An Investigation into the Perspectives of Elementary Pre-Service Teachers on Sustainable Development

: Teachers in elementary schools play a crucial role in educating children about sustainable development. Therefore, the concept of sustainable development is essential for elementary pre-service teachers to be ready for their future careers. This study used the sequential mixed methods design to investigate the relationship between the sustainable development perspectives of elementary pre-service teachers in Southern Vietnam and the current teacher training curriculum. The EDINSOST sustainability questionnaire was distributed to 392 participants in order to measure four dimensions of sustainable development: critical contextualization of knowledge, sustainable use of resources, participation in community processes, and ethics. A set of six interview questions was conducted with eight participants, focused on their understandings of sustainable development and the evaluation of the curriculum on sustainable development. Quantitative ﬁndings revealed that elementary pre-service teachers rate moderately on all dimensions; however, the knowledge is slightly higher than the behavior. Moreover, there were variations in perspectives based on pre-service teachers’ grade level and gender. Qualitative ﬁndings suggested the lack of sustainable development perspectives amongst pre-service teachers and the gaps in the teacher training curriculums regarding sustainable development. Finally, detailed discussions regarding the implementation of sustainable development in the training curriculum were provided.


Introduction
Environmental and social challenges have always captured the focus of global public opinion for many decades. The greenhouse effect, global warming, and pollution have had a significant impact on biodiversity in the natural world [1,2]. Furthermore, the overuse of natural resources to promote economic development and fulfill the requirements of a rising population is one of the hazards to our world [3][4][5][6][7]. As a result, sustainability has emerged as the new paradigm of development to tackle these issues [1,2,8]. In this context, UNESCO stated that education is the essential tool for achieving sustainability by increasing knowledge, skills, and attitudes and by empowering future generations to make changes in our world [9].
Sustainable development was defined as "the development that meets the needs of the present without compromising the ability of future generations to meet their own needs" [10] (p. 43). The 2030 Agenda proposes a worldwide action plan with seventeen Sustainable Development Goals (SDGs). Quality education (Goal 4) is the key to attaining all the SDGs by promoting responsible actions for environmental integrity, economic viability, and just society [9,11]. To become a practical solution, education for sustainable been highlighted as having more positive environmental attitudes [45] and having more eco-friendlier behaviors than their male peers [21]. For instance, females were more likely to engage in sustainable consumer behavior and judge environmental issues to be more problematic than males, because they consider safety and health more [46]. Meanwhile, results from another quantitative study showed that male pre-service teachers have more environmental knowledge than females [21]. This can be explained by the fact that men seem to be more disciplined and tend to seek sustainability information [47,48].
In Vietnam, the government has adopted the World Commission on Environment and Development (1987) [10] definition of sustainable development, which makes close associations between the concepts of economic development, social progress, and environmental protection [49]. Vietnam has shown an explicit commitment to this movement in the form of a policy statement [49,50]. The Vietnamese National Action Plan for ESD stated: "As a member of the United Nations and UNESCO, Vietnam will continue its active involvement in activities under the framework of the ESD" [51]. The Ministry of Education and Training has integrated environmental issues into the education curriculum since 2009 and has promoted some ESD principles. However, this is not a comprehension approach [52] because only a few topics related to sustainable development were integrated into teaching, such as population, saving energy, and heritage. Additionally, they appeared to be "add-on knowledge" in an overloaded curriculum [53].
Most studies in Vietnam on sustainable development focused on the perspectives of general teachers and education stakeholders, such as school administrators and policymakers [54]. ESD implementation challenges in teacher education institutions were considered [41]. However, the perspectives of elementary pre-service teachers on this issue were expanding but were still not as broad as in other areas. Meanwhile, they will be the main influencers and decision-makers of the future for children and the community [55]. Implementing ESD for elementary students does not guarantee quality unless the pre-service teachers' views on this issue are noted. Therefore, an investigation of their perspectives on sustainable development in the Vietnamese context is worth conducting.
Based on this view, this paper aims to examine the perspectives of elementary preservice teachers on sustainable development and analyze their sustainable development competencies through the current teacher training curriculum. This paper seeks to answer the following research questions:

1.
Which level are the perspectives and competencies of elementary pre-service teachers on sustainable development? 2.
How are elementary pre-service teachers' perspectives and competencies on sustainable development varied by gender? 3. To what extent do elementary pre-service teachers receive sustainable development from the training curriculum?

Research Design
This study adopted a sequential mixed-methods design to explore research questions. A mixed-method approach builds a more robust study than only qualitative or quantitative research [56]. In the field of ESD, mixed methods research provided a wealth of information and allowed a more thorough analysis of individual perspectives toward sustainable development [57,58]. For this study, the quantitative method would provide a basis and broader picture of the level of elementary pre-service teachers' competencies and perspectives on sustainable development based on different dimensions, but to gain deeper insight, the qualitative method was added. The qualitative method would complement this picture by adding more detailed information about pre-service teachers' correct and incorrect (or incomplete) understanding of sustainable development and how they apply strategies for sustainable development in their own teaching as an explanation for the quantitative results. Moreover, it is important to develop a qualitative method aimed at assessing whether an effective integration of sustainability into the training curriculum and into university teaching exists and to determine whether or not pre-service teachers achieve better learning outcomes in sustainable competencies from the enacted curriculum. In other words, the quantitative data were collected first. The qualitative data were collected after the quantitative data and were meant to explain the results of the quantitative data.

