1. Introduction
The increase of urban sprawling is a common phenomenon in recent decades due to the rapid urban population growth [
1]. This implies various anthropogenic activities including industrial operations, municipal processes, urban gardening, and construction among others, which may affect soil quality [
2]. Consequently, in many cases, soils allocated in urban and periurban areas became technosols [
3] thereby acquiring several problematics, such as an increased concentration of metal(loid)s, which requires attention regarding human health risks [
4,
5,
6]. This is partly explained because, unlike natural soils, technosols typically contain materials such as slags, clinker, ashes, and construction debris, which often carry a significant metal (loid) contamination [
7,
8]. Due to this alteration, a thorough risk assessment is usually necessary to select remediation approaches taking into account future soil uses and the reduction of threats to human health and the environment [
9].
One of the most common and hazardous soil contaminants is arsenic (As). In fact, As contamination is a widespread problem because of its negative impact on living organisms and human health [
10]. This metalloid usually appears in urban-type technosols and represents a severe threat because of its potential accumulation in the human food chain, essentially by plant uptake and animal transfer [
11]. This could affect human health given the carcinogenic and toxic character of As [
12]. Furthermore, different precautions should be taken into account when treating soil due to the anionic form of this metalloid [
13,
14,
15].
Nature-based solutions (NBS) is an umbrella concept used to apprehend nature-based, cost-effective and eco-friendly treatment technologies, as well as redevelopment strategies that are socially inclusive, economically viable, and with good public acceptance [
16]. The NBS can offer a great variety of benefits, ranging from less energy usage and higher material efficiency to increased resilience to global environmental change [
17,
18]. Therefore, these technologies are very suitable for soil treatment in urban and peri-urban areas. Some of the proposed nature-based remediation technologies, all of them applicable to As pollution, are phytoremediation, bioremediation, stabilisation with amendments such as biochar, green mulch or compost, and nanoremediation [
16,
17,
18,
19,
20].
Currently, there are two main NBS trends to treat soils contaminated with metal(loid)s, such as As [
20]. The first one consists of the immobilisation of the metal(loid)s in the soil trying to avoid the As enter into the trophic chain. For this purpose, the selection of amendments is critical, and it is done according to the metal(loid) to be immobilised [
14]. In the case of As immobilisation, nanoremediation is a modern technology [
21] that is beginning to be used through the application of zero valent iron nanoparticles (nZVI). This technology has already provided good results in water [
10,
22] and soils [
23,
24,
25], even at field scale [
26]. The second approach for As-polluted soil remediation consists on the mobilisation of the contaminant so that it can be progressively removed by means of sustainable techniques such as phytoextraction, alone or combined with the application of organic soil amendments [
27,
28]. As several authors demonstrated, the organic amendments (compost, biochar) due to their negative surface charge and dissolved organic carbon mobilise As [
14,
15,
29,
30], facilitating the capture of As in soil solution by phytoextraction plant species and thus, favouring its accumulation in biomass [
31]. Consequently, this process would lead to a gradual decrease in the available As concentration in soil [
28]. Also, the use of phytoremediation combined with amendments made with by-products is concordant with circular economy principles [
32]. Within the potential As–phytoextracting plants reported by several authors, Li et al. [
33] demonstrated that
Lolium perenne L. can grow under the stress caused by high concentrations of As.
Lolium perenne L. was also used for As phytoremediation by Clemente et al. [
27], while Karczewska et al. [
34] evaluated the effects of different amendments on its growth.
Following the preceding considerations, the main objective of this work is to test the two aforementioned strategies, mobilisation and immobilisation, in an As-polluted technosol located in the surroundings of a peri-urban area. This work compares an inorganic treatment (nZVI) to decrease As mobility, which could affect negatively plant development and soil properties, with two organic treatments, compost and biochar, which can improve plant development and soil quality, although they can mobilise the As. The potential reduction of human health risks, the amelioration of soil properties, and the reduction/increase of the incorporation of As into the trophic chain were examined.
2. Materials and Methods
2.1. Soil Sampling and Characterisation
The technosol sampled in this study is located in a periurban area of the municipality of Madrid, Spain, which according to the land use planning, will be harnessed in the future for residential use. Initial analyses of several soil samples (data not shown) revealed As concentration exceeding the Soil Screening Levels in force for the urban and industrial land uses (24 and 40 mg∙kg
−1 respectively) [
35]. To characterise the technosol, a composite representative sample of 20 kg was obtained, air-dried and sieved through 2 mm mesh. After homogenisation, subsamples were obtained with an aluminium riffler and subjected to the following analyses according to [
36]: Soil pH was determined using a pH electrode in a water to soil extract of 1:2.5. The quantitative determination of organic matter was carried out by dry route by difference in weight after a 24 hour combustion in a muffle at 550 °C, whereas available P was determined by Mehlich 3 method, and total nitrogen (TN) content was quantified by the Kjeldahl method. Pseudo-total As concentration was measured by ICP-MS (7700 Agilent Technologies equipment) after extraction using aqua regia by acid digestion in a microwave oven (Milestone ETHOS 1, Italy). Ca, K, Mg, Na, Al, and exchangeable cations (Ca
2+, K
+, Mg
2+, Na
+, Al
3+) were extracted with 0.1 M BaCl
2, and ICP-MS determined element concentrations. Effective cation exchange capacity (CEC) was calculated using the sum of exchangeable cation concentrations.
