Quality of heavy metal-contaminated soil before and after column flushing with washing agents derived from municipal sewage sludge

Residual HMs in the flushed soil and their mobility

One of the aims of soil remediation is a permanent and substantial reduction in the amount, toxicity or mobility of pollutants. In this study, many factors affected HM removal, such as the type of WA, the flow rate of the WA and the type of HM. In general, the residual HM contents in soil flushed at a flow rate of 1.0 ml/min. were significantly lower than those in soil flushed at 0.5 ml/min (p < 0.05).

With all WAs, the decrease in Cu content (2.2–3.7-fold) was larger than that of the other HMs (Fig. 1a). In soil flushed with DOM or HLS, the residual Cu content was lower than that in soil flushed with SHS or Na₂EDTA. Although the amount of Cu removed from the soil was very high (5060.0 mg/kg, on average), its residual content exceeded the permissible level for soil according to national legislation (600.0 mg/kg)²⁸.

Flushing with HLS, SHS and Na₂EDTA decreased Pb content below the limit (600.0 mg/kg)²⁸, but after flushing with DOM, the residual content of Pb in the soil exceeded the permissible value by 2.2-fold (Fig. 1b). This is because DOM extracted from sewage sludge contained low-molecular-weight organics and fulvic acids which have a stronger affinity for Cu than Pb^13,29. HLS and Na₂EDTA removed Pb most effectively, which means that HLS can be considered for remediating Pb-contaminated soil and can serve as a substitute for Na₂EDTA, but DOM should not be used for remediating soil contaminated with this HM.

Zn is a coexisting HM in soil affected by the smelting industry in Poland, but its content is lower than that of Cu and Pb and below the permissible value (2000.0 mg/kg)²⁸. In this study, all WAs decreased the total content of this HM. Soil flushed with DOM had a higher residual Zn content than soil flushed with other WAs (Fig. 1c).

It is not sufficient to assess soil quality only on the basis of total HM content, as the environment risk of a HM stems from its availability and mobility, not its total content. The content of HMs in the F1 fraction can be a good indicator of the quality of HM-contaminated soil and remediated soil, as HMs in this fraction have the highest mobility and availability and pose the greatest risk to the environment. In the contaminated soil in this study, the respective percent contents of HMs in the F1 fraction, termed the mobility factor (MF), were 86.0%, 74.0% and 76.0% for Cu, Pb and Zn, indicating very high environmental risk. Thus, the contents of the F1 fraction made the largest contribution to the overall removal of HMs from the soil.

Although the total contents of HMs in contaminated soil differed considerably, the HMs posed similar risks to the environment, based on the MF values. Soil flushing markedly reduced the concentrations of HMs in the F1 fraction and the MF, except for Pb flushed with DOM (Fig. 2). After soil flushing at 1.0 ml/min, the MF was significantly lower than after soil flushing at 0.5 ml/min (p < 0.05). In soil washed with DOM at 1.0 ml/min, the Cu concentration in the F1 fraction (1236.0 mg/kg) was lower than in soil washed with other WAs, but its mobility was still very high (MF = 61.0%). HLS lowered Pb mobility more efficiently than the other WAs (MF = 35.0% at 0.5 ml/min, MF = 41.0% at 1.0 ml/min) (Fig. 2b). Particularly at the higher flow rate, HLS were also the most effective WA for decreasing Zn mobility from a very high (76.0%) to a medium level (MF = 17.0%) (Fig. 2c). Thus, for simultaneously removing all tested HMs and decreasing their mobility, HLS seem to be the most appropriate WA.

HMs were not completely removed under column flushing conditions. This could be a result of competition between individual HMs for active sites on the WAs, relatively quick saturation of the active sites with exchangeable HMs and sorption of the WAs in soil. Under batch conditions, in contrast, Borggaard et al.³⁰ observed that EDTA and HS from processed cow slurry almost completely removed Cu from the exchangeable, carbonate-bound and oxide-bound fractions. Similarly, our previous study revealed that most of the mobile F1 fraction of Cu, Pb and Zn was removed with most of the tested SS_WAs and Na₂EDTA under both batch and dynamic washing conditions. In the flushed soil, however, the respective MFs for Cu, Pb and Zn were 1.5–4.1-fold, 1.3–10.7-fold and 3.6–9.9-fold higher than in the washed soil⁶. Elimination of the most reactive HM fractions is desirable because these fractions are the most available to plants and the most easily leached.