Participants and Sampling Design
For the quantitative method, a random sample of 480 elementary pre-service teachers enrolled in a four-year program in the elementary education department at a university of education in Southern Vietnam was invited by email to participate in this research. In total, we received 392 responses (81.67%) from the invited sample. Participants consisted of 101 freshmen, 95 sophomores, 98 juniors, and 98 seniors, with no missing data. There are 307 females (78.32%) and 85 males (21.68%), which reflects the distribution of gender in the teaching sources in Vietnamese elementary education [59], with over 70% of female teachers in elementary education in Vietnam from 2010 to 2020. The detailed demographic information is described in Table 1. For the qualitative method, the interview sampling literature suggests that the ideal individual size is between 6 and 12 people [56]. Most themes in the study were identified within six interviews, and no new codes emerged after conducting ten interviews [60]. A number of new codes were identified in the first eight individual interviews with over 80% saturation [61]. Based on these results, we started the interview and continued until we recognized data saturation to have been reached. After the analyzing process, the interview data were gathered from eight Vietnamese elementary pre-service teachers.
The criterion sampling method was used in selecting interviewees for this research. The main criterion in the selection of pre-service teachers is that they have experience learning with the elementary education curriculum for at least one semester. Another criterion was that pre-service teachers were concerned about reflecting on the current training curriculum because this study attempted to both understand pre-service teachers' perspectives on sustainable development and assess the effectiveness of implementing sustainable development in the training curriculum. The demographic features of the elementary pre-service teachers are shown in Table 2. For the collection of the quantitative data, this study used the EDINSOST sustainability questionnaire [28]. This questionnaire consists of 18 questions concerning four dimensions of sustainability competencies: critical contextualization of knowledge, sustainable use of resources, participation in community processes, and ethics. The items of each dimension were sorted by three competency levels, including "Know" (level 1), "Know-how" (level 2), and "Demonstrate and Do" (level 3) (see Table 3). The participants were asked to state how much they agree with these statements on a four-point Likert scale ranging from "strongly disagree" to "strongly agree". Because the original questionnaire [33], which is in English, has not been translated to Vietnamese, the survey was first translated into Vietnamese by all the authors. Then the translated survey was translated back into English as a validity check by a university lecturer that is an expert in ESD and one university lecturer that is from the English Department. After checking the transferability and accuracy of the translation, a pilot study with N = 30 was conducted to check if the instrument was understood by the participants. The information on the reliability and validity of the questionnaire is shown in Table 3. Table 3, the reliability statistic indicated that the internal consistency of all dimensions was reliable enough; Cronbach's Alpha was from 0.70 to 0.81. Overall, the Cronbach's alpha coefficient in the questionnaire of this study was 0.91, showing that the reliability level of this research is high enough. It means the reliability of this questionnaire was considered acceptable.

The Set of Interview Questions
For the collection of the qualitative data, this study used a set of open-ended questions adapted from a previous study [62]. Of the eight initial questions, this research used four questions. Furthermore, we decided to add two more questions based on the literature review regarding the effectiveness of transferring sustainability in the curriculum [32,33]. These added questions related to the perspectives on sustainable development and the evaluation of the curriculum on sustainable development. Finally, a full set of interview questions with a total of six questions (see Table 4).

Data Collection and Data Analysis
For the quantitative data collection, an online survey was distributed by the authors to all participants in March 2022 by email. Before administering the study, the researchers emphasized that the participation was voluntary. Their personal information would be treated confidentially, and all data would be used for research purposes only. The data collection occurred from 7 March to 31 March 2022. Finally, we gathered 392 responses.
The quantitative data were analyzed by SPSS Statistics 27.0. The analytic strategy was as follows: The means (M) and standard deviations (SD) were calculated for sustainable development perspective variables of male and female pre-service teachers across four years at university. An independent t-test was performed to test the differences between these two genders, while a one-way ANOVA was conducted to determine if there was a statistically significant difference among the groups of four years at university. We are not only interested in knowing if there are differences, but also how big those differences are. Thus, Cohen's d was calculated for each question to measure the size of the effect.
For the qualitative data collection, semi-structured interviews were used. Before the interviews were conducted, the participants were explained the purpose of the interviews by the interviewer (in that case, the author). It was emphasized that there were no right or wrong answers, and the participants were encouraged to share as much of their views as possible. The privacy of the participants was ensured, and pseudonyms were used for all the participants. Due to the COVID-19 pandemic, all the interviews were individually conducted online using Zoom meetings on different days from 12 April to 7 May 2022. Each interview lasted around 40 min. All the interviews were video-recorded for the subsequent transcription and analysis of the data.
The qualitative data were analyzed following thematic analysis guidelines [63] to investigate the answers of participants related to their understanding and application of sustainable development, as well as their evaluations of the implementation of sustainable development in the training curriculum. Firstly, the interview audio recordings were transcribed verbatim. To have an overall understanding of the data, the researchers listened to the recordings and read the manuscript repeatedly before the coding and analysis began. Next, the codes were then generated by grouping sentences and phrases with similar meanings. Once the coding of the first set of data (the interview of one of the participants) was completed, the researchers revised and refined the codes to eliminate any overlap and redundancy. After that, the researchers continued coding the remaining data set. The final set of codes was then revised and refined to form broader themes for report and discussion.
The research procedure is illustrated in Figure 1. sustainable development, as well as their evaluations of the implementation of sustainable development in the training curriculum. Firstly, the interview audio recordings were transcribed verbatim. To have an overall understanding of the data, the researchers listened to the recordings and read the manuscript repeatedly before the coding and analysis began. Next, the codes were then generated by grouping sentences and phrases with similar meanings. Once the coding of the first set of data (the interview of one of the participants) was completed, the researchers revised and refined the codes to eliminate any overlap and redundancy. After that, the researchers continued coding the remaining data set. The final set of codes was then revised and refined to form broader themes for report and discussion.
The research procedure is illustrated in Figure 1.

