1 Introduction

Chromium is an element of significant environmental concern (Fruchter 2002; Oze et al. 2007; Kazakis et al. 2018). The toxicity of chromium to biota depends on its oxidation state of presence with free hexavalent chromium (Cr(VI)) as the most toxic form among the chromium pools (Abreu et al. 2014; Antoniadis et al. 2018; Patra et al. 2018). Immobilization of aqueous Cr(VI) or transformation of Cr(VI) to other less toxic species, mainly trivalent chromium (Cr(III)), is among the major strategies for reducing the environmental impacts from chromium. Cr(VI) usually occurs in oxy-anionic forms as chromate (CrO42−) or dichromate (Cr2O72−). Conversion of Cr(VI) into Cr(III) via reduction results in the change of chromium from an oxy-anionic form to a cationic form (Owlad et al. 2009). Unlike chromate or dichromate, cationic Cr3+ is subject to immobilization through hydrolysis or adsorption by negatively charged colloids under non-acidic environmental conditions (Tytłak et al. 2015).

Organic acids act as reductants to reduce Cr(VI) in environmental media (Deng and Stone 1996). It was shown that Cr(VI) could be transformed to Cr(III) by Fe2+ in the presence of fulvic acids (Agrawal et al. 2009). Cr(VI) can also be photochemically reduced to Cr(III) by low-molecular-weight organic acids in the presence of either dissolved Fe3+ or adsorbed Fe3+ (Sun et al. 2009). Biochar has the capacity to immobilize trace elements (Heaney et al. 2018; Qin et al. 2020a, b). Chen et al. (2015) compared the immobilization of Cr(III) and Cr(VI) by biochar and found that the biochar material removed more Cr(III) than Cr(VI). This was attributed to cation exchange and precipitation of Cr3+ on the biochar surface due to biochar-driven increase in pH, which, on the other hand, disfavoured adsorption of Cr(VI) that was present in oxy-anionic forms (Saha and Orvig 2010).

In the co-presence of biochar and LMWOAs, the surfaces of biochar could be protonated by LMWOAs (Heaney et al. 2020), which complicates the processes regulating immobilization of trace elements by the biochar (Alozie et al. 2018; Achor et al. 2020). Qin et al. (2020a, b) investigated the effects of pyrolysis temperature and types of LMWOAs on biochar-driven reduction of Cr(VI) and found that oxalic acid and malic acid tended to have better effects on enhancing biochar-driven Cr(VI) reduction compared to citric acid, and biochar produced at 300 °C was more favourable for Cr(VI) reduction compared to the higher-temperature biochars. However, the above work was limited to a fixed dose of biochar and a relatively short period of reaction time. Increase in biochar application rate and reaction time is likely to markedly affect the behaviour of chromium in the combined biochar and LMWOA reaction systems but such effects are still unclear. This knowledge gap needs to be filled to optimize the reaction conditions for developing cost-effective procedures for treating Cr(VI)-containing wastewater. The objective of this work was therefore to examine the effects of biochar dose and reaction time on the fate of chromium in the biochar–LMWOA–Cr(VI) systems.

2 Materials and methods

2.1 The biochar materials used in the experiment

Three biochar materials made from the stalks of Pennisetum hydridum at a pyrolysis temperature of 300, 500 and 700 °C (labelled as Biochar300, Biochar500 and Biochar700, respectively) were used in the experiment. The details of the procedure for producing these biochar materials have been reported previously (Qin et al. 2020a, b). The physiochemical characteristics of the three biochar materials are shown in Table 1. Biochar300 had a porous structure with thick pore walls. In contrast, Biochar500 and Biochar700 showed much thinner pore walls and greater uniformity. The amount of flakes inside the pores of biochar increased with increasing pyrolysis temperature.

Table 1 The pH, EC, BET and elemental composition of the biochar materials produced under different pyrolysis temperatures
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The biochar materials were ground with a pestle and mortar to pass a 2 mm sieve prior to use in the experiment. The major characteristics of the biochar materials are provided in the Supplementary document.

2.2 Batch experiment design

Consistent with the previously reported work (Qin et al. 2020a, b), the concentration of citric acid, oxalic acid and malic acid was set at 0.01 M; the reacting solution was fixed at 100 mL and the concentration of Cr(VI) in the solution was set at 100 mg/L. However, the dose of biochar materials was doubled to 20 g/L for the current experiment. The experimental set-up is detailed in Table 2.

