Introduction

Printing and dyeing is an important and traditional economic industry in China, according to statistics in 2021. It produced 60.581 billion meters of printed and dyed fabric, an increase of 11.76%1, and with the increase in the demand for printing and dyeing products, it also produces a large amount of printing and dyeing wastewater. China’s printing and dyeing enterprises discharged about 4 million tonnes of printing and dyeing wastewater every year2, becoming the main of industrial wastewater outpouring. Printing and dyeing wastewater mainly includes desizing wastewater, boiling wastewater, bleaching wastewater, mercerising wastewater and dyeing wastewater, etc3. Printing and dyeing wastewater has a complex composition, high chromaticity, high content of difficult-to-biodegrade substances and contains a variety of organic substances that are biotoxic and lead to the “three causes”4, especially the residual dye components in the printing and dyeing wastewater, which can cause serious damage to the ecosystem in waters. The environmental pollution problem caused by China’s textile printing and dyeing industry has become an important bottleneck limiting development of the industry, and the search for a fast, simple and effective way of printing and dyeing wastewater treatment has become an important initiative to deal with the wastewater in textile printing and dyeing industry. Effective solution of printing and dyeing wastewater can not only accelerate the transformation of social economy to green economy, but also actively establish resource recycling system. The technology for treating printing and dyeing wastewater mainly includes chemical, physical and biological methods, of which the adsorption method in the physical method is most widely used, using activated carbon to adsorb the printing and dyeing wastewater, destroying the chromogenic tissue in printing and dyeing wastewater, decreasing the chromaticity of printing and dyeing wastewater, removing biotoxicity of printing and dyeing wastewater and improving the biochemistry of printing and dyeing wastewater, so as to enable the printing and dyeing wastewater to meet the emission standards5.

Olea europaea is an oilseed plant of the genus Olea europaea in the family of Lignaceae, and olive in China is mainly distributed in the areas of Sichuan, Gansu, Guangdong and other regions6, and the planting area of olive in China accounted for about 1,350,000 acres in 2022, and the annual output of olive oil is about 13,000 tonnes7. People are increasingly recognizing olive oil as a healthier ingredient, which has led to an increase in demand for olive oil and a large number of by-products. In producing olive oil, olive pomace comes out, which is composed of a mixture of olive shells, olive pulp and plant moisture8. Olive pomace has a high organic load and a long residue cycle, which predisposes it to a number of derived risks such as water contamination, damage to microbiota and harm to protozoa and plants9. Olive pomace contains not only residual oil, but also cellulose, with high chemical stability and mechanical strength, so it is a natural biomass resource10. Preparation of activated carbon from olive oil waste has the advantages of low cost and simple source of raw materials. At present, main methods for treating dye wastewater are photocatalytic degradation, membrane filtration, advanced oxidation and adsorption11,12, among which adsorption method has advantages of simple operation, low cost, adsorption material recycling so it is widely used13. Adsorbent election is particularly important, and activated carbon (OP-AC) has advantages of large adsorption capacity and fast adsorption rate, so it is one of the most widely used adsorbents14. In this paper, olive pomace-based activated carbon was prepared from olive pomace by high-temperature calcination using alkaline thermal activation method, and then used as an adsorbent to adsorb seven dyes, namely, methylene blue (MB), Congo red (CR), methyl orange (MO), rhodamine B (RHB), malachite green (MG), crystalline violet (CV), and neutral red (BL), to investigate the adsorption performance and mechanism of organic dyes, with aims of treating waste with waste and turning waste into “treasure” and providing reference for resourceful use of olive pomace, and new materials for treatment of organic printing and dyeing wastewater, and realizing social, economic and environmental benefit in olive industry.

Experiment

Raw materials and reagents

Olive pomace from an olive oil production plant in Gansu province, China; MB, CR, MO, RHB, MG, CV, BL, potassium hydroxide, concentrated hydrochloric acid, sodium hydroxide (analytically pure).

Instruments and equipment

SX2-4-10 box-type resistance furnace, TDZ5-WS low-speed multi-tube rack automatic balancing centrifuge, 17,000 electromagnetic heating churn, 75,500 UV-spectrophotometer, 38,000 multi-functional programming drying oven, 24,900 electronic analytical balance, BC-1000 multi-tube vortex mixer, 3520PHMETER precision desktop pH meter.

