1. Introduction
The generation of a large amount of waste is a characteristic of human civilization development. One of the most troublesome wastes systematically generated by humans is sewage sludge (SS). According to the Local Data Bank [
1], in the last 12 years (2010–2022), the amount of SS increased by 10% in Poland. This may not be a dramatic pattern, but the proper management of this waste is needed, especially since the landfill of SS is prohibited in Poland, and its incineration is not the most popular method. The above data [
1] indicate that the use of SS for agricultural and reclamation purposes is a popular method of utilization, in addition to its use for composting and the production of organic fertilizer, i.e., compost. The basis for this direction of SS management is its chemical properties related to its significant and attractive fertilization value [
2,
3,
4,
5,
6,
7]. On the other hand, the possibility of the presence of pollutants that have a toxic effect on the environment, such as heavy metals, PAHs, PCBs, and AOX, should be considered [
8,
9]. In particular, the last negative aspect resulted in the introduction of standards tightening the conditions for the use of SS in agriculture in some European countries (Germany, Norway, and Sweden), up to including a complete ban (e.g., in Switzerland) [
10]. However, in many countries around the world (the USA, China, and South Korea), SS has been successfully used for agricultural purposes because the chemical composition of SS is rich in organic matter and nutrients; thus, it contributes significantly to the improvement in soil fertility [
11]. However, when SS is applied repeatedly, there is a possibility of heavy metal accumulation in the soil and its subsequent incorporation into the food chain [
12,
13,
14]. Considering the dual nature of SS, an alternative is composting. It should be emphasized that the composting process is an environmentally friendly technique that is relatively inexpensive and attractive for agriculture.
At the household scale, composting has been used since immemorial times and is likely the oldest form of recycling [
15]. As a biological process, composting involves simultaneous biochemical and microbiological processes (mineralization and humification), which lead to the transformation of organic matter and physicochemical changes in the composted materials. Additionally, the composting of SS solves many environmental problems. The most frequently mentioned are the avoidance of waste disposal, elimination of bothersome odors, reduction in the mass of waste, degradation of toxic organic compounds, reduction in the bioavailability of heavy metals or transformation of organic matter, and maintenance of nutrient circulation [
2]. Moreover, during the composting process of SS, the quality of organic matter improves, and the sludge is dewatered and sanitized [
15]. The most important product of the entire process of composting SS is the production of organic fertilizer, compost, which is a valuable source of both macro- and micronutrients and organic matter [
11,
16,
17,
18]. However, the use of compost from organic waste is not only related to the assessment of its effectiveness as a fertilizer but also to concerns about whether it can be safely used in agriculture due to the potential content of heavy metals. The origin of the raw materials used for compost production is the most important factor when determining the level of heavy metals in the mature product. In general, composts made from plant waste contain small amounts of metals, and the highest content is found in composts with a high proportion of sludge sewage [
19,
20]. This raises concerns about their possible excessive accumulation in the soil when using composts prepared from sewage sludge. When assessing the value of compost, we consider, among other factors, the total amount of metals that cannot exceed the permissible standards for use as fertilizer. However, as was proven by Jakubus [
12,
21,
22], the total amounts of metals in SS do not necessarily mean that their use both as fertilizers and co-substrate for composting is a threat to soils. Using only a total content, it is assumed that all forms of metals have the same impact on the environment, which is contrary to the actual situation. Therefore, research on the chemical forms of metals in SS-derived composts requires great attention, with particular emphasis on the degree of their bioavailability to plants. Many authors [
12,
14,
21,
22,
23] also claim that, compared to the total metal content, the chemical forms of metals in the solid phase are more important in determining their bioavailability for plants and leaching into groundwater. This approach is of great practical importance because it provides the opportunity to predict the possible behavior of the introduced metal along with the compost in the environment. During a proper process, metals are strongly complexed with stabilized organic matter. As a result, metals from composts enter the soil in more chemically stable combinations, and therefore, they become less available to plants [
15]. As reported by Barthod et al. [
19], this is noticeable in the case of composting with various additives that significantly reduce the bioavailability of metals, resulting in a marketable, safe material. The use of various additives (organic and inorganic) in the process of composting sewage sludge is also highly important because of its high moisture content, small particle size, and thick texture [
19,
24]. According to Zhao et al. [
24], bulking agents, especially those with a high level of recalcitrant carbon, reduce organic matter degradation, enhance the humification process, and consequently improve the quality of final compost products. Additionally, bulking agents are desirable because they significantly improve the structure of the mixture, ensuring proper gas exchange and reducing moisture. For this purpose, various types of straw, sawdust, and bark are most often recommended [
15].