Table 2 Experimental setup details
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Plastic bottles with a capacity of 150 mL were used as batch reactors. For the controls, 100 mL of Cr(VI) solution at a concentration of 100 mg/L was used. For the LMWOA treatments, 50 mL of Cr(VI) solution at a concentration of 200 mg/L was mixed with 50 mL of a respective organic acid at 0.02 mM. For the added biochar treatments, 2 g of biochar was added into each batch reactor. After adding all the ingredients, the batch reactors were shaken in a rotary shaker for 2 h and then allowed to stand in dark during the period of the experiment. The pH and electrical conductivity (EC) of the solutions were determined after 1 and 7 days of the experiment. An aliquot of solution in each batch reactor was also taken after pH and EC measurements for determination of total Cr and Cr(VI). The spent biochar materials for the control and various treatments were washed with deionized water three times for preparation of samples for XPS and SEM–EDS analyses.

2.3 Analytical methods

The pH and EC in the solution samples were measured with a PHSJ-5 pH meter and PXSJ-216F EC meter, respectively. After filtration with a 0.45 µm membrane filter, Cr(VI) in the solutions was determined by the diphenylcarbazide colorimetric method (Deng and Stone 1996). The total Cr in the solutions was measured by inductively coupled plasma mass spectrometry (Agilent 7700 ICP-MS) and the operating conditions for the ICP–MS are provided in Supplementary Table S1. The surface morphology and structure of the spent biochar samples were observed using a scanning electron microscope (SEM, Merlin, Zeiss, Germany). The chemical composition of the spent biochar surfaces was determined by energy-dispersive X-ray spectroscopy (Quantax200 with X-Flash 6/100, Brüker, USA). XPS analysis of the spent biochar surfaces was performed using a Kratos X-ray photoelectron spectrometer (Axis Ultra DLD).

2.4 QA/QC and statistical analysis methods

The experiment was performed in triplicate. All chemical reagents used in the experiments were of analytical reagent grade. Ultrapure water (18.2 MΩ/cm) was used throughout the entire course of all the experiments. The statistically significant difference between the treatment means was determined by Duncan’s multiple range test using SPSS (IBM SPSS software Version 23.0).

3 Results

3.1 No-added biochar reaction system

The control had a pH of 2.20 after 1 day of the experiment, which was significantly higher (p < 0.05) than that in the citric acid treatment and the oxalic acid treatment, but not significantly (p > 0.05) different from that in the malic acid treatment. The oxalic acid treatment had the lowest pH (p < 0.05) among the control and treatments. The solution pH tended to increase after 7 days for both the control and the treatments except for the citric acid treatment (Fig. 1a).

Fig. 1
figure 1

EC (a), pH (b) and concentration of Cr(VI) (c) and total Cr (d) in the control and various LMWOA treatments in the absence of biochar. All values are presented as mean ± standard error (n = 3). Means with different letters in the same column for each parameter are significantly different at p < 0.05

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The value of EC after 1 day of experiment showed the following decreasing order (significant at p < 0.05): oxalic acid treatment > citric acid treatment > malic acid treatment > control. The pattern was not changed after 7 days of the experiment (Fig. 1b).

The total Cr concentration in all the solutions was around 100 mg/L after both 1 and 7 days of the experiment, comparable to the theoretical concentration of the added Cr(VI). However, Cr(VI) concentration was significantly lower in all the treatments than that in the control. There was no significant difference in Cr(VI) concentration between the oxalic acid treatment and the malic acid treatment, and the concentration of Cr(VI) in both was significantly lower (P < 0.05) than that in the citric acid treatment. After 7 days of the experiment, the Cr(VI) concentration was markedly reduced for the oxalic acid and malic acid treatments, while the concentration of Cr(VI) in the control and the citric acid treatment remained at around 100 mg/L (Fig. 1c, d).

3.2 Added biochar reaction system

3.2.1 1-day reaction time

For Biochar300, the EC value in the control was 4.9 dS/m. The addition of a LMWOA raised the EC to nearly 6.0 dS/m. The solution EC increased to nearly or above 11.0 dS/m in the Biochar500 treatments and nearly or 13.0 dS/m in the Biochar700 treatments. The pH in the control was 5.60 for Biochar300, but significantly dropped to 4.27, 4.13 and 4.50 in the citric acid treatment, oxalic acid treatment and malic acid treatment, respectively. For the control and any LMWOA treatments, the pH significantly increased from the Biochar300 treatment to the Biochar500 treatment to the Biochar700 treatment, with the pH being always significantly higher in the control than in any of the LMWOA treatments (Table 3).