Preparation of olive pomace activated carbon

Olive pomace is sieved and separated, most of the olive flesh being removed, and the olive pomace mainly consists of olive kernel, which is cleaned and put into the constant temperature oven for drying at 105°C to constant weight, and the dried olive pomace is weighed and put into a porcelain crucible, and then placed in an electromagnetic heating stirrer, heated to carbonisation at 200°C, when a large amount of smoke escapes, and then close the electromagnetic heating stirrer when there is no smoke, waiting for the temperature to drop to room temperature to form olive pomace charcoal powder for later use. The powder, placed into the porcelain crucible, with 2:3 solid-liquid ratio, being added in 20% potassium hydroxide solution impregnated for 24 h, is put into the box-type resistance furnace, heated to 550°C, and calcined in the box-type resistance furnace at high temperature for 20 min and then removed, dropping to room temperature. 10% hydrochloric acid solution is used for washing the residue twice, and then washed with deionised water until the water is colourless and transparent. Subsequently, the remain was dried in a drying oven at 105 °C to obtain olive pomace activated carbon (OP-AC). Potassium hydroxide, as an activator, can be an effective reagent to increase the specific surface area of activated carbon and reduce ash content. During impregnation of raw material with potassium hydroxide solution, potassium compounds are embedded into the internal carbon structure of raw material, which enlarges the carbon atom layer. In a high temperature environment, potassium hydroxide undergoes dehydration reaction to produce potassium oxide as metal oxide, which under inert gas atmosphere undergoes reduction reaction with carbon to produce potassium, whereas the elemental carbon escapes in the form of gas to form pore structure inside raw material11.

Exploring adsorption properties

Standard curves for seven dyes

Preparing seven dye stock solution: 0.0500 g (0.0001 g) of MB, CR, MO, RHB, MG, CV and BL were accurately weighed and dissolved in deionised water, transferred into a 500mL volumetric flask and calibrated to the scale line to obtain 100 mg/L standard stock solution of MB, CR, MO, RHB, MG, CV and BL, respectively. The standard curve of the dye was drawn, and the results are shown in Table 1.

Table 1 Regression parameters of seven organic printing and dyeing dyes.
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Determining adsorption capacity and removal rate

A certain amount of olive pomace activated carbon was weighed to remove seven printing and dyeing dyes at different specific concentrations. After static adsorption, the upper solution was taken out for the measurement of absorbance, and the adsorption capacity and removal rate of the activated carbon for the seven printing dyes were calculated based on the measured values according to the formula (1) and (2).

$$q_{t} = \frac{{(\rho _{0} - \rho )v}}{m}$$
(1)
$$\eta = \frac{{\rho _{0} - \rho }}{{\rho _{0} }} \times 100\%$$
(2)

ρ0 — mass concentration of the dye before adsorption by the adsorbent, mg/L; Ρ— mass concentration of the dye after adsorption by the adsorbent, mg/L; V — volume of dye, L; m— mass of adsorbent, g.

Adsorption test

  1. (1)

    Adsorption isothermal test: at room temperature (25°C), the concentrations of 10, 20, 40, 60, 80, 100, 150 mg/L dye (40mL) and 0.6 g OP-AC were prepared and put into centrifuge tubes, maintaining original pH constant, and then oscillated in a thermostatic oscillator for 24 h, and then the absorbance was measured in the supernatant at the corresponding wavelength of absorption, and removal rate was calculated. The adsorption isothermal models of OP-AC on seven kinds of printing and dyeing wastewater at different initial concentrations were examined, and the adsorption process was fitted by two isothermal models, Langmuir and Freundlich, respectively.

  2. (2)

    Adsorption kinetics: At room temperature (25°C), 40 mL of 20 mg/L printing dye was taken in 100 mL centrifuge tubes, OP-AC (0.6 g) added, and the supernatants were taken to measure the absorbance at the corresponding absorption wavelengths after oscillation under a constant temperature shaker for 0, 4, 8, 12, 16, 20, and 24 h, and removal rates were calculated. The adsorption kinetics of OP-AC on seven printing and dyeing dyes were examined, which were reflected by linear fitting of quasi-primary and quasi-secondary kinetic models, respectively.