Considering that some metals, such as Cu, Zn, Mn, Ni, and Fe, are also micronutrients and necessary for plants, it is important to consider both their availability for plants and the undesirable nature of excess amounts. Therefore, the analysis of metal combinations becomes necessary for the soil application of compost, especially when the compost is made from SS with various additives as bulking agents. It should be noted that there are no specific methods for evaluating compost. The same techniques, as in the case of soils, are routinely used, and their usefulness and verifiability are satisfactory [
12,
21,
22]. Considering the assessment of the mobility and bioavailability of metals in composts, two independent groups of analyses are proposed: sequential and single. Each of these methods has advantages and disadvantages. Sequential methods are time-consuming but provide precise and comprehensive information. The general idea of sequential extraction is based on subjecting the analyzed sample to subsequent sample “attacks” of solutions (reagents with different chemical properties: pH, redox potential, and complexing abilities) with increasing aggressiveness. In this way, the basic goal of sequential extraction is to achieve the successive washing of the component from various compounds and its various connections with the phase constant of the sample, with extraction taking place from the most mobile, easily soluble forms and ending with permanent, hard-to-dissolve forms. Such a goal can be achieved using the BCR sequential extraction method, which allows the separation of four fractions describing the combinations of elements with the solid phase of the tested sample with different degrees of solubility [
25]. In turn, single extractions are quick and useful in routine monitoring, and although these methods can extract bioavailable amounts of metals, they allow only for general conclusions. There are many single extractants used in the discussed technique, and in this group, both strong mineral acids and neutral salts can be distinguished, but it is customary to use them to determine the bioavailability of buffered salts or complexing reagents, which is related to their ability to create stable complexes with a wide spectrum of elements. Due to the large number of extraction methods used, efforts are being made to standardize them, and common single extraction procedures involving 0.005 mol·L
−1 DTPA + 0.01 mol·L
−1 CaCl
2 + 0.1 mol·L
−1 TEA have been proposed [
26].
Despite the differences between the sequential and single methods described above, in the opinion of the authors of this study, it is appropriate to use them simultaneously when assessing changes in the bioavailability of metals from composts. As a result of these various extractions, we can obtain a complete database with complementary information, which allows for accurate interpretation and conclusions. Unfortunately, this is not a popular approach to the issue in the literature, and studies of this type are few [
27]. Researchers focus on either method, and the use of sequential extraction methods dominates. However, as reported by Zimmerman and Weindorf [
28], the application of this technique is controversial due to the nonselectivity of used reagents in protocols. In the opinion of the cited authors, reliance on sequential extraction alone is not feasible and needs to be complemented with some other kind of analytical technique to positively identify the solid components involved. Therefore, this work was undertaken to determine the following: 1. the fractional distribution of microelements (Cu, Zn, and Ni) during the composting process of biodegradable wastes using sequential methods (BCR) as well as a single extractant (DTPA solution); 2. the rate of the microelements’ quantitative changes during the composting process depending on bulking agents and the process stage; and 3. the assessment of potential metal pollution in composts on the basis of various indices.
We assumed that the bulking agents used have a small impact on the quantitative changes and rate of microelement transformations during the composting process and that the main driving force of transformations is the naturally occurring processes during composting.