Subsamples above 2 mm were also observed using a Dino-Lite Digital Microscope to obtain preliminary mineralogical data. To corroborate the microscopy study, they were also studied by X-ray diffraction (XRD) using a Phillips X’ Pert Pro diffractometer with Cu kα1 radiation (1.540598 Å); after determining the position of Bragg peaks observed over the range of 2θ = 5–90°, the minerals were identified using databases of the International Centre for Diffraction Data. Furthermore, the major compounds of the soil were measured using X-ray fluorescence (XRF) employing a Philips PW2404 X-ray fluorescence spectrometer. Both XRD and XRF were carried out after grounding materials above 2 mm to ensure the homogeneity of the rock sample. Finally, grain size distribution of the fraction below 2 mm was determined by wet-sieving (ASTM D-422-63, Standard Test Method for Particle Size Analysis of Soils) in order to obtain the different soil fractions (2000–1000, 1000–500, 500–250, 250–125, 125–63, <63 microns). Subsequently As contents in the different fractions were determined by ICP–MS after acid digestion as described above.
2.2. Organic and Inorganic Amendments
The compost (C) used was made from animal manure mixed with plant debris and provided by Piensos Lago S.L. (Asturias, Spain). Biochar (B), which was provided by PYREG Carbon Technology Solutions (Dörth, Germany), was made from wood (remains of pruning) following the PYREG® methodology. Parameters studied in organic amendments were the same as in soil samples (see above), excluding mineralogy and grain size studies; i.e., EC, pH, total carbon, nitrogen, available phosphorus, pseudo-total concentrations (As, Cd, Cu, Pb, Zn), available concentrations of As, nutrients (Ca, K, Mg, Na, Al) and cation exchange capacity.
ZVI nanoparticles (nZVI), namely NANOFER 25S, were supplied by NANO IRON s.r.o. (Brno, Czech Republic). According to commercial specifications, this product has an iron content of 14–18%, and 2–6% of magnetite. The particles have an average size of around 60 nm, the suspension is strongly alkaline (pH 11–12), and the active surface area is 20 m
2/g (additional details are available at
www.nanoiron.cz). These nanoparticles were deeply characterised in previous works [
37], revealing that the zeta potential of nZVI was negative due to the polyacrylic acid (PAA) used as a coating to stabilise the nanoparticles and prevent agglomeration.
2.3. Lolium perenne L.
Lolium perenne L. seeds, supplied by Piensos Lago S.L. (Asturias, Spain), were sown in pots, which were watered to field capacity throughout the experiment. Lolium perenne L. was grown in all pots for 30 days.
2.4. Greenhouse Experiment and Monitoring
The one-month experiment was performed in a greenhouse where twelve plant pots, three per treatment, were prepared and distributed randomly in the greenhouse. Non-amended pots containing only the polluted soil (S) were used as controls. Three treatments were chosen, the first (SN) consisted of the application of nZVI in order to know if just the decrease in As concentration in the soil is sufficient to improve soil conditions and to allow a proper vegetation growth. In the second treatment, two organic treatments were chosen. One of them (SC treatment) consisted of compost application, which has been shown to improve soil conditions for plant development but may increase As mobility [
13]. The second organic treatment (SCB) was carried out by a blend of compost and biochar since biochar can foster the positive effects of compost. It must be noted that according to several authors, this latter procedure, can decrease As mobility, whereas according to others, it can enhance it [
30,
38]. The amendments were mixed with the polluted soil up to 0.5 kg per pot. The dose of nZVI suspension applied to the soil was 2.5%, based on prior works with As-polluted soils [
26,
31,
37,
39,
40]. In the case of SC and SCB treatments, the proportions were 12.5% of compost and 2.5% of biochar (
Table 1). These doses were based on previous works with similar treatments [
41,
42,
43].
Throughout the experiment, greenhouse average temperature was maintained at 13 ± 4 °C, and pots were watered to field capacity, while plant growth was supervised under visual examination to detect toxicological effects. After the incubation time, the pots were dismantled, the aerial part was harvested, and the soil samples were air-dried and sieved through a 2 mm mesh. Soil pH, organic matter, available P, pseudo-total As concentration, As RBA extraction, exchangeable cations (Ca2+, K+, Mg2+, Na+, Al3+) were determined following the procedures described above.