  3. (3)

    One-way adsorption experiment: a certain amount of OP-AC, in a 50mL centrifugal bottle, adding 40mL of dye solution with a concentration of 20 mg/L, adjusting different pH values, stands up for adsorption for a period of time, then centrifugates for 3 min to take the supernatant and measure the absorbance at different absorption wavelengths, and then calculate the removal rate and adsorption amount. The removal rates of various dyes were investigated by dosage of OP-AC (0.2, 0.4, 0.6, 0.8, 1.0, 1.2 g), the initial pH of the dyes (2, 4, 6, 8, 10, 12) and the adsorption time (0, 6, 12, 18, 24, 48 h).

Results and discussion

Characterization of results analysis

SEM analysis

Figure 1a, b shows the morphology of the olive fruit residue and OP-AC obtained by scanning electron microscope, and the adsorption properties can be roughly understood through its pore structure. It can be observed from Fig. 1a that the raw material of olive fruit residue is a relatively complete individual with no pore structure on the material surface, while in Fig. 1b, the OP-AC surface has irregular pore distribution, rich pores, and overlapping layer honeycomb structure appears. Because KOH corroded the carbon powder under high temperature conditions, so that a large number of pores in the carbon powder, and in the activated process, KOH will promote the carbon powder to produce CO2, water vapor and other gases. Gas escaping through pores will also increase number of pores in the activated carbon. OP-AC activated at high temperature has more cracks and pores, which will make the adsorbed dye molecules more easily enter cracks under the action of van der Waals force, being adsorbed into OP-AC. All above will finally improve adsorption efficiency of OP-AC.

Fig. 1
figure 1

SEM plots of the olive fruit residue (a) and OP-AC (b).

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XPS and FT-IR analysis

To investigate surface properties and functional groups of OP-AC, chemical states of C and O in activated carbon of olive fruit residue were determined using an X-ray photoelectron spectrometer and OP-AC functional groups were determined using a Fourier transform infrared spectrometer. Figure 2a shows the high-resolution total XPS map of the OP-AC, containing the C and O elements. Figure 2b shows that two typical high-resolution XPS peaks of O1s can be classified as carboxylic acid groups (531 eV) and carbon-oxygen atomic clusters (533 eV). The C1s XPS spectrum of OP-AC in Fig. 2c is divided into three associated peaks: 284 eV, 285 eV and 288 eV, representing C-C, carbon-oxygen single bond C-O and C = O radicals on the benzene ring, respectively. Figure 2d shows FT-IR spectrum of the prepared OP-AC in the range of (500–4000 cm− 1). The figure shows that: 3415.29 cm− 1 corresponds to the tensile vibration peak of the intermolecular hydrogen bond O-H, 2918.91 cm− 1 corresponding to the expansion peak of C-H in lignin or the peak of O-H. At 1574 nm− 1, C = C or the oscillation peak of the C = O hydroxyl group, it shows that a relatively stable conjugated aromatic ring system exists in the activated carbon14. Two distinct peaks were also present at 1233.63 cm− 1 and 832.64 cm− 1, respectively. It can be inferred that alkoxide (C-O) and aromatic groups (C-H) are present respectively. The analysis shows that OP-AC contains rich oxygen-containing functional groups, and hydroxyl group and carboxyl groups facilitate chemisorption of OP-AC and provide more adsorption sites for adsorbed dyes.

Fig. 2
figure 2

(a) Total XPS profile of OP-AC, and the FT-IR profile of (b) O1s, (c) C1s, and (d) OP-AC.

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pHpzc of OP-AC

pH (pHpzc) of the OP-AC was determined by the solid addition method14. A 0.2 mol/L NaCl solution of pH 2 ~ 12 was prepared with 2 mol/L HCl and 2 mol/L NaOH conditioning, recorded as pH1. Taking 50 mL of each solution with different pH values to a 125 mL beaker and adding 0.6 g OP-AC, supernatant pH was measured at 30°C at constant temperature oscillator (180 rpm) after.

2 h, recorded as pH2. Curves are drawn with pH1 as abscissa and pH1-pH2 as ordinate, and the intersection of the figure with abscissa is pHpzc value of OP-AC. The results are shown in Fig. 3.

