the raw materials and give way to the modifying photocatalyst. Silver nanoparticles were loaded into the modifying anatase TiO2 nanopowder to give Ag–TiO2 (STOX); this was intended to extend the photo-activity of the ceramic powder well into the visible region. Silver nitrate (AgNO3) was used as the silver ions source. For the preparation, 10 g of anatase TiO2 powder was placed into a 500 ml beaker with 100 ml ethanol as the dispersion medium. Then, 0.1 M solution of AgNO3 and 1% (w/v) solution of sodium carbonate (reducing agent) were prepared separately. Hence, 4.6 ml of the prepared silver nitrate solution and 5 ml of sodium carbonate were added into the beaker containing the dispersed TiO2. The mixture was stirred vigorously for 2 h with magnetic stirrer to form a slurry solution and afterwards the solid material (Ag–TiO2) was collected by centrifugation. The collected modified TiO2 nanoparticle was dried in the furnace at 100 °C for about 24 h; thereafter, the powder was calcined at 400 °C for 12 h to remove leftover organics and also for thermal diffusion of the material.

Modified diatomaceous ceramic adsorbents (ZEO-T and STOX-Z)

Ag–TiO2-modified diatomaceous ceramic adsorbents were prepared using two procedural routes. For the preparation of adsorbent ZEO-T, the prepared Ag–TiO2 powder (STOX) was ring milled with the HNO3-treated diatomaceous aluminosilicate powder (ZEO) in other to achieve mechano-chemical interactions and uniform distribution of constituents in the ceramic composite. This was followed by solid-state fusion; the milled mixture was subjected to high temperature–pressure treatment at 800 °C. To prepare ceramic adsorbent STOX-Z, dispersion of modified Ag-modified TiO2 was prepared as earlier described in Sect. 2.2. Then, depending on the quantity proposed, 25 g of HNO3-treated diatomaceous aluminosilicate powder was added slowly into the continuously stirred Ag–TiO2 colloidal solution (see Fig. 1). The slurry solution formed was stirred and mildly heated simultaneously for 2 h on the hot plate. The final modified diatomaceous aluminosilicate powder adsorbent STOX-Z was recovered from the slurry solution via centrifugation as shown in Fig. 1. Conspicuous visible light-induced color change was observed for the STOX-Z particles when exposed to daylight due to photoreaction. The constituents of the ceramic powder are highlighted in Table 1. The micrographs of the prepared diatomaceous clay adsorbents ZEO-T, STOX-Z and STOX are shown in Fig. 2a, b and c, respectively. In the SEM images, Ag–TiO2 nanoparticles are shown to be well distributed and coalesced around the surface and micropore structure of the diatomaceous aluminosilicate.

Fig. 1
figure 1

Ag–TiO2-modified diatomaceous aluminosilicate ceramic membrane preparation steps

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Table 1 Constituents of the ceramic membrane precursor materials
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Fig. 2
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SEM micrographs for diatomaceous ceramic adsorbents a ZEO-T b STOX-Z and c STOX

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Adsorption experiments

The stock solutions of Cu (II) and Co (II) were prepared from their commercial salts (sulphates and nitrates, respectively) and the solutions were normalized titrimetrically. Other required concentrations were made with appropriate dilutions from the stock. Batch adsorption tests were carried out in 50 ml Pyrex beakers. For each adsorption test, 0.125 g of raw and modified ceramic powder adsorbents was added to 20 ml of aqueous solutions at room temperature. The ion-adsorbent mixtures were shaken for 2 min for even distribution of the adsorbent particles in the solution and then kept for 12 h for saturation. The high concentration aqueous solutions resembled effluents obtainable in real life contamination scenarios. The aqueous phase was separated from the ceramic adsorbent by centrifugation after 12 h and the new equilibrium concentrations of the cation in final solution were monitored using UV–visible spectrophotometer (Thermoscientific: Helio Omega). The proper absorbance–concentration calibrations based on Beer–Lambert’s law were carried out at every stage of the spectrophotometric measurement to determine the equilibrium concentrations. The effect of initial concentrations of the aqueous solutions of Cu and Co on the adsorption capacity of the ceramic adsorbents was also determined from the solutions with concentrations ranging from 100 to 1000 mg/l. The amount of adsorbed Cu2+ and Co2+ (mg ion/g ceramic adsorbent) was calculated from the decrease in concentration in the media. The initial and equilibrium concentrations of the adsorbates were measured and compared to know the extent of the removal. The initial and final concentrations of the adsorbates are displayed in Table 2.