Sustainable Development Perspectives and Competency Levels across Grade Levels
Research question 1: Which level are the perspectives and competencies of elementary pre-service teachers on sustainable development? This study applies the one-way ANOVA to determine if there is a statistically significant difference among the groups of four years at university. Table 5 indicates that elementary pre-service teachers' perspectives on sustainable development from four dimensions were moderate for all grade levels, ranging from 2.67 to 2.98 in total. It could be said that these participants' self-reflection on their own perspectives of sustainable development is not high. Despite the moderate scores, their perspectives had a rising trend from the first year to the fourth year. With the dimension of sustainable use of resources, there was t (392) = 3.27, p < 0.05. With the dimension of participation in community processes, there was t (392) = 4.12, p < 0.01. With the dimension

Sustainable Development Perspectives and Competency Levels across Grade Levels
Research question 1: Which level are the perspectives and competencies of elementary pre-service teachers on sustainable development? This study applies the one-way ANOVA to determine if there is a statistically significant difference among the groups of four years at university. Table 5 indicates that elementary pre-service teachers' perspectives on sustainable development from four dimensions were moderate for all grade levels, ranging from 2.67 to 2.98 in total. It could be said that these participants' self-reflection on their own perspectives of sustainable development is not high. Despite the moderate scores, their perspectives had a rising trend from the first year to the fourth year. With the dimension of sustainable use of resources, there was t (392) = 3.27, p < 0.05. With the dimension of participation in community processes, there was t (392) = 4.12, p < 0.01. With the dimension of ethics, there was t (392) = 3.72, p < 0.05. That meant these three dimensions had statistically significant differences over the four years of university. It could be understood that the ideas of freshmen seem to be more simplistic than those of seniors, since they have yet to complete a course or receive extracurricular training in sustainability [64][65][66]. However, the improvement of pre-service teachers' sustainability perspectives from the first year to the fourth year was not as much as expected because all items scored lower than 3.00 on the  Moreover, the Tukey post hoc test results suggested that the difference between the sophomore and junior groups was insignificant. Only the comparison between the senior and freshman groups was significant among the four groups of grade level.  Table 5). It could be explained that the sophomores and juniors were in the learning process with a general knowledge base optimized for being ready to join specialized sustainability courses in the future, so their sustainability perspective did not show development in the two middle years of university [65].
Additionally, Table 5 also described the competency dimension of sustainable development in three levels: Know, Know-how, Demonstrate and Do. Pre-service teachers rated Know level with the highest scores (2.91 < M < 3.11), while they rated the Demonstrate and do level with the lowest scores (2.54 < M < 2.79). With the dimension of the Know-how level, there was t (392) = 5.50, p < 0.01. With the dimension of the Demonstrate and Do level, there was t (392) = 2.74, p < 0.05 (see Table 5). That meant there were statistically significant differences in two dimensions during the four years of university. However, the data did not show statistically significant changes in the dimension of the Know level with t = 2.25, p > 0.05. Noticeably, as can be seen from the data presented in Table 5, the sophomore group states that in some dimensions of perspectives (Critical contextualization of knowledge, sustainable use of resources, participation in community processes) and some levels of competencies (Know, Know-how) they have higher scores than the junior group. This situation can be explained by the effect detailed by Kruger and Dunnin [67], according to which an individual with fewer competencies and less knowledge has an illusory feeling of superiority, considering themselves to be more intelligent than another, better-prepared individual. The freshmen and the seniors were more consistent in their answers.
For more details, the result of pre-service perspectives is applied to each question. As shown in Figure 2, the X-axis contains the 18 questions of the EDINSOST Sustainability Questionnaire. The Y-axis on the left identifies the mean scores of sustainable development perspectives corresponding to each question, while the Y-axis on the right presents the p-value obtained for each question.
an illusory feeling of superiority, considering themselves to be more intelligent than another, better-prepared individual. The freshmen and the seniors were more consistent in their answers.
For more details, the result of pre-service perspectives is applied to each question. As shown in Figure 2, the X-axis contains the 18 questions of the EDINSOST Sustainability Questionnaire. The Y-axis on the left identifies the mean scores of sustainable development perspectives corresponding to each question, while the Y-axis on the right presents the p-value obtained for each question.  Figure 2 shows the senior group rated higher scores than the freshman, sophomore, and junior groups for all 18 items. However, there are only significant differences (p < 0.05) in eight questions, including Q1, Q8, Q9, Q11, Q12, Q16, Q17, and Q18. Therefore, the data corresponding to these eight questions will be analyzed in more detail by Cohen's d to know how different they are (see Figure 3). In these questions, pre-service teachers are asked whether they know how to interrelate between natural, social, and economic systems (Q1); develop educational actions to minimize negative socio-environmental impacts (Q8), and design educational activities with concern about the negative socio-environmental impacts and corrective actions (Q9), develop myself in community educational projects (Q11), carry out socio-educational activities in the community (Q12), consider the sustainable human development as a fundamental purpose of citizen education (Q16), analyze the consequences that personal performance may have on the integral development of students (Q17), design educational intervention proposals that integrate sustainability values (Q18). Figure 3 illustrates the Cohen's d for eight questions to measure the effect size. The size of all measured effects was small, with the Cohen's d ranging from 0.13 to 0.39. However, the effect sizes corresponding to questions Q9, Q11, Q12, and Q17 were quite larger than the remaining questions, with significant differences at 0.22, 0.33, 0.26, and 0.39, respectively.
Sophomore Junior Senior p-value  Figure 2 shows the senior group rated higher scores than the freshman, sophomore, and junior groups for all 18 items. However, there are only significant differences (p < 0.05) in eight questions, including Q1, Q8, Q9, Q11, Q12, Q16, Q17, and Q18. Therefore, the data corresponding to these eight questions will be analyzed in more detail by Cohen's d to know how different they are (see Figure 3). In these questions, pre-service teachers are asked whether they know how to interrelate between natural, social, and economic systems (Q1); develop educational actions to minimize negative socio-environmental impacts (Q8), and design educational activities with concern about the negative socio-environmental impacts and corrective actions (Q9), develop myself in community educational projects (Q11), carry out socio-educational activities in the community (Q12), consider the sustainable human development as a fundamental purpose of citizen education (Q16), analyze the consequences that personal performance may have on the integral development of students (Q17), design educational intervention proposals that integrate sustainability values (Q18). Figure 3 illustrates the Cohen's d for eight questions to measure the effect size. The size of all measured effects was small, with the Cohen's d ranging from 0.13 to 0.39. However, the effect sizes corresponding to questions Q9, Q11, Q12, and Q17 were quite larger than the remaining questions, with significant differences at 0.22, 0.33, 0.26, and 0.39, respectively.
It could be concluded that elementary pre-service teachers' perspectives on the sustainable development concept have a noticeable improvement over the four years at university, especially in the following aspects: participation in community processes (d = 0.33 for Q11 and d = 0.26 for Q12), ethics (d = 0.39 for Q17), and sustainable use of resources (d = 0.22 for Q9) (see Figure 3). Taken together, the data of this study should not be interpreted as a generalized increase in the sustainability perspectives and competencies of pre-service teachers corresponding to all the eighteen questions analyzed, but rather that a minority of questions in which participants have greatly increased their learning. Sustainability 2022, 14, x FOR PEER REVIEW 10 of 22 It could be concluded that elementary pre-service teachers' perspectives on the sustainable development concept have a noticeable improvement over the four years at university, especially in the following aspects: participation in community processes (d = 0.33 for Q11 and d = 0.26 for Q12), ethics (d = 0.39 for Q17), and sustainable use of resources (d = 0.22 for Q9) (see Figure 3). Taken together, the data of this study should not be interpreted as a generalized increase in the sustainability perspectives and competencies of pre-service teachers corresponding to all the eighteen questions analyzed, but rather that a minority of questions in which participants have greatly increased their learning.

