Sustainable bio-hydrothermal sequencing treatment for asbestos-cement wastes Danilo Spasianoa*, Vincenzo Luongob,d, Marco Raceb, Andrea Petrellaa, Saverio Fiorec, Ciro Apollonioa, Francesco Pirozzib, Umberto Fratinoa, Alberto F. Piccinnia
a
Dipartimento di Ingegneria Civile, Ambientale, Edile, del Territorio e di Chimica, Politecnico di
Bari, Via E. Orabona, 4, 70125, Bari, Italy. b
Dipartimento di Ingegneria Civile, Edile ed Ambientale, Università di Napoli Federico II, Via
Claudio, 21, 80125, Napoli, Italy. c
Institute of Methodologies for Environmental Analysis, National Research Council of Italy, Tito
Scalo, Potenza, Italy. d
Dipartimento di Matematica e Applicazioni “Renato Caccioppoli”, Università di Napoli Federico
II, Via Cintia, Monte S. Angelo, 80126, Napoli, Italy. * Corresponding Author: tel.: (+39) 0805963240;
[email protected].
Keywords: dark fermentation; hydrothermal treatment; anaerobic digestion; glucose; chrysotile.
Abstract In this paper, the treatment of asbestos-cement waste (ACW) has been attempted by a dark fermentation (DF) pre-treatment followed by hydrothermal and anaerobic digestion (AD) treatments. During DF, glucose, employed as a biodegradable substrate, was mainly converted to H2-rich biogas and organic acids (OAs). The latter caused the dissolution of the cement matrix and the partial structural collapse of chrysotile (white asbestos). To complete the chrysotile degradation, hydrothermal treatment of the DF effluents was performed under varying operating conditions (temperature, acid type, and load). After the addition of 5.0 g/L sulfuric acid, a temperature decrease, from 80 °C to 40 °C, slowed down the treatment. Similarly, at 100 °C, a decrease of sulfuric, lactic 1
or malic acid load from 5.0 g/L to 1.0 g/L slowed down the process, regardless of acid type. The acid type did not affect the hydrothermal treatment but influenced the AD of the hydrothermal effluents. Indeed, when malic acid was used, the AD of the hydrothermally treated effluents resulted in the highest production of methane. At the end of the AD treatment, some magnesium ions derived from ACW dissolution participated in the crystallization of struvite, an ecofriendly phosphorous-based fertilizer.
1. Introduction Due to their versatility and low cost, cement-asbestos materials have been widely used since the beginning of the 20th century. However, following the ban of asbestos due to the high toxicity of these fibrous minerals, the production of asbestos artefacts saw a marked decline after 1977 [1,2]. Despite numerous studies on asbestos toxicity and the ban of asbestos in many countries, the worldwide annual asbestos extraction is actually equal to 2.0×106 Mg [3]. This material is nowadays mainly employed in emerging countries to produce pipelines, flat sheets, corrugated roof sheeting, and insulation boards. Moreover, since the extraction of crocidolite (blue asbestos) and amosite (brown asbestos) ceased in the mid-1990s, asbestos mining is now based on chrysotile (Mg3Si2O5(OH)4) [4]. Chrysotile consists of trioctahedral sheets of Mg(OH)2 (brucite-like sheets) covalently bonded to tetrahedral sheets of SiO2 and has a layered structure wrapped around itself to form a tubular fibre structure [5]. Although inhalation of white asbestos is considered less hazardous than that of other asbestos fibres, many studies have documented a strong relationship between the exposure to chrysotile fibres and fatal diseases [6-8]. In contrast, many studies justify the use or presence of non-friable cement-asbestos products in public and private buildings since airborne chrysotile can not generate airborne fibres [9,10]. However, these products have released hazardous airborne asbestos following damage from disasters, including hurricanes and earthquakes, or terrorist attacks [11-13]. Thus, the EU Parliament has encouraged a draft plan of action aimed at the removal of all the products containing asbestos from private and public buildings, the correct management of 2