4. Discussion
Understanding changes in metal bioavailability during the composting of SS with various bulking agents largely depends on the changes in composting organic matter that occur during the process. Composting can be divided into four basic phases: mesophilic, thermophilic, cooling, and maturation [
40]. They differ in temperature and pH, which have a direct impact on the microorganisms dominating in particular stages. In the mesophilic phase, at a temperature close to the ambient temperature, mesophilic bacteria predominate and use easily degradable organic compounds, such as simple sugars, proteins, starch, and fats. Thanks to thermophilic bacteria, the second phase of the process is the most active stage of the composting, during which most readily available organic compounds are decomposed into carbon dioxide and converted into humus. The material composted in the cooling phase consists of, among others, substances that are difficult to decompose: cellulose, chitin, lignin, and hemicellulose. Due to their structure and low susceptibility to degradation, the listed compounds can only be broken down by extracellular enzymes secreted by fungi and actinomycetes [
41]. Therefore, the population of these organisms begins to dominate during the maturation phase. This stage of the process is the longest, and during this stage, the stabilization of the composted mixture occurs, expressed, among other things, by an increased share of humus and a lack of phytotoxic substances [
42]. As can be seen from this general description of the individual composting phases, a number of microbiological and biochemical transformations occur during the process, which are reflected in the transformation of organic matter, pH, and temperature. These factors also have a direct and indirect impact on the mobility of metals and, consequently, their potential bioavailability expressed by their individual distribution in specific fractions and combinations. In general, the largest changes in the amounts of microelements were in the thermophilic phase, and the smallest changes were in the cooling phase. Changes in the organic matter of composts are one of the main drivers of the variable distribution of metals in their fractions. Therefore, special attention, especially in waste composting, should be paid to reducing the mobility and availability of metals to plants, even when their total content increases as a result of reducing the composted mass. During the composting process, a decrease in the amount of organic matter can cause either dilution or an increase in the concentration of nutrients, depending on the combinations in which metals occur [
43]. The heterogeneity and dynamics of the transformations of individual metals were demonstrated in this research.
The amounts of Cu and Zn in the separated fractions generally increased during the composting process of the individual mixtures. Additionally, the bioavailable amounts of Cu and Zn were greater in mature composts than at the beginning of the composting process, which the chain relative increment data show. The pattern of quantitative changes in Ni was slightly different because the content of Ni in Fr. I decreased while that in the remaining connections increased, which was particularly visible in the organic and residual bonds. The amounts of bioavailable Ni, apart from those in C1, were at a constant level. The bioavailable amounts of Ni in C1 decreased significantly during composting. An experiment conducted with composted sewage sludge by Jakubus [
27] confirmed a significant reduced mobility of this metal during the composting process. Zorpas and Loizidou [
44] also noted a significant share (75%) of nickel in organic and residual connections of sewage sludge composted with sawdust and zeolites, although here, the decisive role in binding metals may have been that of zeolites. Rehana et al. [
45] and Pecorini et al. [
46] also found the largest amounts of Ni in Fr. IV and observed a significant reduction in the mobile fraction of Ni. According to the authors of the cited studies, the additives used for composting SS, such as sawdust, zeolites, and fly ash, strongly stabilized the amount of this metal, reducing the mobility of Ni and its probable uptake by the plant. This statement is acceptable and can be interpreted in relation to the changes in the distribution of Ni in the sequentially separated fractions that were found for the mixtures examined in this paper in relation to those determined for SS alone, so that each compost had the same percentage of SS (45%) in its composition. Jakubus [