Sustainable Development Perspectives and Competency Levels across the Genders
Research question 2: How are elementary pre-service teachers' perspectives and competencies on sustainable development varied by gender? An independent t-test was performed to test the differences between these two genders regarding perspectives on sustainable development. Table 6 presented the perspectives and competency levels of sustainable development between male and female elementary pre-service teachers. In general, the total score of both perspective and competency level dimensions rated by the male group (M = 3.06, SD = 0.61) were higher than the female group (M = 2.81, SD = 0.52).

Sustainable Development Perspectives and Competency Levels across the Genders
Research question 2: How are elementary pre-service teachers' perspectives and competencies on sustainable development varied by gender? An independent t-test was performed to test the differences between these two genders regarding perspectives on sustainable development. Table 6 presented the perspectives and competency levels of sustainable development between male and female elementary pre-service teachers. In general, the total score of both perspective and competency level dimensions rated by the male group (M = 3.06, SD = 0.61) were higher than the female group (M = 2.81, SD = 0.52). Regarding the perspective dimensions, the male group showed significantly higher scores than the female group on three dimensions: critical contextualization of knowledge, sustainable use of resources, and ethics. With the dimension of critical contextualization of knowledge, there was t (392) = 3.57, p < 0.01. With the dimension of sustainable use of resources, there was t (392) = 2.26, p < 0.05. With the dimension of ethics, there was t (392) = 4.40, p < 0.01. The only third dimension, participation in community processes, did not show a statistically significant difference between males and females with t = 1.87, p > 0.05. That meant the perspectives on sustainable development of the female group were significantly lower than those of the male group, except for how they participate in community processes that promote sustainability.
Regarding the competency level dimensions, all three dimensions of competency levels showed statistically significant differences across the genders. Thus, gender differences were observed in all levels of sustainability competency. Male pre-service teachers' competencies were at higher levels than those of females following the gradation: demonstrate and do level < Know-how level < Know level. The findings of this study confirmed the findings of others [21], that male pre-service teachers have better environmental competencies than females, especially in their knowledge. This can be explained by the fact that men seem to be more disciplined and tend to seek sustainability information [47,48].
To add more detail about elementary pre-service teachers' perspectives, Figure 4 depicts the scores of the sustainable development perspectives of males and females, corresponding to 18 questions on the left Y-axis and the p-value obtained for each question on the right Y-axis.
Regarding the perspective dimensions, the male group showed significantly higher scores than the female group on three dimensions: critical contextualization of knowledge, sustainable use of resources, and ethics. With the dimension of critical contextualization of knowledge, there was t (392) = 3.57, p < 0.01. With the dimension of sustainable use of resources, there was t (392) = 2.26, p < 0.05. With the dimension of ethics, there was t (392) = 4.40, p < 0.01. The only third dimension, participation in community processes, did not show a statistically significant difference between males and females with t = 1.87, p > 0.05. That meant the perspectives on sustainable development of the female group were significantly lower than those of the male group, except for how they participate in community processes that promote sustainability.
Regarding the competency level dimensions, all three dimensions of competency levels showed statistically significant differences across the genders. Male pre-service teachers' competencies were at higher levels than those of females following the gradation: demonstrate and do level < Know-how level < Know level. The findings of this study confirmed the findings of others [21], that male pre-service teachers have better environmental competencies than females, especially in their knowledge. This can be explained by the fact that men seem to be more disciplined and tend to seek sustainability information [47,48].
To add more detail about elementary pre-service teachers' perspectives, Figure 4 depicts the scores of the sustainable development perspectives of males and females, corresponding to 18 questions on the left Y-axis and the p-value obtained for each question on the right Y-axis.