asbestos-containing waste, and the research of eco-compatible alternative technologies for the complete and definitive conversion of asbestos fibres in non-harmful compounds [14]. The complete destruction of these fibres has recently been achieved by means of thermic, mechanical, chemical, and hydrothermal treatments [15-19]. However, these processes require high energy and/or reagent consumption; thus, their adoption at industrial scale is not feasible and the ACW landfilling continues to be the main solution [20]. However, by a DF pre-treatment followed by a hydrothermal phase (100 °C and ambient pressure) the destruction of 5 g/L ACW was achieved with the addition of only 1.25 g/L sulfuric acid [21-22]. Notably, the DF step led to the production of 465.4 mmol/L bio-hydrogen, useful to reduce the energy cost of the acid-infused hydrothermal treatment, and halved the sulfuric acid consumption during hydrothermal treatment [18]. Indeed, during this biological pre-treatment, the glucose, added as biodegradable substrate, was mainly converted to H2, CO2 and organic acids (OAs); this is well in agreement with previous findings [23-25]. The generation of OAs led to the reduction of acid consumption during the final fibres decomposition stage in the hydrothermal treatment. Indeed, these OAs dissolved all the Ca-containing compounds of the cement matrix in the ACW sample, such as portlandite (1) and calcium carbonate (2). − (1) Ca(OH)2(s) + 2OA(l) → Ca2+ (l) + 2OA (l) + 2H2 O − (2) CaCO3(s) + 2OA(l) → Ca2+ (l) + 2OA (l) + H2 O + CO2
Additionally, the OAs produced during the DF led to the dissolution of 50% brucite-like sheets of the chrysotile (3), leading to structural collapse of the fibres. − (3) Mg 3 Si2 O5 (OH)4(s) + 6OA(l) → 3Mg 2+ (l) + 6OA (l) + 2SiO2(s) + 5H2 O
Indeed, the chrysotile peaks disappeared from the X-ray diffractograms of the solids present in the DF effluents. However, hydrothermal treatment aimed at completing the dissolution of all the brucitic sheets was necessary to avoid the presence of chrysotile fibres in the effluents [21].
3
This work is an in-depth study of some fundamental issues related to this novel ACW treatment. Since the cost of the whole process is mainly ascribed to the energy consumption during the hydrothermal step, the effect of operative temperatures ranging from 100 °C to 40 °C was evaluated. Additionally, the effect of the acid types and loads on the brucitic sheet dissolution trend were examined from both an economic and ecological point of view. Finally, mesophilic anaerobic digestion (AD) of hydrothermally treated effluents was carried out to produce bio-methane, which could further reduce the cost of the whole treatment train.
2. Materials and methods 2.1. Materials Chemicals used in the experiments (sulfuric acid (98%), lactic acid (85%), malic acid (99%), HNO3 (70%), HCl (35%), H2O2 (30%), and glucose (99.5%)) were manufactured by Sigma Aldrich (USA), except HPLC grade acetonitrile which was produced by Carlo Erba (Italy). The solvent used in all the experiments and sample dilutions was distilled water. The sludge deriving from the AD plant of the ‘Davide Colangelo’ dairy farm in Capaccio (Salerno, Italy) was used to perform the two biological processes of the proposed treatment train. Indeed, it was adopted for the production of DF inoculum and as inoculum for AD tests aimed at evaluating the amount of additional energy derived from final treatment of the hydrothermal effluents. The characterization of the adopted sludge is reported in Table 1S. The ACW sample used in this study derives from an Eternit slate and after the characterization, elsewhere reported [21], chrysotile fibres were found. In particular, Mg2+ and Ca2+ concentrations were equal to 3.1 %w/w and 30.0 %w/w, respectively.