Changes in soil properties before and after flushing

As indicated by the analysis of soil characteristics (pH, OM, HS and their fractions, ammonium nitrogen and available micro- and macronutrients), soil flushing affected the properties of the soil, but the magnitude and direction of the change depended on the WA that was used.

The WAs that were used had acidic reactions, and as a result, the pH in the soil was lower after flushing (the pH decreased by 0.4, on average, after flushing with SS_WAs, and by 0.6 after flushing with Na₂EDTA). With a particular WA (DOM, Na₂EDTA), the pH after flushing at the tested flow rates did not differ significantly (p > 0.05) (Table 2).

OM content was increased significantly (p < 0.05) by soil flushing with the SS_WAs, but it was decreased by flushing with Na₂EDTA (Fig. 3a). In unflushed soil, OM content was 3.4%, and flushing with SHS increased OM content almost 1.6-fold (to 5.5%), whereas flushing with DOM and HLS increased it to a lesser extent (to 3.8%, on average). In contrast, flushing with Na₂EDTA lowered OM content to 2.8%, on average. Flushing intensity (flow rate) did not appear to affect OM content. Other authors have also reported that soil flushing/washing with waste-derived WAs increases OM or organic carbon content. For example, Juwarkar et al.³¹ stated that organic carbon content in soil increased from 0.3% in contaminated soil to 0.4% after washing with a rhamnolipid biosurfactant produced by Pseudomonas aeruginosa strain BS2. However, reports indicate that soil washing with conventional WAs does not affect the OM content. For example, Gao et al.³² stated that in soil washed with FeCl₃, citric acid and FeCl₃ with citric acid, OM content did not change compared to unwashed soil. Zupanc et al.³³ also reported unchanged soil OM content after soil washing with EDTA: the OM contents of the unwashed and remediated soil were 7.8 and 7.7%, respectively. Similar results were reported by Jelusic and Lestan³⁴. Soil properties such as OM and CEC can influence the removal efficiency of HMs. An increase in OM in soil can enhance pollutant retention in soil³⁵ and diminish their removal. An increase in OM in the flushed soil has a positive effect on soil fertility and soil health³⁶.

This study investigated not only OM content in soil, but also that of humic substances (HS), humic acids (HA) and the fulvic fraction. Although it is known that HS positively affect the physicochemical properties of soil, an analysis of their content in washed/flushed soil is often omitted. It was found that soil flushing with HLS and SHS clearly increased soil HS content by 1.3-fold and over twofold, respectively (Fig. 3b). Flushing with DOM increased HS content only slightly. In contrast, flushing with Na₂EDTA caused 0.7-fold decrease in HS content.

Whereas soil flushing with HLS and SHS increased soil HS content, it decreased the share of HA in HS. In unflushed soil, HA constituted 55.9% of HS and this decreased to 49.5% and 48.9% after flushing with HLS and SHS, respectively. However, it should be emphasized that despite the percent decreases, HA contents were higher in the flushed soils (6.8 mg/g, on average, after HLS and 11.1 mg/g, on average, after SHS) than in the unflushed soil (5.8 mg/g). Similar changes in FF content were noted, i.e., increases after flushing with HLS and SHS and a decrease after flushing with Na₂EDTA. All these changes are consistent with what was observed in our previous study on soil washing with the same SS_WAs, but in batch conditions¹³.

HS content in soil is very important as these substances increase the quality and productivity of soil by improving its structure and ability to retain water and ensuring constant access to nutrients. HS affect the physical, chemical and biological properties of soils to a greater extent than other soil constituents. In the presence of Ca, an increase in HS content, especially that of HA, causes stable aggregates to form, which in turn, improves field water capacity, air capacity, soil porosity and permeability. Therefore, a loss of HS, especially HA, can increase soil compactness, decrease soil aeration, and lead to the presence of reductive chemical conditions³⁷. In this context, it can be seen that soil flushing/soil washing with SS_WAs should not only remove HMs from soil, but also improve soil properties.

In soil, apart from OM and HS content, it is also important to consider the content of nutrients necessary for plant growth. The three principal macroelements are N, P and K, but Ca and Mg are also important. A lack of a clear decrease in ammonium content after soil remediation with Na₂EDTA (Table 2) is a rather unusual phenomenon, as most studies have reported clear decreases in ammonium content. This is probably connected with the fact that most studies examined soil washing, not flushing. Our previous study that used the same soil also found a clear decrease in ammonium content after washing with Na₂EDTA¹³.