Male
Female p-value As shown in Figure 4, the analysis by gender groups showed that male pre-service teachers rated the sustainable development perspectives with higher scores than female pre-service teachers for all 18 items. However, the significant differences (p < 0.05) only appeared in four questions, including Q3, Q7, Q13, and Q18. These questions came from level 1 and level 3 of the three competency levels, in which Q7 and Q13 are at level 1 (the Know level), while Q3 and Q18 are at level 3 (the Demonstrate and Do level). This means the significant difference between males and females is the level of knowledge and the level of action toward sustainable development issues.
Based on the significant difference questions, the data corresponding to these four questions were analyzed in more detail by Cohen's d (see Figure 5). In these questions, elementary pre-service teachers are asked whether they know how to anticipate the repercussions of changes in natural, social, and economic systems (Q3); identify the possible socio-environmental impacts derived from my educational activities (Q7); know the ethical principles of sustainability (Q13); and design educational intervention proposals that integrate sustainability values (Q18). Figure 4, the analysis by gender groups showed that male pre-service teachers rated the sustainable development perspectives with higher scores than female pre-service teachers for all 18 items. However, the significant differences (p < 0.05) only appeared in four questions, including Q3, Q7, Q13, and Q18. These questions came from level 1 and level 3 of the three competency levels, in which Q7 and Q13 are at level 1 (the Know level), while Q3 and Q18 are at level 3 (the Demonstrate and Do level). This means the significant difference between males and females is the level of knowledge and the level of action toward sustainable development issues.

As shown in
Based on the significant difference questions, the data corresponding to these four questions were analyzed in more detail by Cohen's d (see Figure 5). In these questions, elementary pre-service teachers are asked whether they know how to anticipate the repercussions of changes in natural, social, and economic systems (Q3); identify the possible socio-environmental impacts derived from my educational activities (Q7); know the ethical principles of sustainability (Q13); and design educational intervention proposals that integrate sustainability values (Q18). Although there is a significant difference between genders in sustainable perspectives from these four questions, Figure 5 indicates that there was no large effect size on these questions with Cohen's d ranging from 0.29 to 0.53. The effect sizes of Q7 and Q13 are medium between two groups of genders, ranging from 0.45 to 0.53, but they are larger than those of Q3 and Q18. In contrast, Cohen's d of questions Q3 and Q18 shows small size effects between two groups of genders at 0.32 and 0.29, respectively.
Hence, it could be concluded that the result of this study showed a significant difference between the male and female pre-service teachers in the sustainable perspectives. However, the differences were more significant in terms of knowledge (level 1-Know) than action (level 3-Demonstrate and Do) between male and female pre-service teachers.

Qualitative Findings
Research question 3: To what extent do elementary pre-service teachers receive the sustainable development from the training curriculum? From eight interviews, the interview data have been analyzed with coding. The lead question for the analysis was: What do preservice teachers understand ESD, apply ESD and assess the integration of ESD into the enacted curriculum? Altogether, fourteen categories could have been found in Table 7. Although there is a significant difference between genders in sustainable perspectives from these four questions, Figure 5 indicates that there was no large effect size on these questions with Cohen's d ranging from 0.29 to 0.53. The effect sizes of Q7 and Q13 are medium between two groups of genders, ranging from 0.45 to 0.53, but they are larger than those of Q3 and Q18. In contrast, Cohen's d of questions Q3 and Q18 shows small size effects between two groups of genders at 0.32 and 0.29, respectively.
Hence, it could be concluded that the result of this study showed a significant difference between the male and female pre-service teachers in the sustainable perspectives. However, the differences were more significant in terms of knowledge (level 1-Know) than action (level 3-Demonstrate and Do) between male and female pre-service teachers.

Qualitative Findings
Research question 3: To what extent do elementary pre-service teachers receive sustainable development from the training curriculum? From eight interviews, the interview data have been analyzed with coding. The lead question for the analysis was: What do pre-service teachers understand ESD, apply ESD and assess the integration of ESD into the enacted curriculum? Altogether, fourteen categories could have been found in Table 7.
The categories can be sorted according to the following six sub-themes: definitions of sustainable development, role of sustainable development, applying ESD within the classroom, applying ESD within the community, curriculum assessment, and curriculum suggestions. Six sub-themes were organized into three main themes: understanding sustainable development, applying sustainable development, and integrating sustainable development. Table 8 lists and defines each major theme and sub-theme with representative quotes from the participants.

Understanding Sustainable Development
Definitions of sustainable development: elementary pre-service teachers from the first year to the fourth year at the university shared fundamental understandings of sustainable development. Although their presented definitions were different, the content showed that sustainable development was related to "reasonable use of natural resources" (A17, M01), "ensuring harmonization and balance between economy and environment" (M04, M07), "individuals and organizations need to work together to find answers to problems like pollution, poverty, and the loss of biodiversity" (M05), and "addressing global challenges based on a multidisciplinary approach" (M07). These answers expressed the extensive range of sustainable development definitions, which consists of social, environmental, and economic issues. However, most of these answers came from juniors and seniors.
Most freshmen and sophomores did not define the sustainable development concept comprehensively. They mainly focused on environmental issues: "sustainable development refers to environmental protection" (A12) and "increasing recycling activities" (A24, A30), while sustainable development includes three components: society, the environment, and the economy. Moreover, interviewees frequently mentioned the environmental and natural resource goals as well as climate change goals among the 17 SDGs of the United Nations, such as "Climate Action" (A12, A17, A24, M04), "Life on Land" (A30), and "Clean Water and Sanitation" (M12, M07). In contrast, the remaining SDGs were not mentioned by interviewed participants. This qualitative data showed that pre-service teachers had insufficient knowledge about the "sustainable development" concept and the 17 SDGs of the United Nations.
Roles of sustainable development: All eight interviewees agreed that sustainable development issues should be included in education. However, when asked why it is important, most pre-service teachers could not answer clearly. There were four out of the eight pre-service teachers who could provide an explanation, such as "connected to economic, social, and environmental development" (M05), "expressing solutions to social challenges which children need to tackle at present and in the future" (A12), "connecting learning content to realworld problems" (M01) and "promoting the development of critical thinking science and making decisions based on scientific knowledge" (M04, M07). Most answers came from the juniors and seniors. These findings suggest that pre-service teachers begin their higher education studies with a confused conception of sustainability, which could influence their perception of the importance of sustainable development and their professional development.