2.2. Reactors and experimental procedures The inoculum adopted for DF was attained via thermal treatment of the AD sludge at 105 °C for 60 min, as reported in literature [26]. Around 0.5 L of the inoculum was diluted with 0.5 L distilled water 4
and poured in a 2.0 L glass bottle containing 31 g of glucose. Consequently, the feed to microorganism ratio (F/M), expressed as COD substrate (g)/VS inoculum (g), equalled 2.64, a value close to that adopted in a previous study [22]. Subsequently, the solution was purged with bubbling pure nitrogen for 30 min and the bottle was closed with an airtight screw cap equipped for the withdrawal of liquid and gaseous samples. The DF reactor was magnetically stirred (370 rpm) and maintained at mesophilic conditions (35 ± 1 °C). Once H2 production stopped, the cap was temporarily removed to add 5.0 g dried ACW sample together with other 31 g glucose under N2 atmosphere. This procedure was repeated in triplicate. The three solutions derived from the bioreactors were mixed in a 3.0 L glass bottle and stored at 4 °C in a refrigerator. The resulting solution, which is representative of the DF process effluent, was used for the hydrothermal experimental campaign as follows: 100 mL of the afforded suspension were poured in a flask equipped with a Graham condenser to avoid the loss of water vapor and volatile compounds. The flask, immersed into a thermostatic oil bath, was magnetically stirred and each experiment was carried out in duplicate. Once the solution reached the required temperature, sulfuric acid, lactic acid, or malic acid were added with concentrations in the range of 0-5.0 g/L. Once the samples were collected, the suspended solids were removed after centrifugation and filtration procedures. The so treated samples were finally diluted with distilled water and analyzed. More information on the experimental procedures and laboratory reactors adopted in this study can be found in literature [21]. To evaluate the production of bio-methane from the anaerobic treatment of the hydrothermal effluents some AD tests were carried out. In particular, the AD of the effluents of the hydrothermal treatments carried out at 100 °C in presence of 5.0 g/L sulfuric acid, lactic acid, and malic acid were performed in triplicate in 150 mL bottles closed with natural rubber sleeve caps maintained at 38 °C for 30 d. The F/M ratio was fixed at values lower than 0.5 to avoid anabolic reactions. To this purpose, 9 mL hydrothermal effluents were mixed with 91 mL of the same AD sludge used for the preparation of the DF inoculum. During the AD experiments, a 100-mL graduated eudiometer was used to monitor 5
biogas production and withdraw gaseous samples. These samples were collected by connecting the eudiometer to an AD bio-reactor with a silicon tube equipped with a syringe needle (Figure 1S); the gaseous samples were immediately analysed. At the end of the AD experiments almost 30 mL of the final solution were lyophilized and the resulting solids were characterized by XRD analysis.
2.3. Analytical methods The quantification of dissolved calcium and magnesium ionic concentrations was carried out by atomic absorption spectrometry (FAAS). To this purpose, a Varian (Varian Australia Pty Ltd., Victoria, Australia) Model 55B SpectrAA was used. Due to the importance of this parameter, each sample was analysed two times. The biogas derived from the DF and the AD experiments was analyzed with a Varian (Varian Australia Pty Ltd., Victoria, Australia) Star 3400 gas chromatograph (GC) equipped with a thermal conductivity detector (TCD) and using Ar as carrier gas. The column used for the gas separation was a Restek (Restek Corporation, Bellafonte, PA, USA) ShinCarbon ST 80/100 column. A Dionex (Dionex Corporation, Sunnyvale, CA, USA) HPLC was used for OA concentration measurements. The system comprised a chromatography oven (Dionex LC 25) equipped with a Metrohm (Herisau, Switzerland) Metrosep Organic Acids - 250/7.8 column and an absorbance detector (Dionex AD25) connected with a gradient pump (Dionex GP 50). The samples were eluted with 0.5 mmol/L sulfuric acid pumped 0.7 mL/min. Phosphate ion concentrations were quantified by a Metrohm (Herisau, Switzerland) 761 compact Ion Chromatograph. The eluent, consisting of an aqueous solution containing 3.2 mmol/L Na2CO3 and 1.0 mmol/L NaHCO3, was pumped at 0.6 mL/min. The separation column was a Metrosep A Supp 5 (4.0 mm × 250 mm) produced by Metrohm (Herisau, Switzerland). The suppressed conductivity detector was thermostated at 25 ± 0.5 °C.