Fig. 3
figure 3

OP-AC zero-charge point curve.

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Effect of OP-AC dosage on removal rate of seven dyes

Six 40mL of 20 mg/L MB aqueous solution was collected in each centrifuge tube, and different amounts of OP-AC were added to make the concentrations of 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2, respectively, and the adsorption was carried out by oscillation at room temperature (25°C) for 24 h to investigate effect of OP-AC dosage on removal rate of seven dyes in water (the other dyes’ experimental steps were the same as that of MB). The results are shown in Fig. 4. It can be seen that with increase of OP-AC dosage, removal rate of printing and dyeing dyes gradually increased, and when the dosage of OP-AC reached 0.6 g, removal rate came to more than 90%, and adsorption effect was better. Removal rate of printing dye and effective contact area between OP-AC and printing dye had a great relationship. When dosage of OP-AC was small, the direct collision between OP-AC and printing dye molecules was less, resulting in weak van der Waals force between dye molecules and OP-AC. The adsorption sites on OP-AC also increased with increasing dosage of OP-AC, and adsorption force on dye molecules also increased. When dosage of OP-AC was high enough, adsorption of limited dye molecules was gradually saturated, and there was a great relationship between pore size of OP-AC and molecular size of adsorbed substances, because molecular masses of CR and RHB molecules were larger, which made it more difficult for them to be adsorbed by OP-AC than the other five dye molecules. Based on the above discussion, in order to save OP-AC, the dosage of OP-AC was selected to be 0.6 g for the subsequent optimizing one-factor conditions15.

Fig. 4
figure 4

Effect of OP-AC dosage on removal rate of seven dyes.

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Effect of pH on removal rate of printing and dyeing dyes

Six 40mL of 20 mg/L MB dye were collected in each centrifuge tube, a certain amount of OP-AC (0.6 g) added, pH of the dye solution adjusted, the initial pH of the dye solution was 2, 4, 6, 8, 10, and 12, respectively, and the dye solution was oscillated at room temperature for 24 h, and absorbance of the supernatant was taken out and measured at the corresponding absorption wavelength, and finally removal rate was calculated16. As can be seen from Fig. 5, removal rate of OP-AC on MO, CR and BL reached the maximum value at pH = 4. With the best removal effect, and with the increase of pH value, removal rate began to decline gradually. Among them, MO and CR are anionic dyes, which include reactive group and are easy to interact to form covalent bonds. When pH of the dye solution is low, solution interface is less negatively charged and the OP-AC surface is positively charged, making it easier to bind to anionic dyes. For BL cationic dyes, when pH is high, it will be higher than its dissociation constant17, which produces a certain electrostatic repulsion and blocks binding its dye ions to the internal pores of OP-AC. The effect of electrostatic repulsion is larger than the adsorption force of BL ions on the surface of OP-AC, and therefore the adsorption force for BL is lower when pH is lower. Therefore, for BL, MO and CR dye molecules, the best removal effect was achieved when the dye’s pH equals 4. And the removal of MB, RHB, CV, MG dyes by OP-AC is increased with increase of pH. When the pH is low, the large amount of H+ present in the dye solution competes with the dye cations and inhibits adsorption of dye cations by ionic groups on the surface of OP-AC. When pH is high, dye solution contains a large amount of OH−, which will be attached to surface of OP-AC and generate electrostatic attraction between it and the dye cation, contributing to an increase in adsorption of dye by OP-AC. Therefore, for MO, BL and CR organic dyes, acidic condition (pH = 4) is more suitable for removal of dye molecules, and conversely, for MB, RHB, MG and CV organic dyes are more efficiently removed under alkaline conditions (pH = 12).

Fig. 5
figure 5

Effect of pH value on removal rate of seven dyes.