Table 3 Solution EC, pH, Cr(VI) and total Cr after the 1-day reaction in the control and the treatments with different LMWOAs in the presence of different biochar types
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For Biochar300, no Cr(VI) was detected for any of the LMWOA treatments but there was still 41.5 mg/L of Cr(VI) in the solution for the control. The concentration of total Cr in the control and all the LMWOA treatments was lower than the theoretical concentration of added Cr(VI). For Biochar500, the concentration of Cr(VI) in both the control and all the treatments was lower than the theoretical concentration of the added Cr(VI) with the following decreasing order: oxalic acid > (insignificant) malic acid > (significant) citric acid > (insignificant) control. The concentration of Cr(VI) in the control for Biochar700 was lower than the theoretical concentration of the added Cr(VI) and the same decreasing order as that of Biochar500 was observed. Overall, there was a general trend in that Cr(VI) and the total Cr increased with the pyrolysis temperature for the control or a respective organic acid treatment (Table 3).

For Biochar300, no Cr(VI) was detected for any of the LMWOA treatments, but there was still 41.5 mg/L of Cr(VI) in the solution for the control. The concentration of total Cr in the control and all the LMWOA treatments was lower than the theoretical concentration of added Cr(VI). For Biochar500, the concentration of Cr(VI) in both the control and all the treatments was lower than the theoretical concentration of the added Cr(VI) with the following decreasing order: oxalic acid > (insignificant) malic acid > (significant) citric acid > (insignificant) control. The concentration of Cr(VI) in the control for Biochar700 was lower than the theoretical concentration of the added Cr(VI) and the same decreasing order as that of Biochar500 was observed. Overall, there was a general trend in that Cr(VI) and the total Cr increased with the pyrolysis temperature for the control or a respective organic acid treatment (Table 3).

3.2.2 7-day reaction time

After 7 days of reaction, there was only little change in pH and EC in all reaction systems except for the pH of the Biochar300 system, which showed an increase, as compared to the 1-day system. Cr(VI) in the control disappeared. For Biochar 500, Cr(VI) in the control and the citric acid treatment increased compared to the 1-day treatment. For Biochar700, increase in the control and all the LMWOA treatments was observed especially for the control, which had a concentration of Cr(VI) close to the theoretical concentration of the added Cr(VI). The concentration of the total Cr in all the Biochar300 treatments decreased compared to the 1-day treatment, while the Biochar500 and Biochar700 treatments all had a total concentration of Cr close to the theoretical concentration of the added Cr(VI) (Table 4).

Table 4 Solution EC, pH, Cr(VI) and total Cr after the 7-day reaction in the control and the treatments with different LMWOAs in the presence of different biochar types
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3.3 XPS and SEM–EDS analyses of the spent Biochar300

Since no significant Cr adsorption was observed for Biochar500 and Biochar700, XPS and SEM–EDS results presented here are only limited to the spent Biochar300. No Cr(III) oxide peak at ~ 576 and Cr(VI) oxide peak at ~ 580 eV were identifiable from the XPS spectra for the control and any of the three LMWOA treatments (Fig. 2). EDS analysis showed the presence of C, O, Si and K on the surfaces of all the spent biochar materials. However, Cr was not detectable on the spent biochar surfaces. Cu was detected in the control and the citric acid treatment, while Ca was detected in the malic acid treatment (Fig. 3).

Fig. 2
figure 2

XPS spectra in the area from 570 to 595 eV for the spent Biochar300 in the control and the three LMWOA treatments

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Fig. 3
figure 3

SEM images and EDS results for the spent Biochar300 in a the control, b citric acid treatment, c oxalic acid treatment and d malic acid treatment