6
The ammonium concentration was evaluated by a modified distillation and titration method [27] using 4% boric acid, instead of the reported 2%, with a UDK 132 semiautomatic distillation unit (Velp Scientifica srl, Usmate Velate, Italy). Panalytical X'Pert Pro diffractometer (Malvern Panalytical Ltd, Malvern, UK) with a solid-state detector (X'Celerator) and a sample spinner (revolutions: 1 mm/s) was used to perform mineralogical investigations. The following experimental conditions were adopted: radiation = Cu-Kα; V = 40 kV; I = 40 mA; divergence slit = 0.5; scanned 2θ range: 3–70°. TS, VS, and COD concentrations were quantified according to the procedures suggested by APHA [28]. A pH-meter (HI 98190 pH/ORP, Hanna Instruments, Woonsocket, RI, USA) was used to monitor the pH. Statistical analyses were conducted in Microsoft® Excel 2013/XLSTAT©-Pro (Version 7.2, 2003, Addinsoft, Inc., Brooklyn, NY, USA) with the post-hoc Tukey’s Honestly Significant Difference HSD test, assuming a statistical significance p < 0.05.
3. Results and discussion 3.1 DF pre-treatment At the same operative conditions, the results of the DF pre-treatment are well in agreement with recently reported results [21]. This highlights the reproducibility of the reference pre-treatment in terms of H2 and OA production and ACW dissolution. Indeed, as a result of glucose bio-degradation, the average H2 production at the end of the start-up phase (63 h) and the whole pre-treatment process (133 h) was equal to 246 mmol and 489 mmol, respectively (Figure 1a). According to a previous study [29], most of the hydrogen is produced during the first 44 h, after the two glucose additions. On the other hand, the DF of glucose afforded OAs that lowered the pH to values less than or equal to 4.5 (Figure 1b-c).
7
Figure 1. Monitoring of the main parameters during dark fermentation pre-treatment: a) bio-H2 production; b) organic acid (OA) production; c) pH trend; d) net and normalized Ca2+ and Mg2+ dissolved concentrations. Vsol = 1.0 L, T = 35 °C, pH0 = 7.8, [C6H12O6]0 = 31 g/L.
The afforded OAs led to the dissolution of the Ca-containing minerals in the cement matrix and the brucitic sheets in the chrysotile. Indeed, since 5 g/L of ACW were characterized by Ca and Mg contents of 30 %w/w and 3.0 %w/w, respectively, the theoretical concentrations [C]t of Ca and Mg suspended in the DF bioreactor were respectively determined as 1500 mg/L and 155 mg/L. Consequently, the normalized net concentrations of calcium and magnesium dissolved ions ([C]/[C]t) reported in Figure 1d confirm the complete dissolution of the calcium-containing compounds and the partial dissolution of the brucitic sheets. Indeed, the dissolved Mg+2 concentration in the DF effluents only corresponds to 48% of the total Mg present in chrysotile. In particular, the [C]/[C]t ratios were evaluated as follows: 8
(4)
(5)
[Mg2+ ] [Mg2+ ]t [Ca2+ ] [Ca2+ ]t
= =
[Mg2+ ]−[Mg2+ ]t=62h 155 mg/L [Ca2+ ]−[Ca2+ ]t=62h 1500 mg/L
being [Mg 2+ ]t=62h and [Ca2+ ]t=62h the dissolved magnesium and calcium concentrations, respectively equal to 154 mg/L and 826 mg/L, before the ACW addition. Therefore, further treatment was necessary to completely dissolve the brucitic sheets and guarantee the complete destruction of the chrysotile fibres.