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Effect of oscillation time on removal of printing dye

Collecting 20 mg/L of printing dye 40mL in each centrifuge tube, adding OP-AC (0.6 g), adjusting pH (MO, CR, BL: pH = 4, MB, RHB, MG, BL: pH = 12), wavelength after oscillating in a thermostatic oscillator for 0, 6, 12, 18, 24, and 48 h was measured and the removal rate was calculate. Figure 6 shows effect of different oscillation time on removal rate of the printing dye. When oscillation time was less than 6 h, removal rate of various printing and dyeing dyes by OP-AC increased gradually with increase of oscillation time, which was attributed to existence of a large number of tiny pores on surface of OP-AC, which formed a sufficient number of adsorption sites, and mass fraction of various dyes in water was sufficiently adequate, and mass transfer kinetics of adsorption was large18, and molecules of various dyes were rapidly combined with active sites on surface of OP-AC under effect of oscillation. The dye molecules were rapidly bound to active sites on OP-AC surface under effect of oscillation. With increase of the oscillation time, dye molecules adsorbed on the surface of OP-AC began to enter into the mesopores and macropores of OP-AC, and more and more adsorption sites on surface of OP-AC were vacated, and more and more dye molecules were adsorbed onto surface of OP-AC under effect of van der Waals forces. Adsorption of each dye molecule on OP-AC gradually reached saturation, and removal rate of each dye tended to stabilize after 24 h. Therefore, optimum oscillation time is 24 h19.

Fig. 6
figure 6

Effect of oscillation time on removal rate of seven dyes.

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Adsorption isotherms

Langmuir and Freundlich models are two commonly used isothermal adsorption models that can provide a theoretical basis for adsorption behaviour of activated carbon on its adsorbed substances. The Langmuir model can better express whether the adsorption sites on activated carbon are uniformly distributed on the surface of the activated carbon and whether the adsorption behaviour occurs as a monolayer adsorption or not, while the Freundlich model considers that the adsorption of molecules in the adsorption process are non-homogeneous multi-layer adsorption and molecules directly interact with each other during the adsorption process20.

The adsorption isotherms were fitted using Langmuir model and Freundlich model.

$$\frac{{c_{e} }}{{q_{e} }} = \frac{{c_{e} }}{{q_{m} }} + \frac{1}{{q_{m} k_{1} }}$$
(3)
$$q_{t} = \frac{{(\rho _{0} - \rho )v}}{m}$$
(4)

Qe—equilibrium adsorption amount (mg/g), Qm—maximum adsorption amount (mg/g), Ce—concentration of printing dye when adsorption reaches equilibrium (mg/L), K1—Langmuir’s adsorption constant, K2—Freundlich’s constant, n—adsorption strength.

Two isothermal models, Langmuir and Freundlich, were used to investigate the adsorption performance of OP-AC on seven organic dyes. Figure 7a and b shows Langmuir and Freundlich isotherms, and parameters of the adsorption isotherms are shown in Table 2. The correlation coefficients R2 of the Langmuir model were 0.9883, 0.9967, 0.9903, 0.9907, 0.9993, 0.9952, 0.9953, respectively, whereas the correlation coefficients R2 derived from the Freundlich model were 0.2307, 0.7592, 0.6404, 0.301, 0.8402, 0.7741, 0.6053, which indicates that OP-AC adsorption process for seven printing and dyeing dyes fitted Langmuir model better, indicating that the adsorption process is a homogeneous monolayer molecular adsorption. The Qmax of MB, MO, CR, BL, MG, CV, and RHB of Langmuir model are 1.814, 2.176, 2.205, 2.277, 2.982, 1.959, 2.04 mg/g, which is close to actual value 1.823, 2.126, 2.315, 2.237, 2.912, 1.952, 2.34 mg/g, indicating that the Langmuir model is more suitable to describe the process of dye adsorption by OP-AC. In adsorption process of OP-AC on dye molecules, all ionic groups present in water will not affect each other, and the interaction force between the adsorbed substance and the adsorbent surface is strong, so the adsorbent can form a separate molecular layer, which can be adsorbed to adsorbent surface quickly, and the adsorbent surface after adsorption can still maintain its original properties, and single molecule adsorption is shown in Fig. 8.

Fig. 7
figure 7

(a) Langmuir isothermal model, (b) Freundlich isothermal model.

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Table 2 Adsorption isotherm parameters of seven organic dyes by OP-AC.
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Fig. 8
figure 8

Schematic diagram of dye adsorption by OP-AC.