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4 Discussion

The results from the experiment without added biochar (Fig. 1) suggested that citric acid had no effect on reducing Cr(VI), while reduction of Cr(VI) took place in the presence of oxalic acid and malic acid at a concentration of 0.01 M with nearly 20% and over 80% of the added Cr(VI) being transformed into possibly Cr(III) after 1 day and 7 days, respectively. The capacity of oxalic acid and malic acid to reduce Cr(VI) at the concentration set for the experiment was very similar and the oxalic acid- or malic acid-driven reduction of Cr(VI) was under kinetic control. These results differ from those of some published studies (Kim and Choi 2011; Mu et al. 2018), where at 24 h and 90 min, respectively, Cr(VI) reduction to Cr(III) was only slightly influenced by the presence of organic acids. However, the current findings are consistent with the findings of Chen et al. (2013), who demonstrated negligible citric acid-driven reduction of Cr(VI), but observed that 19% and 68% of Cr(VI) were reduced by malic acid and tartaric acid, respectively, after 10 h contact time. The greater percentage of transformed Cr(VI) in the current study may be due to the longer contact time between Cr(VI) and the organic acids. In addition, the reduction of Cr(VI) to Cr(III) was likely to be via the formation of Cr(III)–malic and oxalic acid complexes, which may be kinetically stable (Uluçinar and Nur Onar 2005). By comparison with the previous results (Qin et al. 2020a, b) with the biochar dose being set at 10 g/L, the increase in the biochar dose to 20 g/L raised the solution pH due to more OH− being added into the reaction system. The pH rise in the control was evident for the Biochar300 treatment that had relatively lower pH status, as compared to the higher-temperature biochar treatments. The mean pH of all the LMWOA-treated solutions was increased with increasing dosage level for the three biochar treatments (Fig. 4a). The pH rise due to increased biochar dosage level favoured the removal of water-borne Cr by Biochar300 in the control and LMWOA treatments (Fig. 4b). This is attributable to the enhanced de-protonation of the biochar surfaces, which allows adsorption of cationic Cr(III) by the increased negatively charged biochar surface sites. Lower-temperature biochar tends to have a stronger reducing capacity (Klüpfel et al. 2014). This explains the dominant presence of Cr(III) in the Biochar300 systems in contrast to the Biochar500 and Biochar700 systems.

Fig. 4
figure 4

Comparison of a pH and b Cr in the solutions between different biochar doses for various treatments [data concerning the treatments with a biochar dose at 10 g/L were derived from Qin et al. (2020a, b)]

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Increase in the length of reaction time markedly raised the pH for Biochar300. But for the higher-temperature biochars, only slight increase or decrease in pH was observed (Fig. 5a). For the control in Biochar300 system, this resulted in the disappearance of all Cr(VI) in the solution after the 7-day reaction time (Fig. 5b). It was likely that increased dosage level of biochar enhanced the reduction of Cr(VI). It is not clear whether the adsorbed Cr on the biochar was in a form of Cr(VI) or Cr(III), because XPS analysis failed to detect the Cr species on the surface of the spent Biochar300. The adsorption rates of Cr in the control and the LMWOA treatment for Biochar300 were approximately 3.5 and 2.5 mg of Cr per gram of biochar. The low density of Cr on the spent biochar surfaces made the Cr 2p peaks unidentifiable in the presence of background noise (Fig. 2). This also explains the absence of any Cr peaks in the EDS spectra (Fig. 3). However, the increase in pH favoured the deprotonation of the biochar surfaces, which could promote the sorption of cationic Cr(III). This also applies to the LMWOA treatments for Biochar300 where the solution pH increased after the 7-day reaction. It is interesting to note that increase in reaction time for the higher-temperature biochars did not bring about further reduction of Cr(VI) to Cr(III). On the contrary, it caused re-oxidation of Cr(III) to Cr(VI) since the proportion of Cr(III) in the total Cr decreased over time (Fig. 5b). This was particularly evident for the controls where the reducing strength was relatively weaker, as compared to the LMWOA counterparts. The re-oxidation of Cr(III) was related to the increased exposure of the Cr(III) to atmospheric oxygen, but the exact mechanism responsible for this process requires further investigation.

Fig. 5
figure 5

Comparison of a pH and b Cr in the solutions at different reaction durations for various treatments

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The research findings obtained from this study have implications for optimizing treatment procedure for wastewater that contains elevated level of toxic Cr(VI). Simulation experiments are required to determine appropriate biochar dose and reaction time to achieve cost-effective treatment goals.

5 Conclusions

Increased biochar dose raised solution pH particularly for the Biochar300 treatments, but did not enhance the reduction of Cr(VI) after the 1-day reaction. Part of the Cr(III) converted from Cr(VI) was adsorbed by the biochar due to increase in the negatively charged sites on the biochar surfaces. The elevated pH due to biochar dose increase tended to slow down the reduction of Cr(VI) to Cr(III), resulting in more Cr(VI) being adsorbed at a higher biochar dose. For the higher-temperature biochars, the increase in biochar dose did not markedly change the transformation and immobilization of the added Cr except for the control for the Biochar700 treatment where the increase in biochar dose enhanced the reduction of Cr(VI) to Cr(III). Increase in the length of reaction time markedly raised the pH for Biochar300. However, only slightly increase or decrease in pH was observed for the higher-temperature biochars. For the Biochar300 treatments, this resulted in the disappearance of all Cr(VI) in the solution after the 7-day reaction, most likely due to sorption of cationic Cr(III) to the biochar surfaces. Increase in reaction time for the higher-temperature biochars caused re-oxidation of Cr(III) to Cr(VI), particularly for the controls. The re-oxidation of Cr(III) was likely related to the increased exposure of atmospheric oxygen.