Table 2 Initial and final concentrations of Cu2+ and Co2+ solutions determined using UV–visible spectrophotometer
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The adsorbed Cu2+ and Co2+ ions (mg ions/g ceramic adsorbent) were determined basically by taking into account, the decrease in the concentrations of Cu and Co ions, the adsorption volume and the amount of ceramic adsorbent used for each process:

$$q_{\text{e}} = \left[ {\left( {C_{\text{i}} - C_{\text{e}} } \right) \times V} \right]/m,$$
(1)

where q e is the amount of metal ions adsorbed onto unit mass of the clay (mg Cu2+ or Co2+/g adsorbent) at equilibrium; C i and C e are the concentrations of the metal ions in the initial solutions and in the aqueous phase after treatment for certain adsorption time; m is the amount of the ceramic powder used (g) and V is the volume of the aqueous solution (liter).

Results and discussion

Influence of concentrations of adsorbate

The study of the influence of initial concentrations of the metal ions in the aqueous solutions was carried out and the upper limit of the adsorption capacities of the diatomaceous aluminosilicate adsorbents for Cu (II) and Co (II) ions was determined. The characteristic behavior of the adsorbents with respect to the initial concentrations of the metal ions is shown in Figs. 3 and 4 for Cu (II) and Co (II), respectively. For Co2+ adsorption, sample ZEO-T exhibited a promising behaviour by attaining the highest Co2+ removal capacity, all through the concentration range in term of quantity of cations absorbed in mg/g. Samples STOX-Z showed a different relationship in terms of their behavior with the highest Co2+ quantity adsorbed being about 30 mg/g. It should be noted that though quantity adsorbed rose with the concentrations, however, considering the removal in percentage term, with the rise in the concentration, the percentage uptake by the adsorbents reduced steadily. Meanwhile, the adsorption capacities of the adsorbents for Cu2+ followed nearly almost the same pattern but with STOX-Z adsorbing the highest quantity of Cu2+ especially at higher adsorbate concentrations. Nonetheless, both adsorbents ZEO-T and STOX-Z share the same adsorption capacity trend at lower solution concentrations up to about 400 mg/l.

Fig. 3
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Effect of initial adsorbate concentrations on the adsorption capacities of Cu2+

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Fig. 4
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Effect of initial adsorbate concentrations on the adsorption capacities of Co2+

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Cu2+ and Co2+ adsorption isotherm

To comprehend the removal mechanism of Cu (II) and Co (II) ions by Ag–TiO2-modified diatomaceous ceramic adsorbent, the study of adsorption mechanisms becomes important. It is understood both in theory and experiments (Lippens and De Boer 1965; Reed and Matsumoto 1993; Malik 2004) that ions and molecules both in liquid and gaseous forms can adsorb on the surface of certain adsorbent. Therefore, the likeliest mechanism by which ceramic adsorbents remove metals ions from solutions are by forming monolayers on the surface of the porous and non-porous adsorbent particles. In this work, adsorption of Cu (II) and Co (II) in the aqueous solutions is described through isotherms, that is, functions which connect the amount of adsorbate on the adsorbent. Distribution of metal ions between the liquid phase and the solid phase can be described by several isotherm models such as Langmuir and Freundlich. In the case of Langmuir isotherm model, it is assumed that monolayer adsorption onto a surface contains a finite number of adsorption sites of uniform strategies with no transmigration of adsorbate in the plane surface, i.e., once a site is filled, no further sorption can take place at that site (Hameed et al. 2007). This indicates that the surface reaches a saturation point where the maximum adsorption of the surface will be achieved. The adsorption of Cu (II) and Co (II) ions in solutions was recorded in the concentration range from 100 to 1000 mg/l mimicking a high concentration contamination conditions. The profiles obtained from the studies of the concentrations at room temperature were used to obtained Langmuir and Freundlich adsorption isotherms using the well-known liquid isotherm equations. The linearized Langmuir isotherm is represented by Eq. (2):

$$\frac{{C_{\text{e}} }}{{q_{\text{e}} }} = \frac{1}{{bq_{ \hbox{max} } }} + \frac{{C_{\text{e}} }}{{q_{ \hbox{max} } }}.$$
(2)