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Adsorption kinetics

Adsorption kinetics is to investigate adsorption rate during adsorption process, based on the quasi-primary kinetic model and quasi-secondary kinetic model to reflect the adsorption rate to further study adsorption mechanism of OP-AC on printing and dyeing dyes.

The linear equation of quasi-primary kinetic model is given as:

$$\ln (Q_{e} - Q_{t} ) = \ln Q_{e} - k_{1} t$$
(5)

Qt —the adsorbed amount per unit mass at time t, mg/g; Qe —the equilibrium adsorption capacity of OP-AC, mg/g; K1 —the rate constant, 1/min.

The linear equation of the quasi-secondary kinetic model is given by:

$$\frac{t}{{Q_{t} }} = \frac{1}{{k_{2} Q_{e} ^{2} }} + \frac{t}{{Q_{e} }}$$
(6)

Qt —the adsorbed amount per unit mass at time t, mg/g; Qe —the equilibrium adsorption capacity of OP-AC, mg/g; K2 —the rate constant, g/(mg.min).

The quasi-primary and quasi-secondary kinetic models are shown in Fig. 9a, b, and kinetic parameters are shown in Table 3. Adsorption process of seven organic dyes by OP-AC was more consistent with the quasi-secondary kinetic model. The relevant parameters’ R2 obtained from the linear fitting of the quasi-secondary kinetic model were 0.992, 0.9792, 0.9974, 0.9855, 0.9937, 0.9802, 0.987, respectively, which indicated that dye molecules were diffused and adsorbed on surface of OP-AC rapidly during adsorption of dyes on surface of OP-AC, and then after entering tiny pores of the OP-AC, they were adsorbed on surface of OP-AC rapidly. After the dye molecules entered the tiny pores inside OP-AC, they combine with the active sites inside OP-AC at a slower rate. The reason for determining adsorption rate of dye molecules during adsorption process of OP-AC is chemical adsorption process, which is mainly accomplished through the direct interaction force between activated carbon and dye molecules.The Qe of the quasi-first-order models MB, MO, CR, BL, MG, CV, and RHB are 0.432, 1.116, 0.858, 1.255, 1.049, 1.449, 1.141 mg/g, respectively, The Qe of the quasi-secondary models are 1.333, 1.396, 1.334, 1.394, 1.37, 1.426, 1.386 mg/g, respectively. Actual adsorption amount of the dye is 1.343, 1.376, 1.328, 1.374, 1.281, 1.521, 1.421 mg/g. It can be seen that predicted value of the quasi-secondary model is closer to the real value of the experimental data. Therefore, adsorption of dyes by OP-AC is more consistent with the quasi-second-order kinetic model.

Fig. 9
figure 9

(a) Quasi-first-order dynamic model, (b) Quasi-second-order dynamic model.

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Table 3 Quasi-first-order and second-order kinetic parameters.
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Removal rate of seven organic dyes

Under the conditions of OP-AC dosage of 0.6 g, adsorption time of 24 h, and pH (MO, CR, BL: pH = 4, and MB, RHB, MG, BL: pH = 12), OP-AC was used to deal with each of the seven organic dye solutions at 20 mg/L, and removal effects are shown in Fig. 10. Removal rates of seven dye solutions were all above 93.27%, with the highest removal efficiency for RHB (99.01%) and the lowest for CR dye (93.27%). The slightly lower removal rate of CR dye could be attributed to relatively larger molecular mass of dye molecules, and pores formed inside the OP-AC were mostly small and medium-sized, and pore sizes determined by size of adsorbent molecules affected removal rate. For adsorption of CV, MB, RHB, MO, BL, and MG dye molecules, adsorption effect was more than 95% due to combined effect of pore size and intermolecular van der Waals force. Removal effect of the seven organic dyes was as follows: CV> MB> RHB> MO> BL> MG> CR.

Fig. 10
figure 10

Comparison of removal rates of seven printing dyes.