Soil flushing with all SS_WAs increased available P, exchangeable K and exchangeable Na content in remediated soils and decreased exchangeable Ca and, in most cases, exchangeable Mg (Table 2). The use of Na₂EDTA also decreased exchangeable Ca and exchangeable Mg content. Gao et al.³² also stated that soil washing with FeCl₃, citric acid and FeCl₃ with citric acid significantly decreased the content of exchangeable Mg. According to those authors, Ca²⁺ and Mg²⁺ compete with HMs for the binding sites of FeCl₃ or citric acid during soil washing. As a result, Ca and Mg are removed along with the HMs. Wasay et al.³⁸ made similar observations, but the amount of macronutrients that were washed out depended on the WA that was used. Those authors used salts of weak organic acids (citrate, tartrate, oxalate with citrate) and chelating agents (EDTA or DTPA) to remediate polluted soils in a column experiment. Four to five times more macronutrients, such as Ca, Mg and Fe, were leached with EDTA and DTPA than with citrate. Salts of tartrate also leached only a small amount of macro-nutrients. EDTA and DTPA form strong chelates with soil macronutrients that have much higher stability constants than those of chelates formed with citrate or tartrate. Therefore, EDTA and DTPA also extracted other soil nutrients, thereby decreasing the chemical value of the soil.

In our study, soil flushing with Na₂EDTA did not noticeably change the content of ammonium nitrogen, available P or exchangeable K. Importantly, the flow rate of all the WAs did not appreciably affect the macronutrient content in the soil. These results indicate that the use of SS_WAs during column flushing can play an important role in preserving nutrients in soil.

Dehydrogenase activity and plant growth in flushed soil

Soil enzyme activity reflects soil contamination but also can serve as an indicator of biogeochemical cycles, OM degradation, and soil remediation processes. Thus, soil enzyme activity can indicate soil quality and may be useful for indicating both the degree of deterioration of soil quality or the recovery of soil function after remediation. Dehydrogenase is a soil enzyme which is found in intact cells, but does not accumulate extracellularly in the soil. It is known that dehydrogenases participate in and assure the correct sequence of biochemical pathways in soil biogeochemical cycles, and thus provide information on the biological activity and microbial populations in soil. Based on the DHA, the quality of the soil and the degree of regeneration of degraded soils can be assessed. Moreover, DHA is sensitive to HMs presented in soil, including Cu, Zn and Pb. Therefore, in this study, DHA was chosen to analyze soil enzymatic activity after soil flushing, as was done in Klik et al.¹³ after soil washing in batch conditions.

In the contaminated soil, DHA was very low, 4.4 µg TPF/g d.w.·h. In soil flushed with Na₂EDTA at 0.5 ml/min and 1.0 ml/min, DHA increased by only ca. 1.7–2.0-fold, to 7.4 µg TPF/g d.w.·h and 8.7 µg TPF/g d.w.·h, respectively. After flushing with SS_WAs, particularly with HLS and SHS, DHA was markedly higher (14.9–16.0 µg TPF/g d.w.·h and 16.7–18.6 µg TPF/g d.w.·h, respectively) (Fig. 4). Lower DHA after flushing with DOM resulted mainly from low efficiency of this WAs in Pb removal, mainly exchangeable and acid soluble fraction (F1), as this fraction have the highest mobility and availability and pose the greatest risk to the environment.

Although, in this study, DHA increased seriously after soil flushing, our previous study with the same soil and WAs, showed that soil washing enabled to obtain higher degree of soil regeneration: in soil after DOM, SHLS and SHS equalled 21.1 µg TPF/g d.w.·h, 30.7 µg TPF/g d.w.·h and 34.8 µg TPF/g d.w.·h, respectively¹³ and were about twofold higher than after soil flushing (this study). The smaller increase in DHA after washing with Na₂EDTA may be caused by the chelating agent adsorbing on the soil surface³⁹ and negatively affecting soil enzyme activity.

The increase in DHA after soil washing/flushing with HLS and SHS may have been due to the decrease in HM content in remediated soil, especially in the F1 fraction, and to the improvement in soil fertility, especially the increase in soil OM. An increase in OM content can increase DHA by increasing the amount of substrate for the enzymes. Although the contents of soil OM were similar after flushing (the present study) and washing¹³, DHA was lower after flushing. This is probably because the contents of HMs after soil remediation were higher in the present study than in Klik et al.¹³.

Soil phytotoxicity

To evaluate the usability of soil remediated by column flushing with SS_WAs and Na₂EDTA, tests involving seed germination (germination rate, GR) and growth of wheat (Triticum aestivum) roots and shoots were used. These tests are commonly used for assessing toxicity in environmental samples^1,7,40.