The linear plots of specific adsorption (C e/q e) against the equilibrium concentrations (C e) are shown in Fig. 4. It indicates that Cu2+ and Co2+ adsorptions on the diatomaceous ceramic adsorbent conveniently obey the Langmuir isotherm model. The constants b and q max relate to the energy of adsorption and maximum adsorption capacity, and their values are obtained from the interception and slope of the plots and are presented in Table 3. The Freundlich isotherm is introduced as an empirical model, where q e represents the amount adsorbed per amount of adsorbent at the equilibrium (mg/g), C e represents the equilibrium concentration (mg/l), and K f and n are parameters that depend on the adsorbate and adsorbent (Fig. 5). Considering the Freundlich equation below:

$$q_{\text{e}} = K_{\text{f}} C_{\text{e}}^{{{\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 n}}\right.\kern-0pt} \!\lower0.7ex\hbox{$n$}}}} .$$
(3)
Table 3 Langmuir and Freundlich parameters for the ceramic adsorbent materials
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Fig. 5
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Langmuir isotherms for Cu2+ and Co2+ adsorption on modified diatomaceous ceramic adsorbents

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The equation can be linearized and the temperature-dependent constants K f and 1/n can be obtained by linear regression:

$${ \ln }q_{\text{e}} = \ln K_{\text{f}} - \frac{1}{n}{ \ln }C_{\text{e}} ,$$
(4)

where K f and n are Freundlich constants which correspond to adsorption capacity and adsorption intensity, respectively. Freundlich equilibrium constants were determined from the plot of lnq e versus ln C e in Fig. 6 on the basis of the linear form of Freundlich equation. The n value indicates the degree of nonlinearity between solution concentration and adsorption as follows: if n = 1, then adsorption is linear; if n < 1, then adsorption is a chemical process; if n > 1, then adsorption is a physical process. The n value obtained from Freundlich isotherm model for Cu2+ and Co2+ was found to be 2.813 and 5.801 for Cu (II) and Co (II), respectively, on adsorbent STOX-Z, as shown in Table 3. The situation n > 1 is most common and may be due to a distribution of surface sites or any factor that causes a decrease in adsorbent–adsorbate interaction with increasing surface density (Reed and Matsumoto 1993) and the values of n within the range of 1–10 represent good adsorption (McKay et al. 1980; Ozer and Pirincci 2006). In the present study, since n lies between values 2 and 6, it indicates a physical adsorption of metal ions onto diatomaceous ceramic adsorbent. In both cases, linear plots were obtained, though the degree of linearity of the Langmuir isotherm plots is higher than that of Freundlich isotherms. However, this reveals the applicability of these isotherms on the ongoing adsorption process. Complete Langmuir and Freundlich constants with corresponding correlation coefficients (R), derived from the isotherm plots for the adsorption of cations on the adsorbents STOX-Z and ZEO-T, are shown in Table 3. To find the most appropriate model for the metal ions adsorption on aluminosilicates ceramic adsorbents, data were fitted to Langmuir and Freundlich isotherm models and the results indicated that Langmuir adsorption isotherm was the best model for the metal ions adsorption with R of up to 0.99953 in the case of Cu (II) ion on ZEO-T. Langmuir adsorption isotherm parameters can be used to predict the affinity between the sorbate and sorbent using a dimensionless constant called separation factor or equilibrium parameter (R L), which is expressed by the following relationship (Hall et al. 1966; Malik 2004):

$$R_{\text{L}} = \frac{1}{{\left( {1 + bC_{\text{i}} } \right)}},$$
(5)

where b is the Langmuir constant and C i is the initial concentration. The value of R L indicated the type of Langmuir isotherm to be irreversible (R L = 0), linear (R L = 1), unfavorable (R L > 1), or favorable (0 < R L < 1) (McKay et al. 1980). The R L values between 0 and 1 indicate favorable adsorption. The R L value in the present investigation was found to be 0.009–0.319, showing that the adsorption of the metal ion the modified materials is favorable.

Fig. 6
figure 6

Freundlich Isotherms for Cu2+ and Co2+ adsorption on modified diatomaceous ceramic adsorbents

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Conclusions

Photocatalytic diatomaceous ceramic adsorbents were successfully prepared using Ag–TiO2 nanoparticles and activated diatomaceous aluminosilicate minerals for photo-degradation and removal of Cu (II) and Co (II) from relatively high concentration aqueous solutions (100–1000 mg/l). The results and the parameters obtained indicated conclusively that photocatalytically modified diatomaceous ceramic adsorbents are capable of removing Cu (II) and Co (II) ions from relatively high concentrated aqueous solutions. Nonetheless, thermally fused ZEO-T offered greater adsorption capacity for both cations. Moreover, it is presented that both Freundlich and Langmuir isotherms can be used to fit the data and estimate model parameters; however, the overall data are slightly better fitted by Langmuir isotherm for both cations. Considering its microstructural characteristics, obtainability and abundance, photocatalytic diatomaceous ceramic adsorbent is economically viable and environmentally friendly for use, especially in developing countries. It can also be effectively applied as a pretreatment antidote for high concentration toxic effluents.