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Applicating OP-AC in treating seven kinds printing and dyeing dyes wastewater

For actual printing and dyeing wastewater, containing difficult to degrade organic substances, with two and more than two kinds of organic dyes, a variety of complex organic dyes coexist and make treatment difficulty. Under the conditions of OP-AC dosage (0.5, 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 g), adsorption time 24 h, and unadjusted pH, the feasibility of OP-AC treatment of mixed dyeing aqueous solution (each dye’s concentration 1.0 mg/L) was explored, and removal effect is shown in Fig. 11. With increasing dosage of OP-AC, removals of seven organic dyes showed a trend of increasing and later decreasing. Removals of RHB, MB, MO, CV and BL were the highest at the dosage of 4.0 g of OP-AC, and removals of CR and MG were as high as 60.2% and 99.4% at 5.0 g. The results were shown in Fig. 11. Because of small amount of ash present in OP-AC, with increase of OP-AC dosage, ash dispersed in the water will affect measurement of dye molecules in aqueous solution by UV spectrophotometer and make removal rate decrease. With OP-AC dosage of.

100 g/L, overall removal effect of OP-AC on mixed dyes was better, with the highest RHB removal rate of 99.6% and the relatively lowest CR of 59.6%. Removal efficiencies of mixed aqueous solutions of seven organic dyes were: RHB> MG> MB> CV> BL> MO> CR, which was different from results of the removal rate of individual dyes adsorbed by OP-AC. Without adjusting pH value of mixed dyes, pH of mixed dyes aqueous solution was measured to be 10.9, which indicated more negative charges were generated by the entry of OP-AC into the aqueous solutions of the dyes. From the conclusion of “3.3 Effect of pH on removal rate of printing and dyeing dyes”, it can be seen that removal of MO, BL and CR is good under acidic environment, while alkaline conditions are favourable for removal of RHB, MB, MG and CV, so removal rate of MO, BL and CR in the mixed dye aqueous solution (pH = 10.9) is lower than that of RHB, MB, MG and CV. In addition, when OP-AC solid enters mixed dye aqueous solution, it generates a surface charge, which is derived from the dissociation of OP-AC surface groups or the adsorption of ions in the solution as well as the pH of the aqueous solution, where the negative charge is generated by dissociation of acidic coordinators of oxygen such as carboxyl and phenol groups on surface, and the positive charge may be the surface oxygen coordinators’ essential characteristics14, and surface charge of OP-AC affects adsorption of dye molecules. Adsorption of mixed dye molecules by OP-AC correlates with the initial pH, forces between each dye molecule, and forces between π-π bonds. Under the combined effects, OP-AC adsorbed RHB, MB, MG, and CV dye molecules in the mixed dye molecules better, and adsorbed MO, BL, and CR dye molecules relatively poorly.

Fig. 11
figure 11

removal rate of seven kinds of mixed printing and dyeing dye water by op-AC.

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Regenerative properties of OP-AC

Reuse performance of OP-AC is an important indice to measure adsorption performance of OP-AC. OP-AC was used in six cycles, and the results are shown in Fig. 12. After 3 cycles, the dye removal rate of OP-AC is still as high as 73.1%. After 6 cycles, removal rate is 68.6%, indicating that OP-AC is good in reuse and a preferred adsorption material with excellent performance. It is possible that surface collapse of reused activated carbon and adsorption site reduces removal rate of dyes.

Fig. 12
figure 12

Reuse analysis of OP-AC.

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Conclusion

OP-AC was obtained after activation and heat treatment with potassium hydroxide using olive pomace as raw material.

  1. (1)

    Removal rate of seven organic dyes was better when the dosage of OP-AC was 0.6 g and adsorption time was 24 h and adsorption effect is CV> MB> RHB> MO> BL> MG> CR. Removal effect of dyes was different under different pH conditions, among which MO, CR and CV were better removed under acidic environment (pH = 4), while the removal of MB, RHB, MG and BL was favourable under alkaline environment (pH = 12).

  2. (2)

    Adsorption process of OP-AC for seven dyes was more in line with the Langmuir isothermal adsorption model, and the correlation coefficients were all greater than 0.98, indicating that the adsorption process of seven dyes was monolayer adsorption. Adsorption kinetics was more in line with the quasi-secondary kinetic model, and the chemical adsorption dominated the adsorption process, with correlation coefficients greater than 0.97.

  3. (3)

    Removal rate of OP-AC was RHB > MG > MB > CV > BL > MO > CR. Reuseable performance of OP-AC still reached 68.6% at the sixth cycle, indicating that OP-AC can be used as a stable adsorbent for adsorption of organic dyes.