21] found that, in SS from the Poznan district, the amount of Ni in the fractions increased in the opposite direction, i.e., Fr. IV < Fr. III < Fr. II < Fr. I. In this study, regardless of the compost, the Ni content in the fractions of the mature composts increased as follows: Fr. I < Fr. II < Fr. III < Fr. IV (based on absolute values not directly presented here). This allows us to assume that the additives used in the form of sawdust, straw, or bark were effective structural agents that reduced the mobility of nickel in the exchangeable, water-soluble, and acid-soluble bonds (Fr. I). The bulking agents used for composting with SS are characterized by different chemical compositions and, therefore, susceptibility to microbiological decomposition. This will also determine their greater or lesser ability to absorb metals. Straw was used in the same proportion (5%), and its significant impact on the sorption process was rather negligible because, compared to sawdust or bark, straw contains less lignin and more water-soluble sugars, proteins, and starch [
47], i.e., compounds that are easily accessible sources of carbon for microorganisms carrying out the mineralization process. Therefore, it can be assumed that, when sawdust and bark are used as wood materials, they do not decompose as dynamically as straw; thus, they are likely to play a more important role in the sorption process. It can be assumed that the decomposition of organic matter in SS contributed to the release of certain amounts of Ni, which in the initial phases of composting was sorbed on both sawdust and bark. Such a possibility is indicated by Kovacova et al. [
48], although their studies were conducted for Cu and Zn. The cited authors demonstrated that sawdust is an effective adsorbent for the removal of these metal ions from model solutions. Xiong et al. [
49] indicated that the addition of swelling agents decreases the heavy metal content and reduces the mobility of heavy metals. Unfortunately, such a possible sorption with respect to Cu and Zn cannot be clearly stated in these studies because, during composting, both the bioavailable amounts and contents in the separated fractions increased, which was a phenomenon independent of the composition of the mixture. The only exceptions were the amounts of Zn Fr. I for C2 and C3, which decreased during composting. Moreover, the distributions of Zn in the separated fractions of SS [
12] or mature composts were very comparable, which raises doubts as to the effectiveness of the additives used, such as stabilizing agents. As cited by the above author, in SS, the following sequence of increasing Zn amounts was detected: Fr. IV < Fr. I < Fr. III < Fr. II, and in the mature composts in this study, increasing amounts of this metal were detected in the following series of fractions extracted: C1, Fr. IV < Fr. I < Fr. III < Fr. II; C2, Fr. I < Fr. IV < Fr. III < Fr. II; and C3, Fr. I < Fr. IV < Fr. II < Fr. III (based on absolute values not directly presented here). Gao et al. [
43] also confirmed such a distribution of Zn because they determined most of the Zn in fractions III and II. On the other hand, Pecorini et al. [
46] reported the highest percentage of Zn in the mobile fractions of compost.
In light of the reports on the high mobility of nickel and zinc in composts [
50,
51], the low metal contents in fraction I of composts and the higher metal contents in stable chemical forms are interesting results. Such changes seem beneficial from the point of view of environmental protection but less so when we consider the fertilizer aspect of the composts. Despite their significant affinity for organic matter, nickel and zinc form organic ligands characterized by a weaker stability than those created through copper [
52,
53]. In this respect, Huang et al. [
54] emphasized the greater affinity of zinc for fulvic acids. Mineralization occurring during composting could cause the breakdown of these organometallic complexes, and the released metal ions were subsequently adsorbed on iron and manganese oxides or created more durable organic compounds within the resulting compost humus. It is necessary to remember that the humification process occurs simultaneously with the mineralization process, as a result of which more stable structures of humus compounds are created with a high sorption capacity in relation to metals [
54]. Amir et al. [
50] particularly emphasized that, during the composting process, there was a reduction in both metals in water-soluble connections (and zinc additionally in the residual fraction) and increase in organic connections. An explanation for this phenomenon was also provided as the release of nickel and zinc ions from easily soluble connections that were the result of the decomposition of organic matter, which were then bound to stable organic connections. This finding is also confirmed by the results of the research by Liu et al. [
55].