3.2 Hydrothermal treatment Hydrothermal treatment of the properly acidified DF pre-treatment effluents can dissolve the remaining chrysotile brucite-like sheets. Thus, the effects of operative temperature, acid type added to the reacting solution, and dosage were thoroughly investigated. The effect of temperature on the process efficiency was evaluated by four experimental runs performed after the addition of 5 g/L sulfuric acid at ambient pressure and temperature in the range of 40-100 °C. Even though complete dissolution was reached within 24 h in every run, the afforded results (Figure 2) highlight the strong temperature dependence of this process. Indeed, while at 80 °C and 100 °C complete dissolution of the brucite-like sheets was achieved after 8 h, at 40 °C similar results were only attained after 24 h. Negligible differences at 80 °C and 100 °C were observed at detection times >2 h. Hence, the following tests, were carried out at 100 °C due to the faster dissolution rate of the brucite-like sheets at the earlier stages of the experiments.
9
Figure 2. Effect of the temperature on Mg removal from chrysotile. [H2SO4] = 5 g/L. Ambient pressure.
The influence of the acid type and concentration was evaluated. Sulfuric, lactic, and malic acid were added to the DF effluents at loads of 5 g/L, 2.5 g/L, 1.25 g/L, 1.0 g/L. Figure 3 shows that at the same acid load, the acid type did not significantly affect the dissolution kinetics of the brucite sheets. This was also confirmed by statistical analysis (Table S1), since no statistically significant differences were observed (p > 0.05). However, the efficiency of the process was influenced by the acid load. Indeed, >90% Mg removal from chrysotile was achieved after 4 h, 8 h, and 12 h when acid concentrations of 5 g/L, 2.5 g/L, 1.25 g/L, respectively, were added to the solution. Notably, the addition of only 1.0 g/L acid did not result in complete ‘brucite’ dissolution, regardless of acid type. Consequently, a concentration ≥1.25 g/L of acid is required to eliminate the chrysotile still present in the DF effluents undergoing 24-h hydrothermal treatment.
10
Figure 3. Results of hydrothermal treatment with varying type and concentration of acid. T = 100 °C; ambient pressure; acid concentrations: a) 5 g/L, b) 2.5 g/L, c) 1.25 g/L, and d) 1.0 g/L.
Taking into account the molar concentration of Mg2+ to be dissolved (3.06 mM), pH of the DF effluents, and the molecular weights and acid dissociation constants of the three selected acids (Table 1S), the most efficient acid was determined as being malic acid. Indeed, considering the hydrothermal treatment results for an acid concentration of 1.25 g/L and the stoichiometry of the reactions (6-8) between each selected acids and the chrysotile still suspended in solution, the lowest experimental [acid]/[Mg2+]s molar ratio was found with malic acid. 11
− (6) SO2− 4 + Mg(OH)2 s ⇌ MgSO4 (l) + 2OH
[H2SO4]/[Mg2+]s = 4.2
(7) 2CH3 CH(OH)COO− + Mg(OH)2 s ⇌ Mg(CH3 CH(OH)COO− )2 (l) + 2OH− [CH3 CH(OH)COO− ]/[Mg2+]s = 4.5
(8) 2HOOCCH2 CH(OH)COO− + Mg(OH)2 s ⇌ Mg(HOOCCH2 CH(OH)COO− )2 (l) + 2OH− [HOOCCH2 CH(OH)COO− ]/[Mg2+]s = 3.05
Specifically, the experimental molar ratios between sulfuric and lactic acid and the Mg2+ to be dissolved were 4.2 and 2.2 times higher, respectively, than the stoichiometric values. However, when malic acid was added to the solution, the experimental [Acid]/[Mg2+]s molar ratio was only 1.5 times higher than the stoichiometric ratio. These results were attributed to the chelating properties of malic acid and partially, of lactic acid [30,31]. Although sulfuric acid is cheaper than the two selected OAs, both malic and lactic acid do not require the severe safety measures necessary for the storage and handling of sulfuric acid (Table 1). Furthermore, the use of renewable, safer and less hazardous compounds represents the main purpose of green chemistry [32-33]. Indeed, the adoption of lactic and malic acid is more environmentally friendly due to their biodegradability and production through biological processes powered by renewable compounds [34-36].
