The raw materials and reagents used during the experiment are shown in Table 1 .

The instruments and analytical equipment used during the experiment are shown in Table 2 .

1) Scanning electron microscopy (SEM)

During the SEM test, the powder sample was coated on the conductive adhesive and blown out with the ear washing ball. Then, the morphology and microscopic surface structure of the sample were observed at the voltage of 20 V and 20 μ A.

2) X-ray diffraction (XRD)

XRD testing was performed using Shimadzu IR Prestige-21. Sample crystal structure was analyzed by X-ray diffractometer. The rotating cathode is the Cu target K α-ray source with tube voltage 40 kV and current 40 mA. XRD scan range according to sample selection.

3) Infrared spectroscopy (FT-IR)

FT-IR spectroscopy test Brooker Fourier transform infrared spectrometer to analyze the functional groups contained in the sample. The powder sample was mixed with potassium bromide, fully ground, pressed, and placed into the sample chamber to start testing in a range of 4000 - 500 cm⁻¹.

The solution containing heavy metal chromium adds nitric acid and sulfuric acid and reacts with the chromoagent (diphenyl dihydrazide), and the solution becomes purple. The maximum absorption wavelength is 540 nm. The upper limit of this method is, the minimum concentration is 1 mg/L, and the minimum amount is 0.2 μg.

The absorbance of the adsorption solution was added into the standard curve to obtain the solution concentration Ce. The calculation formula of the adsorption capacity of the sample is as follows: $Qe=\frac{\left(Co-Ce\right)V}{m}$

where: Qe is the equilibrium adsorption amount of the sample, mg/g;

Co and Ce are Cr () concentration before and after adsorption, mg/L;

V is the solution volume, mL;

and, m is the sample mass, g;

Cr () removal rate is calculated as follows: $\text{removal}\text{rate}\left(\%\right)=\frac{Co-Ce}{Co}\times 100\%$.

Among them: Co and Ce are the same as above.

The modified activated carbon was added to a tapered bottle filled with chromium solution and put into a constant temperature shaker for 10 hours. After the full reaction, the reacted solution took the filtrate from a syringe and determined the hexavalent chromium ion content. From the three variables of adsorbent dose and pH, the effect of different factors on the effect of hexavalent chromium ions of modified activated carbon in water was studied, and the adsorption capacity of modified activated carbon was explored. The comparison between the front and back of adsorption is shown in Figure 1 .

Mix 5 ± 0.01 g activated carbon (powder), thiourea 5 ± 0.01 g and potassium hydroxide 5 ± 0.01 g for one hour and place in tube furnace 500˚C for calcination for more than two hours. After the temperature is reduced, 10 percent hydrochloric acid is added. After the acid solution, wash it to neutral and place it in the electric thermal constant temperature blast drying box. After the sample is dried, put it into the sample bag for backup.

Cigh potassium dichromate (K₂Cr₂O₇, Analytical pure) 0.2829 ± 0.0001 g, dissolved in a beaker with distilled water and moved into a 1000 mL volumetric flask, and diluted to 1000 mL with distilled water. This solution is a 100-mg/L hexavalent chromium solution.

Weigh 0.2 ± 0.0001 g of diphenyl dihydrazine in 50 mL of acetone into a brown volumetric flask, add distilled water and dilute to 100 mL, shake well and store in a 4˚C thermostatic refrigerator.

Transfer 50 mL of phosphoric acid solution into a 100 mL volumetric flask with distilled water to 100 mL.

Transfer 50 mL of sulfuric acid solution to a 100 mL volumetric flask to 100 mL.

1) Effect of initial chromium concentration on adsorption

Transfer a 200 mg/L hexavalent chromium solution of 25 mL, 50 mL, 75 mL, 100 mL, and 125 mL into a 250 mL volumetric flask, and adjust it to 250 mL at 10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L, and 50 mg/L, respectively. Transfer 30 mL of chromium solution at 10 mg/L, 20 mg/L, 30 mg/L, 40 mg/L and 50 mg/L each to the taical flask, and add 0.05 g of adsorbent in a thermostatic shaker for 25˚C for 6 hours. Take out the conical bottle, take the filtrate, according to the determination principle of hexavalent chromium to measure the absorbance, this experiment using diphenyl spectrophotometry of hexavalent chromium content, adsorption of hexavalent chromium solution diluted one hundred times, take 0.1 ml of adsorption solution to 10 mL of color tube, after volume to add 0.5 mL 1:1 phosphoric acid and sulfuric acid, then add 2 mL color agent. The absorbance was measured by a visible spectrophotometer at 540 nm, and then the adsorbed concentration was calculated.