With respect to the fractional distribution of Cu, it should be emphasized that both the sewage sludge [
12] and the tested composts had the same distribution. The amount of Cu, regardless of the composting phase, increased in the following order: Fr. I < Fr. IV < Fr. II < Fr. III (based on absolute values not directly presented here). The amounts of Cu increased everywhere except for Fr. II, where a decrease in the amount of metal was recorded. Leśniańska et al. [
56] and Jakubus [
27] reported the highest Cu amounts for Cu bound with organic matter of the tested composts. On the other hand, Pecorini et al. [
46] reported the highest percentage of Cu in the mobile fractions of compost. The addition of bulking agents resulted in an increase in the amount of Cu labile connections and a decrease in the amount of the organic fraction [
27,
57]. The authors also noted an unclear trend in Cu mobility due to the transfer of the organic fraction into the changeable and carbonate fractions. The explanation may be the biogeochemistry of Cu, which is considered a metal that does not easily create water-soluble forms; therefore, it is not easily activated and is characterized by complexes with organic matter with high durability [
53,
58]. Moreover, as a result of the decomposition of organic matter during composting, copper is released from soluble organometallic ligands, and then, it may be subjected to precipitation in the form of carbonates, which are extracted via the BCR method with an acetic acid solution (a reagent in fraction I). The increased amount of copper in water- and acid-soluble connections can also be interpreted based on the opinion regarding the creation of stable copper complexes with fulvic acids [
59]. Although the composting process stimulated an increase in the copper content in fraction I, the microelement amount in these combinations was lower than that in the other fractions. The quantitative dominance of copper in the organic fraction is in line with the common opinion about the durability and stability of the ligand copper with sulfur compounds and high-molecular-weight humus compounds of the nature of humic acids [
54,
60]. Additionally, Xiong et al. [
60] stated that ligneous bulking agents may promote the formation of HA and reduce FA during the composting process. Ligneous bulking agents, especially sawdust, could improve the complexation ability of HA, but had little influence on that of FA. These findings indicate that organic materials, especially sawdust, may be used as bulking agents to reduce the mobility and bioavailability of metals in solid waste composts. In this context, the strong bonding of copper with carboxyl and carbonyl groups and phenolics is particularly important [
59].
Although the amounts of Ni, Cu, and Zn in various combinations generally increased during composting, these metals mainly formed permanent bonds and were slowly activated in the environment. This finding indicates environmental safety, which is confirmed by metal pollution indices. The calculated values of the applied indices proved the lack of a negative impact of metals introduced into the soil from the composts in the form of potential contamination. Therefore, the process of composting SS with bulking agents should be considered beneficial, although considering that a low rate of quantitative transformation of micronutrients occurring in the forms most available to plants (bioavailable and isolated in fraction I) was detected. It should be borne in mind that composts of this type should be treated as fertilizers from which microelements are slowly released into the environment. On the one hand, this creates environmental safety, but on the other, it may lead to temporary shortages in nutrients for plants.
5. Conclusions
The direction of the quantitative changes in Cu, Zn, and Ni in the separated fractions and bioavailable contents was varied and depended on the experimental factors and the chemical nature of the metals. The used research methodology significantly presented the changes occurring during the successive stages of composting, and the indicators introduced had an important impact on the assessment of the environmental impact of the tested composts. Regardless of the composition of the mixtures, the amounts of Cu, Zn, and Ni generally increased for the majority of fractions during composting. The most substantial increment in copper content was evident in fraction I of C1 during the thermophilic phase (191.76%), as well as in C3 during the zinc bioavailability phase (75.13%) and in C3 within nickel fraction III (93.87%). Regardless of this, it should be emphasized that, throughout the composting process, the shifting levels of metal contents exhibited diverse trends, including both positive and negative values. Specifically, these variations were observed in C1 for Cu Fr I, Zn Fr IV, and bioavailable amounts, and Ni Fr I and II; C2 for Ni Fr IV and bioavailable amounts; and C3 for Znbioava, Ni Fr I, Fr II, Fr III, and bioavailable amounts. The obtained results confirm the initial assumptions about the significant influence of changes occurring during composting on the amount of microelements because the largest changes in copper, nickel, and zinc contents were observed during the thermophilic phase, except for zinc in C1 and nickel in C2. The smallest changes in the content of all discussed micronutrients were found in C1 and C3 during the cooling phase. For C2, the least changes occurred during the mature phase. According to the assumptions, the role of bulking agents in composting with SS was smaller in relation to the quantitative changes in the metals. Regardless of the bulking agent used, there was a reduction in the mobility of the quantity, and consequently, the availability of microelements, which was particularly visible in the case of Ni. Despite significant increases in the bioavailable amounts of Cu and Zn in all the tested mixtures, these composts did not have a potential negative impact on the environment, which was confirmed by the values of metal pollution indices. Valorizing composts as a source of available microelements, the most beneficial properties in this respect were observed in C3 and the least in C2. Regardless of this, it should be remembered and taken into account that composting SS with various bulking agents is justified and purposeful; however, the obtained composts are a slowly releasing source of Cu, Zn, and Ni, which indicates the need for supplementation with other fertilization.