Also, at the end of the experimental time, plant biomass was measured on harvested Lolium perenne L. plants. The biomass was carefully washed with deionised water, immediately weighed, and dry mass was determined after oven-drying for 48 h at 80 °C and cooling at room temperature. As, P, Na, Mg, K, Ca, and Fe contents were determined by ICP-MS after, digestion in a microwave oven (Milestone ETHOS 1, Italy. 1600W, 30 min) using 0.2 g sample and 12 ml of HNO3.
2.5. As Assessment by RBA Extraction
For determining the oral available As, the Solubility/Bioavailability Research Consortium (SRBC) test was performed according to Kelley et al. [
44] and Juhasz et al. [
45]. The method consists on a simple extraction at low pH simulating the gastric liquids; thus soil subsamples of 1 gram with grain size smaller than 250 microns, obtained by sieving, were mixed with a solution of 30.03 g/L glicine at pH 1.5 following a relation 1:100 (w:v). The mixture was shaken at 40 rpm for 1 h at 37 °C, and then samples were centrifuged, and the supernatant was filtered at 0.45 microns before As analysis by ICP-MS. The RBA (Relative Bioavailability factor) value was then calculated by the ratio between this oral available As concentration and the As pseudo–total concentration.
2.6. Human Health Risk Assessment
Risk assessment was done following the US EPA methodology [
46], as recommended by regulations in most European countries, and specifically in Spain [
47]. Initially, and taking into account the planned near future land use of the site, the site–specific exposure scenario corresponds with a residential one. In this context, the most sensitive human receptors to be considered are children.
The Average Daily Dose for ingestion exposure (
ADD, expressed in mg∙kg
−1∙d
−1), according to USEPA [
48], is determined by means of Equation (1):
where:
CS: As concentration in soil (mg·kg−1). This value depends on the soil treatment.
IR: daily ingestion rate (mg∙d
−1). For children, this value is 200 mg∙d
−1 [
46].
EF: exposure frequency (d∙a
−1). This value is 350 d∙a
−1 [
49].
RBA: relative bioavailability factor (adimensional). This value depends on the soil treatment.
CF: conversion factor (10−6 kg∙mg−1).
ED: exposure duration (years). For children, this value is 6 years [
46].
BW: average body weight (kg). For children, this value is 15 kg [
48].
AT: averaging time (days). This value is equal to exposure duration (ED) for non-carcinogens risk analysis and 70 years for carcinogens risk analysis [
49].
To quantify the risk, the calculation was divided into two categories: non-carcinogenic risk and carcinogenic risk. The potential non-carcinogenic risk is defined by the hazard index (
HI), which was determined for As by means of Equation (2):
where
RfD is the oral reference dose for As, 3 × 10
−4 mg∙kg
−1·d
−1 [
49]. In this regard, when the
HI is below 1, it is considered that there is no toxicological risk [
46].
On the other hand, the carcinogenic risk (
CR) due to As is determined as:
where
SF is the slope factor (kg∙d∙mg
−1), provided for As by US EPA [
50] with a value of 1.5 kg∙d∙mg
−1. According to US EPA,
CR values lower than 10
−6 imply that risk is so small as to be negligible; from 10
−6 to 10
−4, the risk is tolerable; and if
CR is higher than 10
−4, the risk becomes unacceptable (1 person among 10,000 is in risk of developing cancer); nevertheless, in Spain the regulations in force [
47] established 10
−5 as the threshold to consider unacceptable risks.
2.7. Statistical Analysis
All analytical determinations were performed in triplicate. The data obtained were statistically treated using the SPSS programme, version 24.0 for Windows. Analysis of variance (ANOVA) and the test of homogeneity of variance were carried out. In the case of homogeneity, a post hoc least significant difference (LSD) test was done. If there was no homogeneity, Dunnett’s T3 test was performed. Bivariate analysis was also carried out by means of Pearson correlation.
4. Conclusions
Organic amendments (compost and biochar) and nZVI were tested for remediation of an As-polluted technosol from an urban area. The nZVI application proved to be a useful strategy for immobilising As, resulting in a reduction in both human health risks, and plant ability for As extraction. However, this was also accompanied by a reduction in plant ability for extraction of nutrients such as P, K, Ca, and Mg, thereby impacting negatively plant growth. On the other hand, the organic amendments were useful for plant development due to nutrient addition, although As was also mobilised and extracted by the plants. Furthermore, human health risk was not reduced after compost or compost plus biochar addition. Overall, after comparing opposite strategies of immobilisation and mobilisation, our results concluded that a combination of compost and nZVI could be a good strategy to improve soil properties and plant growth while allowing for low levels of As mobilisation.