In unflushed soil, the GR was 0, meaning that none of the seeds germinated, due to the very high total Cu and Pb concentrations which inhibited seed germination. After flushing with all the tested WAs, the seeds germinated. However, the percentage of germinated seeds was higher in soil flushed with SS_WAs (GR 76.7–86.7%) than in soil flushed with Na₂EDTA (GR 63.3–66.7%) (Table 3). Other authors have reported that, although soil polluted with Cd, Pb and Zn inhibits seed germination to some extent (GR on the level ca. 40.0–70.0%), it does not inhibit germination completely¹. They found that, after washing with GLDA and ISA, the GR increased by 13–40%. These increases are smaller than those in the present study because, in their study, seed germination was not completely inhibited.

It is worth noting that, in the study presented here, even though the Cu and Pb concentrations in the F1 fraction were lower after flushing with Na₂EDTA than after flushing with SHS, the GR was lower after Na₂EDTA flushing. This may indicate that chelating agents not only decrease soil enzyme activity but also inhibit seed germination. Inhibition of seed germination by residual chelating agents may be caused by a lack of the substances and energy needed for seed germination, as indicated by reduced breakdown of starch and proteins in seed storages in soil containing residual EDTA, which has limited biodegradability^1,8,41.

In the present study, the GR in unflushed soil was 0%. Soil flushing substantially improved the GR, even though the GR values were still lower in the flushed soils (63.0–67.0% after Na₂EDTA; 83.0–87.0% after HLS) than the value in the control (unspiked) sample (100%). There was less improvement in shoot and root lengths after flushing (Fig. 5a,b), and the corresponding inhibition factors remained high (shoot inhibition, I_s, and root inhibition, I_r) with values in range of 88.0–96.0% (Fig. 5c,d). Although some authors have indicated that roots are more sensitive than shoots to changes in soil after washing⁴², in the present study, both appeared to be very sensitive to the effects of the HMs remaining in the flushed soil, as flushing did not substantially stimulate their growth. Similarly, Feng et al.²⁵ reported that the length of rice roots in soil washed with two plant extracts (from Fagopyrum esculentum and Fordiophyton faber) and EDTA did not differ in a statistically significant manner from that of rice roots in contaminated soil.

Comparison of soil quality: soil flushing vs. soil washing

Table 4 presents selected properties in the contaminated soil and the flushed soil in the present study, as well as these properties in soil after batch washing with DOM, HLS, SHS and Na₂EDTA¹³. When using SS_WAs, the decrease in pH was smaller with soil flushing than with soil washing, but the increase in OM content in soil, and in the content of the HS, FF and HA fractions was larger with soil flushing. When applying Na₂EDTA, more soil OM was lost during flushing than during washing.

Overall, the fertility of flushed soil was lower than that of washed soil (with the exception of OM and HS content). For example, although using SS_WAs with both methods increased soil content of NH_4, available P, exchangeable K and exchangeable Na, the increases were smaller with soil flushing. However, the loss of exchangeable Ca was smaller with soil flushing than with soil washing, except when SHS was used. Except for HLS, washing or flushing with all WAs decreased the content of exchangeable Mg. With regard to HLS, soil washing increased exchangeable Mg content to a greater extent than soil flushing.

Although soil flushing decreased total HM content in the soil, soil washing was more effective. The reasons for the lower HM removal in the column flushing may be a shorter time for the HM in soil to react with WA in comparison to batch washing⁴³. With most of the tested HMs and WAs, the HM concentration in the F1 fraction was higher after soil flushing than after soil washing. However, when using SHS, similar amounts of Pb were removed from the F1 fraction with both techniques; likewise, when using HLS, similar amounts of Zn were removed from this fraction with these techniques. DOM should not be recommended for Pb removal via soil flushing or soil washing because of the small amount of this HM that it removed from the F1 fraction and in total.

Both soil flushing and soil washing considerably improved the germination rate of T. aestivum. When flushing the soil, this effect was more pronounced with the SS_WAs than with Na₂EDTA. Due to its greater content of residual HMs and higher HM mobility, soil flushed with the tested WAs was more phytotoxic (based on the I_s and I_r factors) than soil washed with these WAs.

The quality of flushed soil was better than that of washed soil with regard to pH and the content of OM, including HS and their fractions. Despite the considerable decrease in HM content in the flushed soil, the residual HM content in the F1 fraction was relatively high. Thus, soil flushed with these WAs would require more monitoring for potential environmental risk than washed soil.

Source: Ecology - nature.com