Applying Sustainable Development
Applying ESD within the classroom: Pre-service teachers agreed on the important role of future teachers as "role models for their students, and if they act in sustainable ways, it will help elementary students become more aware of the sustainable development concept." (M04), "Elementary students spend most of the day at school, so the school is a great place to learn good habits toward the environment." (M05, M07). Therefore, all the interviewees expressed they need self-training in knowledge and skills about sustainable development at the university to create a positive classroom or school learning environment in their teaching career in the future.
Furthermore, when asked about ideas for ESD in the classroom, the freshmen and sophomores mostly mentioned two topics: "environmental protection" (A12, A24 A30) and "climate change" (A30). While juniors and seniors not only provided topics but also gave example activities such as "a project like making toys, pen boxes, or plant pots out of old things" (M01, M05), "donating leftover clothes and school supplies to the disadvantaged residents" (M07), and "organize tours of handicraft villages like A Glance at Vietnamese Folk Painting or Making Pottery" (M04). From this finding, the ideas of the freshman and sophomore groups were simpler than those of the junior and senior groups when they applied sustainability in their teaching. It could be explained that they have yet to complete the whole training course at university and the internships in sustainability [64][65][66].
However, some pre-service teachers expressed that males were better at organizing field trips than females because it "requires good health for a long trip" (M01, M05) and "the ability to bring the materials" (A12) and "manage large numbers of students outside the school campus" (A30, M07). At the same time, the females are more suitable for types of activities such as arts and crafts "When children reuse the carton to complete their toys, pen boxes, or plant pots, teachers need to evaluate the products and give feedback. I think the female teachers are better than males." (M01, M05). This proves that the difference in sustainable development perspectives of elementary pre-service teachers is not only in their levels of competencies but also in how they choose activities to organize for their class.
Applying ESD within the community: Pre-service teachers believed that engaging in community activities, such as "Earth Hour" or "Earth Day" (A12, A17, M07), was one of the ways to reduce the negative effects on our planet. Individuals could "use of public transportation" (A24), "use ecologically friendly goods" (M05), "limit the use of plastic bags and excessive shopping" (M07) and "move toward a simpler life" (M07). One of the junior interviewees remarked, "In order to contribute to sustainable development, I believe it is vital to minimize waste and purchase the right things in order to become a smart consumer." (M04).

Integrating Sustainable Development
Curriculum assessment: Pre-service teachers recognized a connection between the current Bachelor Curriculum of the Elementary Education Department and the content of sustainable development through a few required and elective courses, including Foundations of Natural Science in elementary schools, Foundations of Social Studies in elementary schools (A17, A24, A30), Teaching natural and social sciences in elementary schools, and environmental education (M01, M04, M05, M07). On the other hand, participants pointed out that some lecturers integrated sustainable development issues into their lectures even though the course content does not go in-depth on ESD (A24, M01, M05, M07). However, participants noted that economic issues were rarely brought up in their attended courses, and that instead, environmental and social issues were the most frequently brought up.
Most interviewees agreed that science would be the most logical area to implement ESD (M01, M05, M07). However, three of them felt ESD could be integrated into the overall curriculum, not only a few courses, because "Science alone would be very challenging due to the diversity of the real-world problems." (M04, M05, M07). Besides the courses in the training curriculum, pre-service teachers also mentioned extracurricular activities that could help them improve awareness of sustainable development issues, such as Green Summer, Books for rural areas, Earth Hour, Earth Day, and World Environment Day (M04, M07). In explaining the need to include sustainable development issues within the teacher training curriculum, the elementary pre-service teachers said that "This is crucial content integrated into elementary schools, so I need this knowledge to be a good teacher in the future." (A30), "Teachers gain confidence in educating students when they have a comprehensive grasp of sustainable development." (M04, M07).
Curriculum suggestions: Pre-service teachers provided three recommendations for the teacher training curriculum in terms of sustainable development. Firstly, learning and teaching about this topic at the university should take an interdisciplinary approach, not only in the science field (M05, M07). Especially, "STEM/STEAM education and project-based learning are good ways to help the pre-service teachers gain more knowledge, attitude and take actions about sustainable development issues." (M07). Secondly, adjusting course content and lecturers' teaching methods at the university (A30, M04, M05, M07). In particular, "social and economic issues should be increased in the syllabus content." (M04, M05, M07) and "lecturers should reduce the traditional teaching methods, increase the project-based learning and experiential learning methods." (M01, M05). Thirdly, changing the sustainable development courses from elective to required credits (M04, M07) because "I really hope sustainable development can be more integrated into the existing curriculum, and all pre-service teachers must learn this topic before graduation. It is better for our teaching career in the future." (M07). This finding explains the expectations of pre-service teachers towards the enacted training curriculum in the Elementary Education Department, which at present has not yet derived enough value to help them achieve their sustainability competencies to meet their expectations.