12
Chemical formula Type Molar mass (g/mol)
Sulfuric acid
Lactic acid
Malic acid
H2SO4
CH3CH(OH)COOH
HOOCCH2CH(OH)COOH
Inorganic
Organic and biodegradable
Organic and biodegradable
98.09
90.08
134.09
pKa1 = -3
pKa1 = 3.9
pKa1 = 3.4
pKa2 = 2.0
pKa2 = 15.1
pKa2 = 5.2
Naturally occurring and
Naturally occurring and
industrially produced
industrially produced
Causes skin irritation; causes
Causes serious eye
serious eye damage
irritation
850 – 1850 (≈88%)
1300 – 2500 (food grade)
pKai
Availability
Industrially produced
May be corrosive to GHS hazard metals; causes severe skin statements burns and eye damage Cost (USD/ton)a
160 – 400 (≈96%)
Table 1. Properties of the acids used during the hydrothermal runs. a https://www.alibaba.com/
Table 2 reports some discrepancies between the concentration of ammonium and phosphate ions in the hydrothermal effluents and digestate. Theoretically, the concentrations at the end of the hydrothermal process should be equal to half the digestate concentrations. This is ascribed to the procedure used for the DF reacting solution preparation and the absence of nitrogen and phosphorous in the biodegradable substrate added during biological pre-treatment. However, the procedure adopted to produce the DF inoculum comprised thermal treatment of the digestate. This probably led to the partial stripping of the ammonia present in the digestate and phosphate desorption from the extracellular polymeric substances [37-38]. Nitrogen bubbling before and during DF probably contributed towards a further decrease in ammonia concentration due to the stripping phenomenon. This can be deduced from the ammonia and phosphate concentrations in the DF effluents that are 13
similar to those in the hydrothermal effluents. Despite the presence of Graham condensers, the high operative temperature of the hydrothermal treatment contributed towards further evaporation of the ammonia contained in the DF effluents.
Acid added during
pH
hydrothermal treatment
[NH4+ ]
[PO3− 4 ]
COD
(mg/L)
(mg/L)
(mg/L)
Digestate
-
7.99
2954
1125
3.60×104
DF effluents
-
4.5
915.3
913.1
5.50×104
Hydrothermal
5 g/L sulfuric acid
3.93
781.2
908.4
5.31×104
treatment
5 g/L lactic acid
4.31
738.6
898.2
5.46×104
effluents
5 g/L malic acid
4.32
739.2
941.3
5.70×104
5 g/L sulfuric acid
7.64
2680
749.5
1.83×104
5 g/L lactic acid
7.85
2787
784.4
1.81×104
5 g/L malic acid
7.69
2779
697.9
1.73×104
AD effluents
Table 2. Characterization of digestate and dark fermentation (DF), hydrothermal, and anaerobic digestion (AD) effluents.
3.3 AD of the hydrothermal effluents AD tests were performed using effluents from the hydrothermal treatments performed at 100 °C with 5 g/L sulfuric, lactic, and malic acid. Considering the COD values of the hydrothermally treated effluents and the concentration of the VS in the digestate (25.0 g/L), 150-mL serum bottles were filled with 9 mL effluent and 91 mL digestate to produce F/M ratios of ~0.23. This value is within the optimal F/M range for AD tests [39,40].
14
According to literature findings [41,42], the AD test results (Figure 4) revealed the possibility of producing bio-methane as a further source of energy. This is generated from the degradation of the OAs produced during the DF process. The type of acid employed during hydrothermal treatment exhibited a minor effect on biogas production. Thus, 312 mL, 301 mL, and 269 mL of methane were respectively produced in presence of malic, lactic, and sulfuric acid. Such discrepancies were attributed to the biodegradability of malic and lactic acids with respect to sulfuric acid, which cannot lead to bio-methane production. This is another advantage of adopting organic and biodegradable acids over inorganic acids during the hydrothermal phase. In fact, both malic and the lactic acid could play a double role in the whole treatment process: they can dissolve the brucite-like layers during hydrothermal treatment and they can be biodegraded with the consequent production of methane during the final anaerobic digestion treatment.