2) The effect of the injection amount of the adsorbent on the adsorption performance

The amount of adsorbent was 10 mg, 20 mg and 30 mg, the chromium concentration of 20 mg/L was transferred to the conical flask, 10 mg, 20 mg and 30 mg adsorbent were added to the conical flask for three sets of parallel tests, and then the conical flask was placed in a constant temperature shaker for 25˚C for 10 hours. Take out the conical bottle, take the filtrate, measure the absorbance according to the determination principle of hexavalent chromium, and calculate the chromium concentration.

3) Effect of pH on the adsorption properties

The chromium solution with a concentration of 20 mg/L was transferred into the tapered flask, 10 mg adsorbent was added, and pH was adjusted to 2, 4, 6, 8 with HCl and NaOH, and three sets of parallel tests were performed. The conical bottle was placed in a temperature shaker for 25˚C for 10 hours. Take the filtrate, measure the absorbance according to the determination principle of hexavalent chromium, and calculate the chromium concentration. The experimental results show that Cr () from activated carbon is mainly removed by electrostatic adsorption, and the increase of pH will reduce the adsorption effect.

The principle of X-ray diffraction is used to analyze the composition of the substance, and the composition of the substance is used to analyze the properties of the adsorbent. According to Figure 2 , four obvious diffraction peaks, respectively at 13˚, 21˚, 27˚ and 29˚, were found, indicating that the activated carbon contains impurities, and the modified X-ray diffraction spectrum is seriously broadened, indicating that the crystallization of the sample is poor, and most of them are amorphous.

Infrared spectroscopy (IR) is an effective method for analyzing chemical functional groups. Black activated carbon is easy to absorb infrared light. The infrared spectrometer is mainly used to analyze the functional groups of the samples to be tested. According to the measurement, it can be seen from Figure 3 that the functional groups before the modification mainly include hydroxyl group, carbon-hydrogen bonds, carbon and oxygen bonds, etc., and the modified adsorbent has an absorption peak at 3400 and a hydroxyl group, still at 2930, 1100, 960 and 650 cm⁻¹. There are infrared absorption peaks, in addition to the functional groups contained in the activated carbon before modification, there are carboxylate and other functional groups after modification [23] .

From Figure 4 , we can see the surface of activated carbon is smooth, enlarge ten thousand times can see activated carbon surface contains some functional groups and activated carbon pore, activated carbon contains a lot of carbon microspheres, gap small spherical degree is higher, relatively uniform distribution, carbon microsphere size mainly concentrated between 1 - 10 μm, the smallest ball diameter of about 1 μm, and the ball diameter of 5 μm ball distribution most.

This Figure 5 shows that the sulfur and nitrogen are attached to the surface of the activated carbon. By a can see the modified activated carbon surface is very rough, magnified by Figure 5(b) ten thousand times, the modified activated carbon surface attached to some small particles compared with Figure 4(b) is more rough, by c can clearly see the modified activated carbon surface attached with many functional groups, visible sulfur nitrogen has doping to the surface of activated carbon, and not modified the modified adsorbent more rough, surface more functional group, pore is more, so the modified activated carbon adsorption effect is better. From Figure 6 , we can see that sulfur and nitrogen are doped on the surface of activated carbon.

According to the calculation, the removal rate reached 80% to 90% at the initial concentration of 10 mg/L, 60% at the initial concentration of 20 mg/L, 40% and 20% at 50 mg/L, respectively. Since the removal rate is the largest when the initial concentration of chromium solution is 10 mg/L, the adsorption capacity is the minimum when the initial chromium concentration is the lowest, the maximum adsorption capacity is about 19 mg/g and the minimum is about 5 mg/g, as shown in Figure 7 .

It can be seen from Figure 8 that when the adsorbent is 10 mg, the removal rate and adsorption capacity are the largest, and the removal rate is 90% and the adsorption capacity is about 54 mg/g. When the addition amount is 30 mg, the removal rate is about 89%, the adsorption capacity is about 16, the addition amount is 10 mg, 20 mg, and the removal rate of 30 mg is not fluctuating.

From Figure 9 , we know the effect of pH on adsorption capacity and removal rate, and the size of pH is inversely proportional to the adsorption effect, and the adsorption effect is best when pH is 2.