General Findings
While the questionnaire offered predefined statements about sustainable development, in the interviews the pre-service teachers were able to elaborate on their views, opinions, and attitudes freely based on six interview questions. Both sets of data complemented each other, and the results can give valuable insight into pre-service teachers' perspectives and competencies towards ESD. Based on both quantitative and qualitative data analysis, the following common findings were drawn: In the questionnaire, the pre-service teachers showed high scores on "sustainable use of resources", followed by "participation in community processes". In contrast, they presented low scores on "ethics" and "critical contextualization of knowledge". Moreover, pre-service teachers' competence on the "Know level" seems to be higher than the "Know-how level", while the lowest score is the "Demonstrate and Do level". Noticeably, juniors and seniors showed a more detailed understanding and application of sustainable development than freshmen and sophomores. Although there was an improvement in the sustainability perspectives of elementary pre-service teachers during their academic studies at the university, they got the average scores of sustainable development perspectives on all eighteen items. This was not further investigated in the questionnaire, but in the interviews, the pre-service teachers explained that their understandings of sustainability were insufficient. In particular, they understood ESD dealt with the SDGs and ecological issues and that ESD is a broad and diverse issue. The interviewees focused more on the environmental component, while the social and economic components of the concept of "sustainable development" were not mentioned deeply in their answers.
Regarding gender in the sustainability competencies of pre-service teachers, the quantitative results concluded that gender was an element that impacted the competence of elementary pre-service teachers' understanding and applying ESD in their future careers. In particular, male pre-service teachers scored higher on all three levels of sustainability competencies and three out of four dimensions of perspectives, except only the dimension "participation in community processes". Furthermore, the interview results complement the findings of the questionnaire results that gender not only impacted the sustainability competencies of elementary pre-service teachers but also the types of potential activities they carried out in their classroom.
Lastly, the majority of the pre-service teachers expressed that ESD was important to include in teacher education to prepare for their teaching careers in the future. There is even a strong suggestion for the inclusion of more ESD content in the enacted curriculum in higher education because the elementary pre-service teachers evaluated the integration of sustainability issues into the training curriculum at a low level at present. They recognized some content courses addressed environmental issues but lacked economic and social content. Therefore, social and economic content should be included more in the teacher training curriculum. It was also necessary to consider changing the type of course from elective to required courses that could help pre-service teachers have adequate knowledge of sustainable development for future teaching. In the interviews, the elementary pre-service teacher even declared ESD as a teaching mission and a duty. They strongly believe that teachers have an important role in solving sustainability-oriented challenges through education. Teachers play a central role in ESD. A critical comment was mentioned that sustainable development is not always obvious in elementary school classrooms. There should also be more content in accordance with the implementation of ESD in higher education.