Figure 4. Effect of the acid added during the hydrothermal process on (a) methane and (b) carbon dioxide produced from anaerobic digestion of the effluents: F/M = 0.23, T = 38 °C; hydrothermal treatment conditions: T = 100 °C; [acid] = 5.0 g/L. 15
The final phosphate ion concentration in the AD solutions (Table 2) was lower than the theoretical amount (~ 1100 mg/L). This variation was ascribed to the formation of phosphate minerals as reported in other studies on digestion sludges [43-44]. To verify this hypothesis, XRD of suspended solids collected at the end of the AD tests was carried out. The results (Figure 5) revealed strong well defined peaks. There were detected the presence of monohydrocalcite (CaCO3 H2O), calcite (CaCO3), Mgcalcite, natron (Na2CO310H2O); quartz (SiO2), struvite (NH4MgPO4·6H2O). Hydroxylapatite (Ca5(PO4)3(OH)) at minor extent was also found.
Figure 5. X-ray diffraction patterns of a suspended solid sample after AD of the hydrothermal effluents with H2SO4 (a), malic acid (b), and lactic acid (c). Hy-Cal: monohydrocalcite; Stu: Struvite; Cal: Calcite; Mg-Cal: magnesian calcite; Stu: struvite; Nat: natron; Qtz: quartz.
16
Notably, struvite and hydroxylapatite are considered as alternative and renewable phosphorous-based fertilizers since orthophosphates may be formed after the AD treatment of biodegradable waste [45,46]. Struvite and hydroxyapatite formation was mainly promoted by the Ca2+ and Mg2+ ions derived from the dissolution of the ACW sample. Hence, after a mechanical dehydration of the AD sludge, the liquor deriving from this treatment may undergo to a chemical precipitation process leading to the production of phosphorous-based fertilizers [47-48]. Consequently, a proper management of this sludge could lead to the production of fertilizers from the combined treatment of biodegradable and asbestos-cement wastes. Although this is only a hypothesis yet to be tested, this approach would reflect the principle of circular economy and satisfy the requests of the EU towards problems related to ACW management and phosphorous emergency. Indeed, in 2016, the EU promoted the production of phosphorous-based from domestic wastes or secondary raw materials, thus transforming waste into nutrients for crops [49].
4. Conclusions A treatment train comprising a DF, a hydrothermal and AD treatments was employed to degrade 5 g/L of cement-asbestos waste. The conversion of glucose, the biodegradable substrate adopted during the DF pre-treatment, resulted in ~500 mmolH2/L together with a significant amount of OAs. These led to the dissolution of the calcium-based minerals, representing the cementitious part of the adopted ACW, and to the partial collapse of chrysotile fibres. The complete destruction of asbestos fibres was achieved within 8 h at 80 °C and 100 °C. Longer treatments were necessary when adopting lower operative temperatures. The effects of both acid type (specifically, sulfuric acid, lactic acid and malic acid) and concentration were also evaluated. In particular, a decrease in acid load, from 5.0 g/L to 1.25 g/L, slowed down the process regardless of acid type. However, the acid type affected the AD of the hydrothermal effluents. Indeed, when lactic, malic, and sulfuric acid were employed AD of the hydrothermally treated effluents resulted in the production of 1.54 molCH4/L, 1.49 molCH4/L, and 1.33 molCH4/L, respectively. However, only a cost analysis based on the results deriving from an 17
experimental campaign carried out at least on pilot scale reactors may individuate the best acid type and load. Finally, the mineralogical studies performed on the AD sludge highlighted the presence of struvite, which was favored by the presence of Ca2+ and Mg2+ ions derived from ACW treatment. Consequently, the proper management of the AD sludge may lead to the production of phosphorousbased fertilizers. These, together with the reagent saving and the productions of H2 and CH4, could make the present ACW treatment extremely competitive.
Acknowledgements This work was funded by Fondo di Sviluppo e Coesione 2007-2013 – APQ Ricerca Regione Puglia “Programma regionale a sostegno della specializzazione intelligente e della sostenibilità sociale ed ambientale – FutureInResearch”. Furthermore, the present investigation was carried out with the support of Fondazione Puglia. In memory of Prof. Ettore Trulli.
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