Discussion
In their role as future teachers, elementary pre-service teachers are the key elements for developing children's awareness, attitudes, and behaviors toward sustainable development [1]. As a result of the survey on sustainable development perspectives, all participants from the first year to the fourth year at the university showed moderate scores. In fact, the Vietnamese Ministry of Education and Training has integrated environmental issues into the education curriculum since 2009 [52]. Only a few topics such as population, saving energy, and heritage were integrated into teaching in higher education [53]. This led to the sustainability competencies of elementary pre-service teachers not being reached as expected. Thus, it is important for higher education in Vietnam to focus on the holistic nature of the concept of "sustainable development", which includes three elements: environmental, social, and economic elements, and introduces the 17 SDGs to pre-service teachers through different methods. Moreover, the teaching skills of lecturers in universities should be discussed. As stated by Kieu et al. [41], lecturers should develop collaborative and competency approaches in university teaching as the active teaching methods not only help pre-service teachers develop solid knowledge but also sustainable competencies.
Despite the moderate scores, seniors showed higher scores than freshmen on the sustainable development perspectives. This result was not surprising since topics related to ESD were included in teacher education in Vietnam, although it was not enough [38]. Overall, this finding is consistent with the previous studies [33,62]. The perspectives of elementary pre-service teachers on sustainable development improved after the training process. It was interesting to find that the difference in mean scores between the sophomores and juniors was insignificant. That means there was no outstanding improvement in these two middle years at the university on sustainable development perspectives. It could be explained that pre-service teachers in the second year and the third year were in the process of gaining their knowledge of sustainable development through courses at the university. Accordingly, their understanding of this concept reached a peak in the fourth year, when they had experienced courses and two internships. Based on this situation, the curriculum developers in higher education need to have a thorough discussion and analysis of the progress and content of the enacted training curriculum for each grade level to meet the 17 goals of sustainable development, especially in the second year and the third year at university.
In particular, the scores of freshmen were not significantly lower than those of seniors in their knowledge of sustainable development. The recent literature showed a similar result and explained this finding [68]. This fact is probably due to the Kruger-Dunnin effect [67], as an individual with less competence and less knowledge has an illusory sense of superiority, considering themselves to be more intelligent than other better-prepared individuals. This meant perhaps that freshmen were confident in their understanding, but it did not ensure that they were knowledgeable about it. This analysis was discussed more in the qualitative data. Pre-service teachers knew the concept of sustainable development, but their understanding was not comprehensive. This finding was an essential basis for administrators to design the curriculum content that meets pre-service teachers' needs. University administrators must create courses that provide more solid knowledge about sustainable development because accurate and sufficient knowledge leads pre-service teachers to have suitable awareness and act in sustainable ways.
In terms of sustainability competencies, level 1 (Know) was the lowest of the three levels, while level 3 (Demonstrate and Do) was the highest. The low learning achieved at level 1 for all four dimensions of perspectives can be understood as a failure in the development of sustainability competence in teacher education since one of the specific competencies to be developed in teacher education is precisely "knowing the interrelation between natural, social, and economic issues". Meanwhile, this result means that preservice teachers know how to function satisfactorily in community educational projects. In a similar way, the pre-service teachers answered in the interview part that they know how to work well in community educational projects through extra activities or volunteer programs. These results were consistent with the conclusions of other studies [69,70] that found little or no transfer from theory to practice in teacher training. The results obtained show, for competency level, the gap that exists between theory and practice. According to these results, pre-service teachers perform in four dimensions of sustainable development perspectives without having solid basic training. Thus, the training curriculum should be distributed with a balanced proportion of sustainability theory and practice because if pre-service teachers' knowledge is insufficient, their attitudes and behaviors will not be suitable [21].
Regarding gender, pre-service teachers showed a significant difference in the perspectives on sustainable development between males and females. These quantitative data supported the findings suggested by previous research [21] and denied the findings of "no significant difference based on gender" [43] in the context of University Sains, Malaysia. However, the interview findings did not present the evidence to conclude a significant difference in the competencies of ESD between male and female pre-service teachers. It requires more research on gender stereotypes. A male seems to be the leader in an elementary school because they are more disciplined and can easily persuade others to act sustainably [47,48]. From this view, gender stereotypes may become a reason to explain the higher scores of male pre-service teachers than female ones in this research.
The results of the enacted training curriculum revealed that elementary pre-service teachers have a basic understanding of sustainable development and have quite a good ability to integrate sustainability issues into their teaching process, particularly juniors and seniors. This could be explained by the efficacy of well-trained courses in universities [68]. However, the teacher training curriculum was still assessed as lacking comprehensiveness about the concept of sustainable development. Hence, higher education institutes need to develop a suitable curriculum framework for ESD and apply more modern teaching methods to real-world problems in order to increase pre-service teachers' understanding, attitudes, and actions in ESD. Moreover, the university should promote the sustainable development model through all the actions of lecturers, pre-service teachers, and other staff in the university.
Besides the value of results from the mixed-method design, this study has several limitations that should be taken into account. First, the sample size is not representative because all participants come from the Department of Elementary Education at a university in Southern Vietnam. It would have better if this study could have conducted and collected data from more universities of education.
Second, the gender distribution of the sample in this study was unequal (85 males and 307 females), although this distribution matched the gender distribution of the population. This study tried to increase the number of male pre-service teachers to make the distribution of gender groups more balanced. However, despite the efforts, this study did not find an equal number of males and females for the quantitative data analysis. So the gender of the sample may well be representative if the number of male participants could be increased.
Third, other types of demographics, such as knowledge about the sociocultural, discipline, including natural science education and social science education fields, living area in rural or in urban, and family background, could influence their responses. These demographic data related to their perspectives on sustainable development are not analyzed in the present study.
Finally, a more detailed analysis of the implementation of sustainable development in the training curriculum needs to be clarified. The number of courses that develop ESD in the analyzed curriculum is also small, and therefore probably few learning outcomes of the sustainability map are developed in the curriculum. This fact makes it difficult to draw clear conclusions due to the limited available data. It would provide more informative results if the sustainability perspective of pre-service teachers were compared with the number or quality of courses and activities in the training curriculum by the survey in order to analyze which courses achieve their learning objectives and which do not. These limitations leave other suggestions for future studies on the sustainable perspective of pre-service teachers.

Conclusions
Both quantitative and qualitative research questions were utilized in this study. Three key research questions were examined during this study.
Regarding the first research question, both quantitative and qualitative results showed that pre-service teachers' perspectives on the sustainable development concept changed positively following four years of their academic study. Seniors had a significantly higher score for most sustainable development perspectives than freshmen; while only the dimensions of "critical contextualization of knowledge" and "Know level" did not show any significant difference in sustainable development perspectives. Significantly, the level of knowledge was rated with higher scores than the level of action by pre-service teachers. Despite the improvement in perspectives about sustainable development, the interview results showed that pre-service teachers had insufficient knowledge about this concept. They focused on environmental issues, while the definition of sustainable development is broader, including environmental, social, and economic components. Moreover, juniors and seniors had a higher ability to make the lesson plans integrating sustainability into their class than freshmen and juniors.
Regarding the second research question, quantitative results showed significant differences in all dimensions of sustainable development perspectives between male and female pre-service teachers, except for the dimension "participation in community processes". Male pre-service teachers rated higher scores than female ones on perspectives of sustainable development, especially knowledge and action. However, the qualitative data found that no major significance when analyzing pre-service teachers' gender. It could be explained that the number of male interview samples was insufficient and gender stereotypes need to be considered.
Regarding the third research question, qualitative data revealed that some courses in the training curriculum integrated sustainable development content. However, they suggested these courses should be changed from elective to required courses because of the essential role of the ESD in their teaching career in the future. In addition, some lecturers in the university have integrated the issue of sustainable development into their lectures. However, they should use more STEM/STEAM, project-based learning, and experiential learning methods to develop pre-service teachers' awareness, attitudes, and behaviors towards sustainable development.
Based on the findings of this study and their limitations, it would also be interesting to examine the interactions between pre-service teachers' other demographics (e.g., discipline, living area, characteristics, and family background) and their perspectives on sustainable development. Moreover, this study did not find an equal number of males and females. It could be suggested to increase male samples to find out if gender factors influenced the preservice teachers' perspective on sustainable development. However, future studies should pay more consideration to the representativeness with the sample distribution matching the population distribution. Last, it would not be enough if this study only measured the presence of sustainable development in the enacted curriculum; future researchers need to measure how many hours are spent developing sustainability competencies and the types of activities that are carried out to achieve them. Since the fact that there are other elements to impact the success of the training curriculum for developing sustainability competencies of pre-service teachers, not only the number of courses is great or small.